Related
What are the differences in implementing interfaces implicitly and explicitly in C#?
When should you use implicit and when should you use explicit?
Are there any pros and/or cons to one or the other?
Microsoft's official guidelines (from first edition Framework Design Guidelines) states that using explicit implementations are not recommended, since it gives the code unexpected behaviour.
I think this guideline is very valid in a pre-IoC-time, when you don't pass things around as interfaces.
Could anyone touch on that aspect as well?
Implicit is when you define your interface via a member on your class. Explicit is when you define methods within your class on the interface. I know that sounds confusing but here is what I mean: IList.CopyTo would be implicitly implemented as:
public void CopyTo(Array array, int index)
{
throw new NotImplementedException();
}
and explicitly as:
void ICollection.CopyTo(Array array, int index)
{
throw new NotImplementedException();
}
The difference is that implicit implementation allows you to access the interface through the class you created by casting the interface as that class and as the interface itself. Explicit implementation allows you to access the interface only by casting it as the interface itself.
MyClass myClass = new MyClass(); // Declared as concrete class
myclass.CopyTo //invalid with explicit
((IList)myClass).CopyTo //valid with explicit.
I use explicit primarily to keep the implementation clean, or when I need two implementations. Regardless, I rarely use it.
I am sure there are more reasons to use/not use explicit that others will post.
See the next post in this thread for excellent reasoning behind each.
Implicit definition would be to just add the methods / properties, etc. demanded by the interface directly to the class as public methods.
Explicit definition forces the members to be exposed only when you are working with the interface directly, and not the underlying implementation. This is preferred in most cases.
By working directly with the interface, you are not acknowledging,
and coupling your code to the underlying implementation.
In the event that you already have, say, a public property Name in
your code and you want to implement an interface that also has a
Name property, doing it explicitly will keep the two separate. Even
if they were doing the same thing I'd still delegate the explicit
call to the Name property. You never know, you may want to change
how Name works for the normal class and how Name, the interface
property works later on.
If you implement an interface implicitly then your class now exposes
new behaviours that might only be relevant to a client of the
interface and it means you aren't keeping your classes succinct
enough (my opinion).
In addition to excellent answers already provided, there are some cases where explicit implementation is REQUIRED for the compiler to be able to figure out what is required. Take a look at IEnumerable<T> as a prime example that will likely come up fairly often.
Here's an example:
public abstract class StringList : IEnumerable<string>
{
private string[] _list = new string[] {"foo", "bar", "baz"};
// ...
#region IEnumerable<string> Members
public IEnumerator<string> GetEnumerator()
{
foreach (string s in _list)
{ yield return s; }
}
#endregion
#region IEnumerable Members
IEnumerator IEnumerable.GetEnumerator()
{
return this.GetEnumerator();
}
#endregion
}
Here, IEnumerable<string> implements IEnumerable, hence we need to too. But hang on, both the generic and the normal version both implement functions with the same method signature (C# ignores return type for this). This is completely legal and fine. How does the compiler resolve which to use? It forces you to only have, at most, one implicit definition, then it can resolve whatever it needs to.
ie.
StringList sl = new StringList();
// uses the implicit definition.
IEnumerator<string> enumerableString = sl.GetEnumerator();
// same as above, only a little more explicit.
IEnumerator<string> enumerableString2 = ((IEnumerable<string>)sl).GetEnumerator();
// returns the same as above, but via the explicit definition
IEnumerator enumerableStuff = ((IEnumerable)sl).GetEnumerator();
PS: The little piece of indirection in the explicit definition for IEnumerable works because inside the function the compiler knows that the actual type of the variable is a StringList, and that's how it resolves the function call. Nifty little fact for implementing some of the layers of abstraction some of the .NET core interfaces seem to have accumulated.
Reason #1
I tend to use explicit interface implementation when I want to discourage "programming to an implementation" (Design Principles from Design Patterns).
For example, in an MVP-based web application:
public interface INavigator {
void Redirect(string url);
}
public sealed class StandardNavigator : INavigator {
void INavigator.Redirect(string url) {
Response.Redirect(url);
}
}
Now another class (such as a presenter) is less likely to depend on the StandardNavigator implementation and more likely to depend on the INavigator interface (since the implementation would need to be cast to an interface to make use of the Redirect method).
Reason #2
Another reason I might go with an explicit interface implementation would be to keep a class's "default" interface cleaner. For example, if I were developing an ASP.NET server control, I might want two interfaces:
The class's primary interface, which is used by web page developers; and
A "hidden" interface used by the presenter that I develop to handle the control's logic
A simple example follows. It's a combo box control that lists customers. In this example, the web page developer isn't interested in populating the list; instead, they just want to be able to select a customer by GUID or to obtain the selected customer's GUID. A presenter would populate the box on the first page load, and this presenter is encapsulated by the control.
public sealed class CustomerComboBox : ComboBox, ICustomerComboBox {
private readonly CustomerComboBoxPresenter presenter;
public CustomerComboBox() {
presenter = new CustomerComboBoxPresenter(this);
}
protected override void OnLoad() {
if (!Page.IsPostBack) presenter.HandleFirstLoad();
}
// Primary interface used by web page developers
public Guid ClientId {
get { return new Guid(SelectedItem.Value); }
set { SelectedItem.Value = value.ToString(); }
}
// "Hidden" interface used by presenter
IEnumerable<CustomerDto> ICustomerComboBox.DataSource { set; }
}
The presenter populates the data source, and the web page developer never needs to be aware of its existence.
But's It's Not a Silver Cannonball
I wouldn't recommend always employing explicit interface implementations. Those are just two examples where they might be helpful.
To quote Jeffrey Richter from CLR via C#
(EIMI means Explicit Interface Method Implementation)
It is critically important for you to
understand some ramifications that
exist when using EIMIs. And because of
these ramifications, you should try to
avoid EIMIs as much as possible.
Fortunately, generic interfaces help
you avoid EIMIs quite a bit. But there
may still be times when you will need
to use them (such as implementing two
interface methods with the same name
and signature). Here are the big
problems with EIMIs:
There is no documentation explaining how a type specifically
implements an EIMI method, and there
is no Microsoft Visual Studio
IntelliSense support.
Value type instances are boxed when cast to an interface.
An EIMI cannot be called by a derived type.
If you use an interface reference ANY virtual chain can be explicitly replaced with EIMI on any derived class and when an object of such type is cast to the interface, your virtual chain is ignored and the explicit implementation is called. That's anything but polymorphism.
EIMIs can also be used to hide non-strongly typed interface members from basic Framework Interfaces' implementations such as IEnumerable<T> so your class doesn't expose a non strongly typed method directly, but is syntactical correct.
I use explicit interface implementation most of the time. Here are the main reasons.
Refactoring is safer
When changing an interface, it's better if the compiler can check it. This is harder with implicit implementations.
Two common cases come to mind:
Adding a function to an interface, where an existing class that implements this interface already happens to have a method with the same signature as the new one. This can lead to unexpected behavior, and has bitten me hard several times. It's difficult to "see" when debugging because that function is likely not located with the other interface methods in the file (the self-documenting issue mentioned below).
Removing a function from an interface. Implicitly implemented methods will be suddenly dead code, but explicitly implemented methods will get caught by compile error. Even if the dead code is good to keep around, I want to be forced to review it and promote it.
It's unfortunate that C# doesn't have a keyword that forces us to mark a method as an implicit implementation, so the compiler could do the extra checks. Virtual methods don't have either of the above problems due to required use of 'override' and 'new'.
