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 came accross the following code:
public interface IFoo { }
Make IFoo do something via an extension method:
public static FooExtensions
{
public static string Foo(this IFoo foo, string bar)
{
// Do work
return bar;
}
}
Is this a good idea? Why not use an abstract class with a virtual Foo() instead? IFoo could have some contract methods but a consumer gets the Foo() extension method also.
My question is: When is something like this a good idea?
The extension method doesn't "make" IFoo do anything. Extension methods just let you extend a type that's closed... it's generally best used in conjunction with code which you don't have the ability to modify, such as framework types or third-party types.
Another possibility is if you have a lot of logic that's absolutely identical across all implementations of your interface, and you want consumers of your interface to have access to that functionality without having to use a base type. Think of LINQ -- it's implemented via extension methods, and you get all the benefits of it just by implementing IEnumerable.
In this case, you're not gaining anything other than an unnecessary layer of indirection. If IFoo should have the ability to do Foo, add Foo to the interface.
Extention methods is a good idea when you don't want or can't change implementation of the class you are extending. IFoo could be declared in a 3rd party library. Or there might be a lot of code dependent on it so that it is very hard to remake it to an abstract class (maybe some reflection rely on interface).
In general from the usage point of view you should use extention methods when it looks more readable than old-school static methods and anyway you would use static method instead of new class member. When considering extention method vs member, consider static method in helper class vs member and if you select static, then consider if it's better to implement it as extention.
But I often see using extention methods where it really isn't required and usually it makes code less readable. So I wouldn't recommend using them when it's easy and obvious how to avoid them.
When is something like this a good idea?
When you need to teach already existing members which implements this interface with new tricks, like this one from the System.Core assembly:
// System.Linq.Enumerable
public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
if (source == null)
{
throw Error.ArgumentNull("source");
}
if (predicate == null)
{
throw Error.ArgumentNull("predicate");
}
foreach (TSource current in source)
{
if (predicate(current))
{
return current;
}
}
throw Error.NoMatch();
}
The reason you might want to do this is when you want an interface to provide a method and the implementation of that method can always be done using the other methods and properties in the interface.
An interface (unlike an abstract base class) give you no way to provide a "default" implementation for a method. By using an extension method you can provide such a method without all implementers of an interface having to provide the same repeated implementation code.
However, a major drawback of this approach is that the method in the extension method is effectively sealed - you cannot implement it differently. Sometimes this is ok, sometimes not - YMMV.
An alternative approach to this is as follows:
Specify your interface as usual, but add the method in question to it.
Provide an abstract base class which provides the default code for the method in question.
Derive from the abstract base class when you want to provide an implementation of this interface.
Another reason you might want to use an extension method is when you either cannot change the existing interface (because it is third-party, for example) or when you don't want to (because it would break existing code).
Extension methods are merely syntax sugar which allow you to change fun(t, x) into t.fun(x). They're useful for discovery (intellisense), or when you want to compose fluent pipelines of functions which follow a "more intuitive" left to right style, rather than right to left. Eg f(x).g(y).h(z) versus h(g(f(x),y),z).
There's not really any downside to using them other than cluttering intellisense.
This is a good idea when you want to give this implementation to any object which implement that interface, regardless of what implementation is that.
An abstract class provide that implementation only to its derived classes.
If that interface is yours, or, you have a single base-abstract class that implements that interface, and it's safe to assume that no implementations which doesn't derive from that class would be in your code - it would be a good idea to implement that functionality in that abstract class (but, you'll have to cast to that abstract class, to use that method, which makes the interface somehow redundant).
However, if you want to provide an implementation (of that method) to all types which implement that interface, regardless of their actual implementation - an extension method would be a better idea.
Moreover, a class can only derive from a single class - which means that by deriving from that abstract class, you cannot derive from any other class. So, if you'll have multiple inheritance chains which implements that interface, the only solution to provide that method to all of them (directly), without duplication of code, is via an extension (although there other solution to provider the functionality, but it wouldn't be directly: objWhichImplIFoo.Foo()).
BTW, there is another reason to want an extension: if you want it to be callable from nulls. A declared method will always throw a NullReferenceException if the object is null. Because extensions are actually static methods - they can be called upon nulls:
IFoo foo = null;
var something = foo.GetSomethingOrDefault();
what is the difference between explicit interface and implicit interface implementation in c#/asp.net? in which scenario we can use the explicit interface and implicit interface implementation.
