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
Interface:
interface IMyInterface{
internal int Property {get; set;}
}
Class:
public class MyClass: IMyInterface{
internal int Property {get; set;}
}
Result:
CS8704 Error: MyClass doesnot implement interface member Property.get MyClass cannot implicitly implement a non-public member.
Why I have to implement the interface explicitly?
The simple answer to "why is a language like this" is "because that's how the language designers specified it".
Now, why did they design it that way? Some of the official notes I found were these. It seems the main question was about what kind of access the implementor must have:
Would we allow non-public interface members to be implemented implicitly? If so, what is required of the accessibility of the implementing method? Some options:
Must be public
Must be the exact same accessibility
Must be at least as accessible
They decided:
For now, let's simply not allow it. Only public interface members can be implicitly implemented (and only by public members).
The "for now" never changed, so as of C# 8 an interface can have non-public virtual members but a class may only implement them explicitly.
I can speculate on a couple of reasons they may have decided against implicit overrides like this:
Non-public virtual methods in interfaces may have been considered a "rare" feature (after all, aren't interfaces supposed to document the public behavior of a class?), not worth putting a lot of resources into in terms of the semantics of implicit overrides.
Unlike with method overridding in class-to-class inheritance, an class method implementing an interface method doesn't use the override keyword. It might have been considered confusing to see a protected and/or internal method and not realize that it's fulfilling an interface contract. (Public methods are presumably considered exempt from this concern because that's the way they've always worked, and public methods are part of the class' public contract anyway so modifying / removing them would already be cause the reader to think about other parts of code that depend on it.)
Interfaces can only override other interface methods explicitly, possibly again because allowing interface-to-interface implicit implementation would be too expensive for the compiler and tooling teams and too confusing for C# users. (Especially since interface-to-interface inheritance is multiple-inheritance.) Since both this and non-public interface methods were introduced in general in C# 8, it may have made sense to make the two features match syntactically.
See also the notes on this question in the default interface method proposal.
Interface members don't have scopes like public or internal. What you have here is a default interface implementation.
So you need to remove the scope on the interface:
interface IMyInterface{
int Property {get; set;}
}
The internal property forces the implementation to be explicit such that the internal members of the interfaces will remain internal to the assembly.
It helps you to keep implementations internal (to an assembly) so that you can update code without breaking changes e.g. renaming the property.
interface IMyInterface
{
internal int Property { get; set; }
}
public class MyClass : IMyInterface
{
int IMyInterface.Property { get; set; }
}
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
When I googled to find the related topics about interface, I found this from MSDN website:
For example, an interface might declare a property that has a get accessor. The class that implements the interface can declare the same property with both a get and set accessor.
from MSDN
Now I have a doubt. When we specifically mentioned that the property should be read only(only 'get' accessor in the interface) why is it allowed to implement 'set' accessor also?
Now I have a doubt. When we specifically mentioned that the property should be read only(only 'get' accessor in the interface) why is it allowed to implement 'set' accessor also?
There's a difference - when you use an interface, you're not "specifying that the property should be read only", but rather specifying that the contract defines a "readable property" of that specific name and type. Basically, the interface defines the minimum requirements for a contract, not the absolute requirements.
If you cast the object to the specific interface, the property setter will not be available. It's really no different than having extra properties or methods on the object that aren't available via the interface.
You can't access the set property from a interface reference, so it doesn't matter if it's implemented or not when revealing the interface to the public.
Of course it's sometimes necessary to implement a set accessor on class side, i.e. when working with a class which allows access of classes which are in the same assembly.
Interface is a minimum set of requirements that needs to be implemented but you can implement more. In this case read-write property is more than just read-only one.
Besides extending beyond requirements of a contract you can add any other methods and/or properties and also implement other interfaces in the same class.
The code which uses the interface does not know that there is a set and so can't use it.
Think of an interface as a contract. Implementers promise to at least comply to the behaviors defined in that contract, but are not restricted to it. Interfaces allow components to interact without being tightly coupled. Therefore, an implementation may allow both get and set, but at the very least must honor the get.
The class is meeting the requirements of the interface, anything else is an implementation detail of the class itself. If you're referring to the object through the interface, you're only going to see the get. So, no, it's not really breaking it, it's as intended.
The interface though is just a declaration of how the object should be used by consumers. It doesn't make any specifications about the implementation. There's no inconsistency there.
public interface IFoo {
string Name { get; }
}
class FooImplementation : IFoo {
public string Name { get; set; }
}
public class FooWorker {
public void WorkOnFoo(IFoo foo) {
if (null == foo) throw new ArgumentNullException("foo");
Console.WriteLine(foo.Name);
}
}
public class Program {
public void Main() {
IFoo foo = new FooImplementation { Name = "Foo" };
new FooWorker().WorkOnFoo(foo);
}
}
As far as FooWorker is concerned, the foo parameter only has a get accessor for the Name property.
It's probably important to remember that that the Name property may still be set on foo via reflection or a cast.
What Felix said is correct.
In more detail, an interface defines a minimum set of functionality that must exist on any object defined as implementing said interface. That provides a "common" set of functionality among all implementations of the interface, so you know that if an object implements the interface, you can call X, Y, and Z on it. Just because something is IDisposable, for instance, doesn't mean that's ALL the object can do. In fact that would make interfaces rather pointless, if they also defined a maximum amount of functionality. That's just all you care about if and when you are working with the object as an implementation of the interface; if all you need is an IDisposable, you only care about calling Dispose(), regardless of what additional members a particular IDisposable implementation may have.
Back to your example, the interface defining the property is stating that it must have a public get accessor. It does not, and cannot, say that it cannot have a public set accessor; it simply doesn't care either way. The set accessor could be public, internal, protected, private, or nonexistent; what consumers of the interface will expect, and thus what implementors of the interface will need, is the get accessor.
It may be helpful to think in terms of three types of things: an abstract ReadableFoo class (or IReadableFoo interface), along with concrete ImmutableFoo and MutableFoo classes (or IImmutableFoo and IChangeableFoo interfaces). Someone who receives a parameter of type ReadableFoo will be able to read it, but will not be able to set it, and will not be able to reliably persist the data therein merely by persisting a reference. Someone who receives a parameter of ImmutableFoo would be able to reliably persist the data by persisting the reference, but would not be able to change it. Someone who receives a parameter of MutableFoo will be able to change the data, but not reliably persist data by persisting the reference.