Note: for fixed or rarely-changing interfaces (typically from vendor API's), this is not a problem. For my own interfaces, though, I can't predict when/how they will change.
It's self-documenting
If I see 'public bool Execute()' in a class, it's going to take extra work to figure out that it's part of an interface. Somebody will probably have to comment it saying so, or put it in a group of other interface implementations, all under a region or grouping comment saying "implementation of ITask". Of course, that only works if the group header isn't offscreen..
Whereas: 'bool ITask.Execute()' is clear and unambiguous.
Clear separation of interface implementation
I think of interfaces as being more 'public' than public methods because they are crafted to expose just a bit of the surface area of the concrete type. They reduce the type to a capability, a behavior, a set of traits, etc. And in the implementation, I think it's useful to keep this separation.
As I am looking through a class's code, when I come across explicit interface implementations, my brain shifts into "code contract" mode. Often these implementations simply forward to other methods, but sometimes they will do extra state/param checking, conversion of incoming parameters to better match internal requirements, or even translation for versioning purposes (i.e. multiple generations of interfaces all punting down to common implementations).
(I realize that publics are also code contracts, but interfaces are much stronger, especially in an interface-driven codebase where direct use of concrete types is usually a sign of internal-only code.)
Related: Reason 2 above by Jon.
And so on
Plus the advantages already mentioned in other answers here:
When required, as per disambiguation or needing an internal interface
Discourages "programming to an implementation" (Reason 1 by Jon)
Problems
It's not all fun and happiness. There are some cases where I stick with implicits:
Value types, because that will require boxing and lower perf. This isn't a strict rule, and depends on the interface and how it's intended to be used. IComparable? Implicit. IFormattable? Probably explicit.
Trivial system interfaces that have methods that are frequently called directly (like IDisposable.Dispose).
Also, it can be a pain to do the casting when you do in fact have the concrete type and want to call an explicit interface method. I deal with this in one of two ways:
Add publics and have the interface methods forward to them for the implementation. Typically happens with simpler interfaces when working internally.
(My preferred method) Add a public IMyInterface I { get { return this; } } (which should get inlined) and call foo.I.InterfaceMethod(). If multiple interfaces that need this ability, expand the name beyond I (in my experience it's rare that I have this need).
In addition to the other reasons already stated, this is the situation in which a class is implementing two different interfaces that have a property/method with the same name and signature.
/// <summary>
/// This is a Book
/// </summary>
interface IBook
{
string Title { get; }
string ISBN { get; }
}
/// <summary>
/// This is a Person
/// </summary>
interface IPerson
{
string Title { get; }
string Forename { get; }
string Surname { get; }
}
/// <summary>
/// This is some freaky book-person.
/// </summary>
class Class1 : IBook, IPerson
{
/// <summary>
/// This method is shared by both Book and Person
/// </summary>
public string Title
{
get
{
string personTitle = "Mr";
string bookTitle = "The Hitchhikers Guide to the Galaxy";
// What do we do here?
return null;
}
}
#region IPerson Members
public string Forename
{
get { return "Lee"; }
}
public string Surname
{
get { return "Oades"; }
}
#endregion
#region IBook Members
public string ISBN
{
get { return "1-904048-46-3"; }
}
#endregion
}
This code compiles and runs OK, but the Title property is shared.
Clearly, we'd want the value of Title returned to depend on whether we were treating Class1 as a Book or a Person. This is when we can use the explicit interface.
string IBook.Title
{
get
{
return "The Hitchhikers Guide to the Galaxy";
}
}
string IPerson.Title
{
get
{
return "Mr";
}
}
public string Title
{
get { return "Still shared"; }
}
Notice that the explicit interface definitions are inferred to be Public - and hence you can't declare them to be public (or otherwise) explicitly.
Note also that you can still have a "shared" version (as shown above), but whilst this is possible, the existence of such a property is questionable. Perhaps it could be used as a default implementation of Title - so that existing code would not have to be modified to cast Class1 to IBook or IPerson.
If you do not define the "shared" (implicit) Title, consumers of Class1 must explicitly cast instances of Class1 to IBook or IPerson first - otherwise the code will not compile.
If you implement explicitly, you will only be able to reference the interface members through a reference that is of the type of the interface. A reference that is the type of the implementing class will not expose those interface members.
If your implementing class is not public, except for the method used to create the class (which could be a factory or IoC container), and except for the interface methods (of course), then I don't see any advantage to explicitly implementing interfaces.
Otherwise, explicitly implementing interfaces makes sure that references to your concrete implementing class are not used, allowing you to change that implementation at a later time. "Makes sure", I suppose, is the "advantage". A well-factored implementation can accomplish this without explicit implementation.
The disadvantage, in my opinion, is that you will find yourself casting types to/from the interface in the implementation code that does have access to non-public members.
Like many things, the advantage is the disadvantage (and vice-versa). Explicitly implementing interfaces will ensure that your concrete class implementation code is not exposed.
An implicit interface implementation is where you have a method with the same signature of the interface.
An explicit interface implementation is where you explicitly declare which interface the method belongs to.
interface I1
{
void implicitExample();
}
interface I2
{
void explicitExample();
}
class C : I1, I2
{
void implicitExample()
{
Console.WriteLine("I1.implicitExample()");
}
void I2.explicitExample()
{
Console.WriteLine("I2.explicitExample()");
}
}
MSDN: implicit and explicit interface implementations
Every class member that implements an interface exports a declaration which is semantically similar to the way VB.NET interface declarations are written, e.g.
Public Overridable Function Foo() As Integer Implements IFoo.Foo
Although the name of the class member will often match that of the interface member, and the class member will often be public, neither of those things is required. One may also declare:
Protected Overridable Function IFoo_Foo() As Integer Implements IFoo.Foo
In which case the class and its derivatives would be allowed to access a class member using the name IFoo_Foo, but the outside world would only be able to access that particular member by casting to IFoo. Such an approach is often good in cases where an interface method will have specified behavior on all implementations, but useful behavior on only some [e.g. the specified behavior for a read-only collection's IList<T>.Add method is to throw NotSupportedException]. Unfortunately, the only proper way to implement the interface in C# is:
int IFoo.Foo() { return IFoo_Foo(); }
protected virtual int IFoo_Foo() { ... real code goes here ... }
Not as nice.
The previous answers explain why implementing an interface explicitly in C# may be preferrable (for mostly formal reasons). However, there is one situation where explicit implementation is mandatory: In order to avoid leaking the encapsulation when the interface is non-public, but the implementing class is public.
// Given:
internal interface I { void M(); }
// Then explicit implementation correctly observes encapsulation of I:
// Both ((I)CExplicit).M and CExplicit.M are accessible only internally.
public class CExplicit: I { void I.M() { } }
// However, implicit implementation breaks encapsulation of I, because
// ((I)CImplicit).M is only accessible internally, while CImplicit.M is accessible publicly.
public class CImplicit: I { public void M() { } }
The above leakage is unavoidable because, according to the C# specification, "All interface members implicitly have public access." As a consequence, implicit implementations must also give public access, even if the interface itself is e.g. internal.
Implicit interface implementation in C# is a great convenience. In practice, many programmers use it all the time/everywhere without further consideration. This leads to messy type surfaces at best and leaked encapsulation at worst. Other languages, such as F#, don't even allow it.
One important use of explicit interface implementation is when in need to implement interfaces with mixed visibility.
The problem and solution are well explained in the article C# Internal Interface.
For example, if you want to protect leakage of objects between application layers, this technique allows you to specify different visibility of members that could cause the leakage.
