Implicit and Explicit Interface Implementations
As I was putting together a post on IEnumerable and IEnumerator I was reminded of the subtleties of implicit and explicit interface implementations. C# does not support multiple inheritance, but a class has the option of implementing one or more interfaces. One challenge with interfaces is that they may include methods that have the same signatures as existing class members or members of other interfaces. Explicit interface implementations can be used to disambiguate class and interface methods that would otherwise conflict. Explicit interfaces can also be used to hide the details of an interface that the class developer considers private.
Disambiguating Methods
Let's look at an example of method disambiguation. In Listing 1 we have started to write a class called C that implements interfaces I1 and I2, each of which defines a method A() .
Listing 1. Class C implements interfaces I1 and I2.
interface I1
{
void A();
}
interface I2
{
void A();
}
class C : I1, I2
{
public void A()
{
Console.WriteLine("C.A()");
}
}
In this case, A() is a public class member that implicitly implements a member of both interfaces. A() can be invoked through either interface or through the class itself as follows:
Listing 2. A() can be invoked from I1, I2, or C.
C c = new C();
I1 i1 = (I1)c;
I2 i2 = (I2)c;
i1.A();
i2.A();
c.A();
The output from this code is
C.A()
C.A()
C.A()
This works fine if you want A() to do the same thing in both interfaces. In most cases, however, methods in different interfaces have distinct purposes requiring wholly different implementations. This is where explicit interface implementations come in handy. To explicitly implement an interface member, just use its fully qualified name in the declaration. A fully qualified interface name takes the form
InterfaceName.MemberName
In Listing 3 we add an explicit implementation of I1's A() method.
Listing 3. Class C explicitly implements I1.A().
class C : I1, I2
{
public void A()
{
Console.WriteLine("C.A()");
}
void I1.A()
{
Console.WriteLine("I1.A()");
}
}
Now when we run the statements from Listing 2 we get
I1.A()
C.A()
C.A()
When an interface method is explicitly implemented, it is no longer visible as a public member of the class. The only way to access it is through the interface. As an example, suppose we deleted the implicit implementation of A() as shown in Listing 4:
Listing 4. Class C does not implicitly implement A()
class C : I1, I2
{
void I1.A()
{
Console.WriteLine("I2.A()");
}
}
In this case we would get a compile error saying that C fails to implement I2.A() . We could fix this error by changing the first line to
class C : I1
but we'd get another compile error when trying to invoke A() as a member of C:
C c = new C();
c.A();
This time the compiler would report that class C does not contain a definition for method A() . We get the error because the explicit implementation of I1.A() hides A() from the class. The only way to call I1.A() now is through C's I1 interface:
C c = new C();
I1 i1 = (I1)c;
i1.A();
Explicit interface implementations are sometimes necessary when classes implement multiple interfaces with conflicting member definitions. A common example involves collection enumerators that implement System.Collections.IEnumerator and System.Collections.Generic.IEnumerator. In this case both interfaces specify a get-accessor for the Current property. To create a class that compiles, at least one of the two get-accessors must be explicitly implemented.
Hiding Interface Details.
In some cases explicit interface implementation can be useful even when disambiguation is unnecessary. One example is to use an explicit implementation to hide the details of an interface that the class developer considers private. While the level of privacy is not as great as that afforded by the private keyword, it can be useful in some circumstances. Listing 5 shows a common pattern involving IDisposable. In this case, the Dispose() method is hidden by explicit implementation because the method is really an implementation detail that is not germane to the users of class MyFile. At the very least, the explicit implementation keeps the interface members out of the class' Intellisense list.
Listing 5. Using explicit implementation to hide the details of IDisposable.
interface IDisposable
{
void Dispose();
}
class MyFile : IDisposable
{
void IDisposable.Dispose()
{
Close();
}
public void Close()
{
// Do what's necessary to close the file
System.GC.SuppressFinalize(this);
}
}
You can read more about implicit and explicit interface implementations in this MSDN Tutorial or in Version 1.2 of the C# Language Specification.
Hope this helps.
-Mike Hopcroft
Comments
Anonymous
December 12, 2006
You've been kicked (a good thing) - Trackback from DotNetKicks.comAnonymous
December 13, 2006
Thanks, it's always useful to remind simple things sometimes.Anonymous
December 14, 2006
The comment has been removedAnonymous
December 29, 2006
Welcome to the sixteenth Community Convergence. This column comes out about once a week and is designedAnonymous
January 15, 2007
Nice, clear, explanation...thanks!Anonymous
November 13, 2007
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December 10, 2007
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April 03, 2008
Thank you, this was very helpful.Anonymous
May 13, 2009
Pretty neat, clear and pragmatic. Thanks a lot!Anonymous
June 29, 2009
This is realy meaning full information. ThanksAnonymous
August 03, 2009
Really good article, Thanks very much MichaelAnonymous
September 18, 2009
Excellent. Very well written article. Thanks!Anonymous
November 02, 2009
Excellent..... It really gives real insight into the implimentation of Interfaces.... Thanks.Anonymous
November 12, 2009
Great teaching! thanks a bunch!Anonymous
February 02, 2010
Hi Sir, This is the perfect explanation to my doubt, which I am am exploring. Thanks a Lot. -- Pavan KumarAnonymous
February 03, 2010
The comment has been removedAnonymous
July 12, 2011
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September 08, 2011
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November 27, 2011
Thanks for posting this, well written and just cleared up a couple of questions I had.Anonymous
June 04, 2014
Awesome.There is great clarity in your explanationAnonymous
November 25, 2014
Excellent explanation!! Thanks a lot.Anonymous
May 15, 2015
Simple, clean and great clarity in it. Thanks!Anonymous
July 17, 2015
You also get a thank you from me. Clear and to the point, cheers for that!