Michael Hopcroft

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

Published Tuesday, December 12, 2006 11:59 PM by mhop