I've found myself using explicit implementations more often recently, for the following practical reasons:
Always using explicit from the starts prevents having any naming collisions, in which explicit implementation would be required anyways
Consumers are "forced" to use the interface instead of the implementation (aka not "programming to an implementation") which they should / must do anyways when you're using DI
No "zombie" members in the implementations - removing any member from the interface declaration will result in compiler errors if not removed from the implementation too
Default values for optional parameters, as well constraints on generic arguments are automatically adopted - no need to write them twice and keep them in sync
What are the differences in implementing interfaces implicitly and explicitly in C#?
When should you use implicit and when should you use explicit?
Are there any pros and/or cons to one or the other?
Microsoft's official guidelines (from first edition Framework Design Guidelines) states that using explicit implementations are not recommended, since it gives the code unexpected behaviour.
I think this guideline is very valid in a pre-IoC-time, when you don't pass things around as interfaces.
Could anyone touch on that aspect as well?
Implicit is when you define your interface via a member on your class. Explicit is when you define methods within your class on the interface. I know that sounds confusing but here is what I mean: IList.CopyTo would be implicitly implemented as:
public void CopyTo(Array array, int index)
{
throw new NotImplementedException();
}
and explicitly as:
void ICollection.CopyTo(Array array, int index)
{
throw new NotImplementedException();
}
The difference is that implicit implementation allows you to access the interface through the class you created by casting the interface as that class and as the interface itself. Explicit implementation allows you to access the interface only by casting it as the interface itself.
MyClass myClass = new MyClass(); // Declared as concrete class
myclass.CopyTo //invalid with explicit
((IList)myClass).CopyTo //valid with explicit.
I use explicit primarily to keep the implementation clean, or when I need two implementations. Regardless, I rarely use it.
I am sure there are more reasons to use/not use explicit that others will post.
See the next post in this thread for excellent reasoning behind each.
Implicit definition would be to just add the methods / properties, etc. demanded by the interface directly to the class as public methods.
Explicit definition forces the members to be exposed only when you are working with the interface directly, and not the underlying implementation. This is preferred in most cases.
By working directly with the interface, you are not acknowledging,
and coupling your code to the underlying implementation.
In the event that you already have, say, a public property Name in
your code and you want to implement an interface that also has a
Name property, doing it explicitly will keep the two separate. Even
if they were doing the same thing I'd still delegate the explicit
call to the Name property. You never know, you may want to change
how Name works for the normal class and how Name, the interface
property works later on.
If you implement an interface implicitly then your class now exposes
new behaviours that might only be relevant to a client of the
interface and it means you aren't keeping your classes succinct
enough (my opinion).
In addition to excellent answers already provided, there are some cases where explicit implementation is REQUIRED for the compiler to be able to figure out what is required. Take a look at IEnumerable<T> as a prime example that will likely come up fairly often.
Here's an example:
public abstract class StringList : IEnumerable<string>
{
private string[] _list = new string[] {"foo", "bar", "baz"};
// ...
#region IEnumerable<string> Members
public IEnumerator<string> GetEnumerator()
{
foreach (string s in _list)
{ yield return s; }
}
#endregion
#region IEnumerable Members
IEnumerator IEnumerable.GetEnumerator()
{
return this.GetEnumerator();
}
#endregion
}
Here, IEnumerable<string> implements IEnumerable, hence we need to too. But hang on, both the generic and the normal version both implement functions with the same method signature (C# ignores return type for this). This is completely legal and fine. How does the compiler resolve which to use? It forces you to only have, at most, one implicit definition, then it can resolve whatever it needs to.
ie.
StringList sl = new StringList();
// uses the implicit definition.
IEnumerator<string> enumerableString = sl.GetEnumerator();
// same as above, only a little more explicit.
IEnumerator<string> enumerableString2 = ((IEnumerable<string>)sl).GetEnumerator();
// returns the same as above, but via the explicit definition
IEnumerator enumerableStuff = ((IEnumerable)sl).GetEnumerator();
PS: The little piece of indirection in the explicit definition for IEnumerable works because inside the function the compiler knows that the actual type of the variable is a StringList, and that's how it resolves the function call. Nifty little fact for implementing some of the layers of abstraction some of the .NET core interfaces seem to have accumulated.
Reason #1
I tend to use explicit interface implementation when I want to discourage "programming to an implementation" (Design Principles from Design Patterns).
For example, in an MVP-based web application:
public interface INavigator {
void Redirect(string url);
}
public sealed class StandardNavigator : INavigator {
void INavigator.Redirect(string url) {
Response.Redirect(url);
}
}
Now another class (such as a presenter) is less likely to depend on the StandardNavigator implementation and more likely to depend on the INavigator interface (since the implementation would need to be cast to an interface to make use of the Redirect method).
Reason #2
Another reason I might go with an explicit interface implementation would be to keep a class's "default" interface cleaner. For example, if I were developing an ASP.NET server control, I might want two interfaces:
The class's primary interface, which is used by web page developers; and
A "hidden" interface used by the presenter that I develop to handle the control's logic
A simple example follows. It's a combo box control that lists customers. In this example, the web page developer isn't interested in populating the list; instead, they just want to be able to select a customer by GUID or to obtain the selected customer's GUID. A presenter would populate the box on the first page load, and this presenter is encapsulated by the control.
public sealed class CustomerComboBox : ComboBox, ICustomerComboBox {
private readonly CustomerComboBoxPresenter presenter;
public CustomerComboBox() {
presenter = new CustomerComboBoxPresenter(this);
}
protected override void OnLoad() {
if (!Page.IsPostBack) presenter.HandleFirstLoad();
}
// Primary interface used by web page developers
public Guid ClientId {
get { return new Guid(SelectedItem.Value); }
set { SelectedItem.Value = value.ToString(); }
}
// "Hidden" interface used by presenter
IEnumerable<CustomerDto> ICustomerComboBox.DataSource { set; }
}
The presenter populates the data source, and the web page developer never needs to be aware of its existence.
But's It's Not a Silver Cannonball
I wouldn't recommend always employing explicit interface implementations. Those are just two examples where they might be helpful.
To quote Jeffrey Richter from CLR via C#
(EIMI means Explicit Interface Method Implementation)
It is critically important for you to
understand some ramifications that
exist when using EIMIs. And because of
these ramifications, you should try to
avoid EIMIs as much as possible.
Fortunately, generic interfaces help
you avoid EIMIs quite a bit. But there
may still be times when you will need
to use them (such as implementing two
interface methods with the same name
and signature). Here are the big
problems with EIMIs:
There is no documentation explaining how a type specifically
implements an EIMI method, and there
is no Microsoft Visual Studio
IntelliSense support.
Value type instances are boxed when cast to an interface.
An EIMI cannot be called by a derived type.
If you use an interface reference ANY virtual chain can be explicitly replaced with EIMI on any derived class and when an object of such type is cast to the interface, your virtual chain is ignored and the explicit implementation is called. That's anything but polymorphism.
EIMIs can also be used to hide non-strongly typed interface members from basic Framework Interfaces' implementations such as IEnumerable<T> so your class doesn't expose a non strongly typed method directly, but is syntactical correct.
I use explicit interface implementation most of the time. Here are the main reasons.
Refactoring is safer
When changing an interface, it's better if the compiler can check it. This is harder with implicit implementations.
Two common cases come to mind:
Adding a function to an interface, where an existing class that implements this interface already happens to have a method with the same signature as the new one. This can lead to unexpected behavior, and has bitten me hard several times. It's difficult to "see" when debugging because that function is likely not located with the other interface methods in the file (the self-documenting issue mentioned below).