Thanks,
Pradeep
The concept behind implicit and explicit implementation is rather simple:
A member with implicit implementation will be accessible through the interface as well as through the class implementing it
A member with explicit implementation will be accessible through the interface only
As for why and when you would use one or another, it depends. In the case you're implementing more than one interface with the same property/method, explicit implementation is your only option, as it is the only way to know which property/method you're intending to call. Obviously in this scenario you can't have that property/method on the class itself: if there is, it will be class only, and will not match any of the interfaces (which will have their explicit implementation of it).
In other scenarios, it really depends on your design choices and what you're trying to accomplish. If you want to force the caller to access interface members only through interface, and not through class declaration, do an explicit implementation.
Say you have two interfaces, IDoStuff<T> and IDoStuff, which your class implements. They both have a method "GetStuff", but one has the signature T GetStuff(), and the other has the signature object GetStuff().
The problem is that .net will not let you have two methods named the same thing that only differ on the return type. But you need to have both of these methods in your class to satisfy both interfaces. If T is, in fact, an object, then you can use explicit implementation like so.
public T GetStuff()
{
T stuff;
//Stuff Is Got
return stuff;
}
IDoStuff.GetStuff()
{
return (object)GetStuff();
}
Note that because IDoStuff mandates the security requirements of GetStuff, IDoStuff.GetStuff will be public/private/protected/internal based on that interface's declaration.
If you wanted, you could do every implantation explicitly, but the full method name for each would be InterfaceName.MethodName, and that gets a little annoying to read and write. Usually this is only used when you want to implement a method with the same signature multiple times to satisfy several interfaces.
This question and Eric Lippert's answer got me wondering: How do you decide whether to use an explicit or implicit implementation when implementing methods of an interface?
(personally) I only see a need for explicit implementations when there is a clash between methods with the same signature.
For example, when implementing IEnumerable<T>, you should implement 2 methods GetEnumerator() which have the same signature, except for the return type. So you'll have to implement IEnumerable.GetEnumerator() explicitly:
public abstract class MyClass<T> : IEnumerable<T>
{
public IEnumerator<T> GetEnumerator()
{
return ...;
}
IEnumerator IEnumerable.GetEnumerator() // explicit implementation required
{
return GetEnumerator();
}
}
Another use for an explicit implementation is if you don't want the method to be called through an object instance, but only through an interface. I personally think this doesn't make much sense, but in some very rare cases, it can be useful.
Philippe's answer is a practical, one, however, there are architectural considerations as well.
Interfaces are used to make classes compatible so that they can be consumed by other objects. If only those consuming objects need the functionality of the interface, then it should be restricted so - by the principle of least privilege. If would be unnecessary to expose that interface method to all other users of the class.
Paul
Another case for explicit interfaces is where the object needs to implement an interface to accept calls from an internal object, but you don't want to expose those as part of your API.
My goal is to make sure that in most scenarios objects are used via a "read-only interface" that is a subset of the full interface.
As an example, if I were using C++, I would just return a const object.
In C#, if I could achieve this with interfaces, I would just implement a read-only interface and use it everywhere. However, I need operator overloading, which is not allowed for interfaces. That's why I have to use an abstract base class.
But if I define an abstract base class, I am not allowed to change accessibility in the derived type.
So, how can I achieve my goal in C#?
How about if you placed your write operations in an interface and then implement in explicitly on the abstract base class? It would not be a 100% perfect solution (you could still cast the object to the modification interface), but for most part it would prevent anyone from accidentally calling the modifying methods.
Do you really need operator overloading? We are talking about syntactic sugar here. Also, not all .NET languages utilize operator overloading in the same way. So, if you use operator overloading, you are effectively making your code language-specific.
I would implement the read-only interface and drop the operator overloading requirement.
You could implement the readonly behaviour via state in the object, and throw an exception if a modifying method is called?
If anyone is interested in what I did, I finally went for an abstract class instead of interface, and I did hide the method in the derived class to get the right accessors:
Like, in the base abstract class (readonly interface):
protected double accuracy;
public double Accuracy { get { return accuracy; } }
In the derived class:
public new double Accuracy
{
get { return accuracy; }
set { accuracy = value; }
}
Of course, ugly with the "new" keyword, but in this case it will do for me.