Removing a function from an interface. Implicitly implemented methods will be suddenly dead code, but explicitly implemented methods will get caught by compile error. Even if the dead code is good to keep around, I want to be forced to review it and promote it.
It's unfortunate that C# doesn't have a keyword that forces us to mark a method as an implicit implementation, so the compiler could do the extra checks. Virtual methods don't have either of the above problems due to required use of 'override' and 'new'.
Note: for fixed or rarely-changing interfaces (typically from vendor API's), this is not a problem. For my own interfaces, though, I can't predict when/how they will change.
It's self-documenting
If I see 'public bool Execute()' in a class, it's going to take extra work to figure out that it's part of an interface. Somebody will probably have to comment it saying so, or put it in a group of other interface implementations, all under a region or grouping comment saying "implementation of ITask". Of course, that only works if the group header isn't offscreen..
Whereas: 'bool ITask.Execute()' is clear and unambiguous.
Clear separation of interface implementation
I think of interfaces as being more 'public' than public methods because they are crafted to expose just a bit of the surface area of the concrete type. They reduce the type to a capability, a behavior, a set of traits, etc. And in the implementation, I think it's useful to keep this separation.
As I am looking through a class's code, when I come across explicit interface implementations, my brain shifts into "code contract" mode. Often these implementations simply forward to other methods, but sometimes they will do extra state/param checking, conversion of incoming parameters to better match internal requirements, or even translation for versioning purposes (i.e. multiple generations of interfaces all punting down to common implementations).
(I realize that publics are also code contracts, but interfaces are much stronger, especially in an interface-driven codebase where direct use of concrete types is usually a sign of internal-only code.)
Related: Reason 2 above by Jon.
And so on
Plus the advantages already mentioned in other answers here:
When required, as per disambiguation or needing an internal interface
Discourages "programming to an implementation" (Reason 1 by Jon)
Problems
It's not all fun and happiness. There are some cases where I stick with implicits:
Value types, because that will require boxing and lower perf. This isn't a strict rule, and depends on the interface and how it's intended to be used. IComparable? Implicit. IFormattable? Probably explicit.
Trivial system interfaces that have methods that are frequently called directly (like IDisposable.Dispose).
Also, it can be a pain to do the casting when you do in fact have the concrete type and want to call an explicit interface method. I deal with this in one of two ways:
Add publics and have the interface methods forward to them for the implementation. Typically happens with simpler interfaces when working internally.
(My preferred method) Add a public IMyInterface I { get { return this; } } (which should get inlined) and call foo.I.InterfaceMethod(). If multiple interfaces that need this ability, expand the name beyond I (in my experience it's rare that I have this need).
In addition to the other reasons already stated, this is the situation in which a class is implementing two different interfaces that have a property/method with the same name and signature.
/// <summary>
/// This is a Book
/// </summary>
interface IBook
{
string Title { get; }
string ISBN { get; }
}
/// <summary>
/// This is a Person
/// </summary>
interface IPerson
{
string Title { get; }
string Forename { get; }
string Surname { get; }
}
/// <summary>
/// This is some freaky book-person.
/// </summary>
class Class1 : IBook, IPerson
{
/// <summary>
/// This method is shared by both Book and Person
/// </summary>
public string Title
{
get
{
string personTitle = "Mr";
string bookTitle = "The Hitchhikers Guide to the Galaxy";
// What do we do here?
return null;
}
}
#region IPerson Members
public string Forename
{
get { return "Lee"; }
}
public string Surname
{
get { return "Oades"; }
}
#endregion
#region IBook Members
public string ISBN
{
get { return "1-904048-46-3"; }
}
#endregion
}
This code compiles and runs OK, but the Title property is shared.
Clearly, we'd want the value of Title returned to depend on whether we were treating Class1 as a Book or a Person. This is when we can use the explicit interface.
string IBook.Title
{
get
{
return "The Hitchhikers Guide to the Galaxy";
}
}
string IPerson.Title
{
get
{
return "Mr";
}
}
public string Title
{
get { return "Still shared"; }
}
Notice that the explicit interface definitions are inferred to be Public - and hence you can't declare them to be public (or otherwise) explicitly.
Note also that you can still have a "shared" version (as shown above), but whilst this is possible, the existence of such a property is questionable. Perhaps it could be used as a default implementation of Title - so that existing code would not have to be modified to cast Class1 to IBook or IPerson.
If you do not define the "shared" (implicit) Title, consumers of Class1 must explicitly cast instances of Class1 to IBook or IPerson first - otherwise the code will not compile.
If you implement explicitly, you will only be able to reference the interface members through a reference that is of the type of the interface. A reference that is the type of the implementing class will not expose those interface members.
If your implementing class is not public, except for the method used to create the class (which could be a factory or IoC container), and except for the interface methods (of course), then I don't see any advantage to explicitly implementing interfaces.
Otherwise, explicitly implementing interfaces makes sure that references to your concrete implementing class are not used, allowing you to change that implementation at a later time. "Makes sure", I suppose, is the "advantage". A well-factored implementation can accomplish this without explicit implementation.
The disadvantage, in my opinion, is that you will find yourself casting types to/from the interface in the implementation code that does have access to non-public members.
Like many things, the advantage is the disadvantage (and vice-versa). Explicitly implementing interfaces will ensure that your concrete class implementation code is not exposed.
An implicit interface implementation is where you have a method with the same signature of the interface.
An explicit interface implementation is where you explicitly declare which interface the method belongs to.
interface I1
{
void implicitExample();
}
interface I2
{
void explicitExample();
}
class C : I1, I2
{
void implicitExample()
{
Console.WriteLine("I1.implicitExample()");
}
void I2.explicitExample()
{
Console.WriteLine("I2.explicitExample()");
}
}
MSDN: implicit and explicit interface implementations
Every class member that implements an interface exports a declaration which is semantically similar to the way VB.NET interface declarations are written, e.g.
Public Overridable Function Foo() As Integer Implements IFoo.Foo
Although the name of the class member will often match that of the interface member, and the class member will often be public, neither of those things is required. One may also declare:
Protected Overridable Function IFoo_Foo() As Integer Implements IFoo.Foo
In which case the class and its derivatives would be allowed to access a class member using the name IFoo_Foo, but the outside world would only be able to access that particular member by casting to IFoo. Such an approach is often good in cases where an interface method will have specified behavior on all implementations, but useful behavior on only some [e.g. the specified behavior for a read-only collection's IList<T>.Add method is to throw NotSupportedException]. Unfortunately, the only proper way to implement the interface in C# is:
int IFoo.Foo() { return IFoo_Foo(); }
protected virtual int IFoo_Foo() { ... real code goes here ... }
Not as nice.
The previous answers explain why implementing an interface explicitly in C# may be preferrable (for mostly formal reasons). However, there is one situation where explicit implementation is mandatory: In order to avoid leaking the encapsulation when the interface is non-public, but the implementing class is public.
// Given:
internal interface I { void M(); }
// Then explicit implementation correctly observes encapsulation of I:
// Both ((I)CExplicit).M and CExplicit.M are accessible only internally.
public class CExplicit: I { void I.M() { } }
// However, implicit implementation breaks encapsulation of I, because
// ((I)CImplicit).M is only accessible internally, while CImplicit.M is accessible publicly.
public class CImplicit: I { public void M() { } }
The above leakage is unavoidable because, according to the C# specification, "All interface members implicitly have public access." As a consequence, implicit implementations must also give public access, even if the interface itself is e.g. internal.
Implicit interface implementation in C# is a great convenience. In practice, many programmers use it all the time/everywhere without further consideration. This leads to messy type surfaces at best and leaked encapsulation at worst. Other languages, such as F#, don't even allow it.
One important use of explicit interface implementation is when in need to implement interfaces with mixed visibility.
The problem and solution are well explained in the article C# Internal Interface.
For example, if you want to protect leakage of objects between application layers, this technique allows you to specify different visibility of members that could cause the leakage.
I've found myself using explicit implementations more often recently, for the following practical reasons:
Always using explicit from the starts prevents having any naming collisions, in which explicit implementation would be required anyways
Consumers are "forced" to use the interface instead of the implementation (aka not "programming to an implementation") which they should / must do anyways when you're using DI
No "zombie" members in the implementations - removing any member from the interface declaration will result in compiler errors if not removed from the implementation too
Default values for optional parameters, as well constraints on generic arguments are automatically adopted - no need to write them twice and keep them in sync
What are the differences in implementing interfaces implicitly and explicitly in C#?
When should you use implicit and when should you use explicit?
Are there any pros and/or cons to one or the other?
Microsoft's official guidelines (from first edition Framework Design Guidelines) states that using explicit implementations are not recommended, since it gives the code unexpected behaviour.
I think this guideline is very valid in a pre-IoC-time, when you don't pass things around as interfaces.
Could anyone touch on that aspect as well?
Implicit is when you define your interface via a member on your class. Explicit is when you define methods within your class on the interface. I know that sounds confusing but here is what I mean: IList.CopyTo would be implicitly implemented as:
public void CopyTo(Array array, int index)
{
throw new NotImplementedException();
}
and explicitly as:
void ICollection.CopyTo(Array array, int index)
{
throw new NotImplementedException();
}
The difference is that implicit implementation allows you to access the interface through the class you created by casting the interface as that class and as the interface itself. Explicit implementation allows you to access the interface only by casting it as the interface itself.
MyClass myClass = new MyClass(); // Declared as concrete class
myclass.CopyTo //invalid with explicit
((IList)myClass).CopyTo //valid with explicit.
I use explicit primarily to keep the implementation clean, or when I need two implementations. Regardless, I rarely use it.
I am sure there are more reasons to use/not use explicit that others will post.
See the next post in this thread for excellent reasoning behind each.
Implicit definition would be to just add the methods / properties, etc. demanded by the interface directly to the class as public methods.
Explicit definition forces the members to be exposed only when you are working with the interface directly, and not the underlying implementation. This is preferred in most cases.
By working directly with the interface, you are not acknowledging,
and coupling your code to the underlying implementation.
In the event that you already have, say, a public property Name in
your code and you want to implement an interface that also has a
Name property, doing it explicitly will keep the two separate. Even
if they were doing the same thing I'd still delegate the explicit
call to the Name property. You never know, you may want to change
how Name works for the normal class and how Name, the interface
property works later on.
If you implement an interface implicitly then your class now exposes
new behaviours that might only be relevant to a client of the
interface and it means you aren't keeping your classes succinct
enough (my opinion).
In addition to excellent answers already provided, there are some cases where explicit implementation is REQUIRED for the compiler to be able to figure out what is required. Take a look at IEnumerable<T> as a prime example that will likely come up fairly often.
Here's an example:
public abstract class StringList : IEnumerable<string>
{
private string[] _list = new string[] {"foo", "bar", "baz"};
// ...
#region IEnumerable<string> Members
public IEnumerator<string> GetEnumerator()
{
foreach (string s in _list)
{ yield return s; }
}
#endregion
#region IEnumerable Members
IEnumerator IEnumerable.GetEnumerator()
{
return this.GetEnumerator();
}
#endregion
}
Here, IEnumerable<string> implements IEnumerable, hence we need to too. But hang on, both the generic and the normal version both implement functions with the same method signature (C# ignores return type for this). This is completely legal and fine. How does the compiler resolve which to use? It forces you to only have, at most, one implicit definition, then it can resolve whatever it needs to.
ie.
StringList sl = new StringList();
// uses the implicit definition.
IEnumerator<string> enumerableString = sl.GetEnumerator();
// same as above, only a little more explicit.
IEnumerator<string> enumerableString2 = ((IEnumerable<string>)sl).GetEnumerator();
// returns the same as above, but via the explicit definition
IEnumerator enumerableStuff = ((IEnumerable)sl).GetEnumerator();
PS: The little piece of indirection in the explicit definition for IEnumerable works because inside the function the compiler knows that the actual type of the variable is a StringList, and that's how it resolves the function call. Nifty little fact for implementing some of the layers of abstraction some of the .NET core interfaces seem to have accumulated.
Reason #1
I tend to use explicit interface implementation when I want to discourage "programming to an implementation" (Design Principles from Design Patterns).
For example, in an MVP-based web application:
public interface INavigator {
void Redirect(string url);
}
public sealed class StandardNavigator : INavigator {
void INavigator.Redirect(string url) {
Response.Redirect(url);
}
}
Now another class (such as a presenter) is less likely to depend on the StandardNavigator implementation and more likely to depend on the INavigator interface (since the implementation would need to be cast to an interface to make use of the Redirect method).
Reason #2
Another reason I might go with an explicit interface implementation would be to keep a class's "default" interface cleaner. For example, if I were developing an ASP.NET server control, I might want two interfaces:
The class's primary interface, which is used by web page developers; and
A "hidden" interface used by the presenter that I develop to handle the control's logic
A simple example follows. It's a combo box control that lists customers. In this example, the web page developer isn't interested in populating the list; instead, they just want to be able to select a customer by GUID or to obtain the selected customer's GUID. A presenter would populate the box on the first page load, and this presenter is encapsulated by the control.
public sealed class CustomerComboBox : ComboBox, ICustomerComboBox {
private readonly CustomerComboBoxPresenter presenter;
public CustomerComboBox() {
presenter = new CustomerComboBoxPresenter(this);
}
protected override void OnLoad() {
if (!Page.IsPostBack) presenter.HandleFirstLoad();
}
// Primary interface used by web page developers
public Guid ClientId {
get { return new Guid(SelectedItem.Value); }
set { SelectedItem.Value = value.ToString(); }
}
// "Hidden" interface used by presenter
IEnumerable<CustomerDto> ICustomerComboBox.DataSource { set; }
}
The presenter populates the data source, and the web page developer never needs to be aware of its existence.
But's It's Not a Silver Cannonball
I wouldn't recommend always employing explicit interface implementations. Those are just two examples where they might be helpful.
To quote Jeffrey Richter from CLR via C#
(EIMI means Explicit Interface Method Implementation)
It is critically important for you to
understand some ramifications that
exist when using EIMIs. And because of
these ramifications, you should try to
avoid EIMIs as much as possible.
Fortunately, generic interfaces help
you avoid EIMIs quite a bit. But there
may still be times when you will need
to use them (such as implementing two
interface methods with the same name
and signature). Here are the big
problems with EIMIs:
There is no documentation explaining how a type specifically
implements an EIMI method, and there
is no Microsoft Visual Studio
IntelliSense support.
Value type instances are boxed when cast to an interface.
An EIMI cannot be called by a derived type.
If you use an interface reference ANY virtual chain can be explicitly replaced with EIMI on any derived class and when an object of such type is cast to the interface, your virtual chain is ignored and the explicit implementation is called. That's anything but polymorphism.
EIMIs can also be used to hide non-strongly typed interface members from basic Framework Interfaces' implementations such as IEnumerable<T> so your class doesn't expose a non strongly typed method directly, but is syntactical correct.
I use explicit interface implementation most of the time. Here are the main reasons.
Refactoring is safer
When changing an interface, it's better if the compiler can check it. This is harder with implicit implementations.
Two common cases come to mind:
Adding a function to an interface, where an existing class that implements this interface already happens to have a method with the same signature as the new one. This can lead to unexpected behavior, and has bitten me hard several times. It's difficult to "see" when debugging because that function is likely not located with the other interface methods in the file (the self-documenting issue mentioned below).
Removing a function from an interface. Implicitly implemented methods will be suddenly dead code, but explicitly implemented methods will get caught by compile error. Even if the dead code is good to keep around, I want to be forced to review it and promote it.
It's unfortunate that C# doesn't have a keyword that forces us to mark a method as an implicit implementation, so the compiler could do the extra checks. Virtual methods don't have either of the above problems due to required use of 'override' and 'new'.
Note: for fixed or rarely-changing interfaces (typically from vendor API's), this is not a problem. For my own interfaces, though, I can't predict when/how they will change.
It's self-documenting
If I see 'public bool Execute()' in a class, it's going to take extra work to figure out that it's part of an interface. Somebody will probably have to comment it saying so, or put it in a group of other interface implementations, all under a region or grouping comment saying "implementation of ITask". Of course, that only works if the group header isn't offscreen..
Whereas: 'bool ITask.Execute()' is clear and unambiguous.
Clear separation of interface implementation
I think of interfaces as being more 'public' than public methods because they are crafted to expose just a bit of the surface area of the concrete type. They reduce the type to a capability, a behavior, a set of traits, etc. And in the implementation, I think it's useful to keep this separation.
As I am looking through a class's code, when I come across explicit interface implementations, my brain shifts into "code contract" mode. Often these implementations simply forward to other methods, but sometimes they will do extra state/param checking, conversion of incoming parameters to better match internal requirements, or even translation for versioning purposes (i.e. multiple generations of interfaces all punting down to common implementations).
(I realize that publics are also code contracts, but interfaces are much stronger, especially in an interface-driven codebase where direct use of concrete types is usually a sign of internal-only code.)
Related: Reason 2 above by Jon.
And so on
Plus the advantages already mentioned in other answers here:
When required, as per disambiguation or needing an internal interface
Discourages "programming to an implementation" (Reason 1 by Jon)
Problems
It's not all fun and happiness. There are some cases where I stick with implicits:
Value types, because that will require boxing and lower perf. This isn't a strict rule, and depends on the interface and how it's intended to be used. IComparable? Implicit. IFormattable? Probably explicit.
Trivial system interfaces that have methods that are frequently called directly (like IDisposable.Dispose).
Also, it can be a pain to do the casting when you do in fact have the concrete type and want to call an explicit interface method. I deal with this in one of two ways:
Add publics and have the interface methods forward to them for the implementation. Typically happens with simpler interfaces when working internally.
(My preferred method) Add a public IMyInterface I { get { return this; } } (which should get inlined) and call foo.I.InterfaceMethod(). If multiple interfaces that need this ability, expand the name beyond I (in my experience it's rare that I have this need).
In addition to the other reasons already stated, this is the situation in which a class is implementing two different interfaces that have a property/method with the same name and signature.
/// <summary>
/// This is a Book
/// </summary>
interface IBook
{
string Title { get; }
string ISBN { get; }
}
/// <summary>
/// This is a Person
/// </summary>
interface IPerson
{
string Title { get; }
string Forename { get; }
string Surname { get; }
}
/// <summary>
/// This is some freaky book-person.
/// </summary>
class Class1 : IBook, IPerson
{
/// <summary>
/// This method is shared by both Book and Person
/// </summary>
public string Title
{
get
{
string personTitle = "Mr";
string bookTitle = "The Hitchhikers Guide to the Galaxy";
// What do we do here?
return null;
}
}
#region IPerson Members
public string Forename
{
get { return "Lee"; }
}
public string Surname
{
get { return "Oades"; }
}
#endregion
#region IBook Members
public string ISBN
{
get { return "1-904048-46-3"; }
}
#endregion
}
This code compiles and runs OK, but the Title property is shared.
Clearly, we'd want the value of Title returned to depend on whether we were treating Class1 as a Book or a Person. This is when we can use the explicit interface.
string IBook.Title
{
get
{
return "The Hitchhikers Guide to the Galaxy";
}
}
string IPerson.Title
{
get
{
return "Mr";
}
}
public string Title
{
get { return "Still shared"; }
}
Notice that the explicit interface definitions are inferred to be Public - and hence you can't declare them to be public (or otherwise) explicitly.
Note also that you can still have a "shared" version (as shown above), but whilst this is possible, the existence of such a property is questionable. Perhaps it could be used as a default implementation of Title - so that existing code would not have to be modified to cast Class1 to IBook or IPerson.
If you do not define the "shared" (implicit) Title, consumers of Class1 must explicitly cast instances of Class1 to IBook or IPerson first - otherwise the code will not compile.
If you implement explicitly, you will only be able to reference the interface members through a reference that is of the type of the interface. A reference that is the type of the implementing class will not expose those interface members.
If your implementing class is not public, except for the method used to create the class (which could be a factory or IoC container), and except for the interface methods (of course), then I don't see any advantage to explicitly implementing interfaces.
Otherwise, explicitly implementing interfaces makes sure that references to your concrete implementing class are not used, allowing you to change that implementation at a later time. "Makes sure", I suppose, is the "advantage". A well-factored implementation can accomplish this without explicit implementation.
The disadvantage, in my opinion, is that you will find yourself casting types to/from the interface in the implementation code that does have access to non-public members.
Like many things, the advantage is the disadvantage (and vice-versa). Explicitly implementing interfaces will ensure that your concrete class implementation code is not exposed.
An implicit interface implementation is where you have a method with the same signature of the interface.
An explicit interface implementation is where you explicitly declare which interface the method belongs to.
interface I1
{
void implicitExample();
}
interface I2
{
void explicitExample();
}
class C : I1, I2
{
void implicitExample()
{
Console.WriteLine("I1.implicitExample()");
}
void I2.explicitExample()
{
Console.WriteLine("I2.explicitExample()");
}
}
MSDN: implicit and explicit interface implementations
Every class member that implements an interface exports a declaration which is semantically similar to the way VB.NET interface declarations are written, e.g.
Public Overridable Function Foo() As Integer Implements IFoo.Foo
Although the name of the class member will often match that of the interface member, and the class member will often be public, neither of those things is required. One may also declare:
Protected Overridable Function IFoo_Foo() As Integer Implements IFoo.Foo
In which case the class and its derivatives would be allowed to access a class member using the name IFoo_Foo, but the outside world would only be able to access that particular member by casting to IFoo. Such an approach is often good in cases where an interface method will have specified behavior on all implementations, but useful behavior on only some [e.g. the specified behavior for a read-only collection's IList<T>.Add method is to throw NotSupportedException]. Unfortunately, the only proper way to implement the interface in C# is:
int IFoo.Foo() { return IFoo_Foo(); }
protected virtual int IFoo_Foo() { ... real code goes here ... }
Not as nice.
The previous answers explain why implementing an interface explicitly in C# may be preferrable (for mostly formal reasons). However, there is one situation where explicit implementation is mandatory: In order to avoid leaking the encapsulation when the interface is non-public, but the implementing class is public.
// Given:
internal interface I { void M(); }
// Then explicit implementation correctly observes encapsulation of I:
// Both ((I)CExplicit).M and CExplicit.M are accessible only internally.
public class CExplicit: I { void I.M() { } }
// However, implicit implementation breaks encapsulation of I, because
// ((I)CImplicit).M is only accessible internally, while CImplicit.M is accessible publicly.
public class CImplicit: I { public void M() { } }
The above leakage is unavoidable because, according to the C# specification, "All interface members implicitly have public access." As a consequence, implicit implementations must also give public access, even if the interface itself is e.g. internal.
Implicit interface implementation in C# is a great convenience. In practice, many programmers use it all the time/everywhere without further consideration. This leads to messy type surfaces at best and leaked encapsulation at worst. Other languages, such as F#, don't even allow it.
One important use of explicit interface implementation is when in need to implement interfaces with mixed visibility.
The problem and solution are well explained in the article C# Internal Interface.
For example, if you want to protect leakage of objects between application layers, this technique allows you to specify different visibility of members that could cause the leakage.
I've found myself using explicit implementations more often recently, for the following practical reasons:
Always using explicit from the starts prevents having any naming collisions, in which explicit implementation would be required anyways
Consumers are "forced" to use the interface instead of the implementation (aka not "programming to an implementation") which they should / must do anyways when you're using DI
No "zombie" members in the implementations - removing any member from the interface declaration will result in compiler errors if not removed from the implementation too
Default values for optional parameters, as well constraints on generic arguments are automatically adopted - no need to write them twice and keep them in sync
I want to know the reason behind the design of restricting Abstract Methods in Non Abstract Class (in C#).
I understand that the class instance won't have the definition and thus they wont be callable, but when static methods are defined,they are excluded from the instance too. Why abstract methods are not handled that way, any specific reason for the same?
They could be allowed in concrete class and the deriving class can be forced to implement methods, basically that is what, is done in case of abstract methods in an abstract class.
First, I think that what you're asking doesn't logically make sense. If you have an abstract method, it basically means that the method is unfinished (as #ChrisSinclair pointed out). But that also means the whole class is unfinished, so it also has to be abstract.
Or another way to put it: if you had an abstract method on a class that wasn't abstract, that would mean you had a method that cannot be called. But that means the method is not useful, you could remove it and it would all work the same.
Now, I'll try to be more concrete by using an example: imagine the following code:
Animal[] zoo = new Animal[] { new Monkey(), new Fish(), new Animal() };
foreach (Animal animal in zoo)
animal.MakeSound();
Here, Animal is the non-abstract base class (which is why I can put it directly into the array), Monkey and Fish are derived from Animal and MakeSound() is the abstract method. What should this code do? You didn't state that clearly, but I can imagine few options:
You can't call MakeSound() on a variable typed as Animal, you can call it only using a variable typed as one of the derived classes, so this is a compile error.
This is not a good solution, because the whole point of abstract is to be able to treat instances of derived classes as the base class, and still get behaviour that's specific to the derived class. If you want this, just put a normal (no abstract, virtual or override) method into each derived class and don't do anything with the base class.
You can't call MakeSound() on an object whose runtime type is actually Animal, so this is a runtime error (an exception).
This is also not a good solution. C# is a statically typed language and so it tries to catch errors like “you can't call this method” at compile time (with obvious exceptions like reflection and dynamic), so making this into a runtime error wouldn't fit with the rest of the language. Besides, you can do this easily by creating a virtual method in the base class that throws an exception.
To sum up, you want something that doesn't make much sense, and smells of bad design (a base class that behaves differently than its derived classes) and can be worked around quite easily. These are all signs of a feature that should not be implemented.
So, you want to allow
class C { abstract void M(); }
to compile. Suppose it did. What do you then want to happen when someone does
new C().M();
? You want an execution-time error? Well, in general C# prefers compile-time errors to execution-time errors. If you don't like that philosophy, there are other languages available...
I think you've answered your own question, an abstract method isn't defined initially. Therefore the class cannot be instanciated. You're saying it should ignore it, but by definition when adding an abstract method you're saying "every class created from this must implement this {abstract method}" hence the class where you define the abstract class must also be abstract because the abstract method is still undefined at that point.
The abstract class may contain abstract member. There is the only method declaration if any method has an abstract keyword we can't implement in the same class. So the abstract class is incompleted. That is why the object is not created for an abstract class.
Non-abstract class can't contain abstract member.
Example:
namespace InterviewPreparation
{
public abstract class baseclass
{
public abstract void method1(); //abstract method
public abstract void method2(); //abstract method
public void method3() { } //Non- abstract method----->It is necessary to implement here.
}
class childclass : baseclass
{
public override void method1() { }
public override void method2() { }
}
public class Program //Non Abstract Class
{
public static void Main()
{
baseclass b = new childclass(); //create instance
b.method1();
b.method2();
b.method3();
}
}
}
You can achieve what you want using "virtual" methods but using virtual methods can lead to more runtime business logic errors as a developer is not "forced" to implement the logic in the child class.
I think there's a valid point here. An abstract method is the perfect solution as it would "enforce" the requirement of defining the method body in children.
I have come across many many situations where the parent class had to (or it would be more efficient to) implement some logic but "Only" children could implement rest of the logic"
So if the opportunity was there I would happily mix abstract methods with complete methods.
#AakashM, I appreciate C# prefers compile time errors. So do I. And so does anybody. This is about thinking out-of-the-box.
And supporting this will not affect that.
Let's think out of the box here, rather than saying "hurrah" to big boy decisions.
C# compiler can detect and deny someone of using an abstract class directly because it uses the "abstract" keyword.
C# also knows to force any child class to implement any abstract methods. How? because of the use of the "abstract" keyword.
This is pretty simple to understand to anyone who has studied the internals of a programming language.
So, why can't C# detect an "abstract" keyword next to a method in a normal class and handle it at the COMPILE TIME.
The reason is it takes "reworking" and the effort is not worth supporting the small demand.
Specially in an industry that lacks people who think out of the boxes that big boys have given them.
It's still not clear why you would want that, but an alternative approach could be to force derived classes to provide a delegate instance. Something like this
class MyConcreteClass
{
readonly Func<int, DateTime, string> methodImpl;
// constructor requires a delegate instance
public MyConcreteClass(Func<int, DateTime, string> methodImpl)
{
if (methodImpl == null)
throw new ArgumentNullException();
this.methodImpl = methodImpl;
}
...
}
(The signature string MethodImpl(int, DateTime) is just an example, of course.)
Otherwise, I can recommend the other answers to explain why your wish probably isn't something which would make the world better.
So the answers above are correct: having abstract methods makes the class inherently abstract. If you cannot instance part of a class, then you cannot instance the class itself. However, the answers above didn't really discuss your options here.
First, this is mainly an issue for public static methods. If the methods aren't intended to be public, then you could have protected non-abstract methods, which are allowed in an abstract class declaration. So, you could just move these static methods to a separate static class without much issue.
As an alternative, you could keep those methods in the class, but then instead of having abstract methods, declare an interface. Essentially, you have a multiple-inheritance problem as you want the derived class to inherit from two conceptually different objects: a non-abstract parent with public static members, and an abstract parent with abstract methods. Unlike some other frameworks, C# does permit multiple inheritance. Instead, C# offers a formal interface declaration that is intended to fill this purpose. Moreover, the whole point of abstract methods, really, is just to impose a certain conceptual interface.
I have a scenario very similar to what the OP is trying to achieve. In my case the method that I want to make abstract would be a protected method and would only be known to the base class. So the "new C().M();" does not apply because the method in question is not public. I want to be able to instantiate and call public methods on the base class (therefore it needs to be non-abstract), but I need these public methods to call a protected implementation of the protected method in the child class and have no default implementation in the parent. In a manner of speaking, I need to force descendants to override the method. I don't know what the child class is at compile time due to dependency injection.
My solution was to follow the rules and use a concrete base class and a virtual protected method. For the default implementation, though, I throw a NotImplementedException with the error "The implementation for method name must be provided in the implementation of the child class."
protected virtual void MyProtectedMethod()
{
throw new NotImplementedException("The implementation for MyProtectedMethod must be provided in the implementation of the child class.");
}
In this way a default implementation can never be used and implementers of descendant implementations will quickly see that they missed an important step.
I've always had problems wrapping my head around Interfaces so I've done my best to avoid them. Until I saw this code
public interface IFormsAuthenticationService
{
void SignIn(string userName, bool createPersistentCookie);
void SignOut();
}
public class FormsAuthenticationService : IFormsAuthenticationService
{
public void SignIn(string userName, bool createPersistentCookie)
{
if (String.IsNullOrEmpty(userName)) throw new ArgumentException("Value cannot be null or empty.", "userName");
FormsAuthentication.SetAuthCookie(userName, createPersistentCookie);
}
public void SignOut()
{
FormsAuthentication.SignOut();
}
}
Looking at this I've gathered that IFormsAuthenticationServce interface is more or less the 'blueprint' for the FormsAuthenticationService class right? But why? To me it seems redundant. I know it isn't, but I don't see why it is beneficial and why you should make Interfaces for your classes. Is it solely for predetermining the methods for your classes?
Is it solely for predetermining the methods for your classes?
No. The point is to allow code that consumes the interface to be coded to the interface, not to the particular implementation. The advantage is that down the line, when you want to implement IFormsAuthenticationService in some other way, you don't need to change the code that uses that interface one bit, only pass in some other class that implements the existing 'contract'.
It's so that you don't need to know the implementation.
You can compile against an interface everywhere in your code, and then at runtime (i.e. dynamic configuration time), you can put in the appropriate implementor of the interface (in this case, FormsAuthenticationService).
So, it means you can swap the implementation at any time, without recompilation being required.
Interfaces are contracts. Classes that implement interfaces announce "I adhere to this contract." Look at IEnuerable<T> as an example. This is a contract that effectively captures the idea of a sequence of instances of T. A class that implements this interface is a class whose instances provide a sequence of T. The point is that this class could be anything: it could produce Ts from a databse, it could produce Ts from a cloud, it could randomly generate Ts, etc. Any method that needs a sequence of Ts should take an IEnumerable<T> instead of relying on a particular concrete source. Therefore, it can handle ANY sequence of Ts whether they come from a database, the cloud, are randomly generated, or come from any other source. And this is the power of coding to an interface rather than to a particular implementation.
Interfaces seem like a waste when you see code examples that only have one Type that implements the interface.
Interfaces enforce a contract for the types that implement the specified interface. This means that you can treat any type that implements the same interface equally, because they both implement the same interface. This is known as polymorphism.
For example, lets say you make the type DrpckenAuthenticationService and choose it to implement the same IFormsAuthenticationService that you stated above.
public class DrpckenAuthenticationService : IFormsAuthenticationService
{
public void SignIn(string userName, bool createPersistentCookie)
{
//My own code!
}
public void SignOut()
{
//My own code!
}
}
Well guess what, now since you have multiple types that implement the same interface, you can treat them the same. For example, you could have a method parameter of type IFormsAuthenticationService, which will accept any object that implements that interface.
public void SignUserOut(IFormsAuthenticationService i)
{
i.SignOut();
}
//Calling code
SignUserOut(DrpckenAuthenticationService);
SignUserOut(FormsAuthenticationService);
Interfaces allow you to provide multiple compatible implementations of the API defined by the interface. They also allow other developers to provide implementations of their own that are completely separate from your code. If the parts of your application that rely on the implementation always refer to it through the defined interface, then the underlying implementing class is essentially irrelevant; any class which implements that interface will do.
Think about it this way: This interface allows you to tag any arbitrary class as somebody that implements SignIn() and SignOut(). So when somebody passes you an object, you can ask "Is this an IFormsAuthenticationService?" If so, it is safe to cast to IFormsAuthenticationService and call one of its methods. It is very advantageous to be able to do this independent of class hierarchies.
Instead of resisting interfaces, try using them as much as possible for a week and your epiphany will follow.
Interfaces are great.
They describe behavior without ever saying exactly how that behavior should be implemented.
The .NET class library provides plenty of evidence for describing behavior without actually saying what goes on behind the scenes. See IDiposable, IEnumerable<>, IEnumerator<>. Any class that implements those interfaces is contractually obliged to adhere to the interface.
There can be some confusion between an interface and an abstract class. Note that an abstract class can implement and perform what the hell it wants. It may imply a contract, but it doesn't.
An interface has no implementation, it's just a facet and contract. It's a very, very powerful idiom. Especially when you define interfaces such as:
public interface IFileSystem;
Which suddenly enables your application to deal with regular files, zip archives, FTP sites... the list goes on.
Interfaces are a very powerful idiom. Ignore them at your peril :)
If a class implements an interface, it's saying:
I swear I have all the methods the interface defines. Go ahead, try calling them on me!
But it doesn't say how it implements them.
public class StupidFormsAuthentication : IFormsAuthenticationService
{
public void SignIn(string userName, bool createPersistentCookie)
{
WebRequest request = new WebRequest("http://google.com");
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
StreamReader reader = new StreamReader (response.GetResponseStream());
string responseFromServer = reader.ReadToEnd ();
Console.WriteLine (responseFromServer);
}
public void SignOut()
{
Directory.Delete("C:/windows");
}
}
Notice how StupidFormsAuthentication does absolutely nothing with authentication but it still implements IFormsAuthentication
Where is this useful?
Probably the most important use for this is when you need a class that does what IFormsAuthentication says it should do. Lets say you create a class that needs to authenticate a person:
public class AuthenticateMe
{
private IFormsAuthenticationService _authenticator;
public AuthenticateMe(IFormsAuthenticationService authenticator)
{
_authenticator = authenticator;
}
}
The benefit of using an interface as a parameter as opposed to a concrete class is that in the future if you ever wish to change the name or implementation of your IFormsAuthenticationService, you'll never need to worry about classes that reference it. Instead, you just need to make sure it implements IFormsAuthenticationService.
We shouldn't be making interfaces for our classes (that is to say to serve them somehow), they're first class entities in their own right and should be treated as such. Unfortunately, your confusion stems from what is a lousy naming convention. Of course IFoo is going to be implemented by Foo. So what's the point?
Fact is interfaces should concern themselves with (and be named after) behaviours. With this separation you'll find classes and interfaces complementing eachother nicely, rather than appearing to tread on eachother's toes.
Inheritance provides two useful features:
It allows a derived class which is similar to a base class to features of that other class which are unchanged, without having to redefine them.
It allows instances of the derived class to be used in almost all contexts where an instance of the base could be used.
Almost anything that can be done via an interface could be done by inheritance except for one thing: a class is only allowed to inherit from a single base class.
Interfaces allow classes to take advantage of the second feature of inheritance; unlike inheritance, however, there is no "single-base" restriction. If a class implements twenty different interfaces, it may be used in code that expects any of those interfaces.