Explicit Interface Implementation (C# Programming Guide)

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09/29/2022

If a class implements two interfaces that contain a member with the same signature, then implementing that member on the class will cause both interfaces to use that member as their implementation. In the following example, all the calls to Paint invoke the same method. This first sample defines the types:

public interface IControl
{
    void Paint();
}
public interface ISurface
{
    void Paint();
}
public class SampleClass : IControl, ISurface
{
    // Both ISurface.Paint and IControl.Paint call this method.
    public void Paint()
    {
        Console.WriteLine("Paint method in SampleClass");
    }
}

The following sample calls the methods:

SampleClass sample = new SampleClass();
IControl control = sample;
ISurface surface = sample;

// The following lines all call the same method.
sample.Paint();
control.Paint();
surface.Paint();

// Output:
// Paint method in SampleClass
// Paint method in SampleClass
// Paint method in SampleClass

But you might not want the same implementation to be called for both interfaces. To call a different implementation depending on which interface is in use, you can implement an interface member explicitly. An explicit interface implementation is a class member that is only called through the specified interface. Name the class member by prefixing it with the name of the interface and a period. For example:

public class SampleClass : IControl, ISurface
{
    void IControl.Paint()
    {
        System.Console.WriteLine("IControl.Paint");
    }
    void ISurface.Paint()
    {
        System.Console.WriteLine("ISurface.Paint");
    }
}

The class member IControl.Paint is only available through the IControl interface, and ISurface.Paint is only available through ISurface. Both method implementations are separate, and neither are available directly on the class. For example:

SampleClass sample = new SampleClass();
IControl control = sample;
ISurface surface = sample;

// The following lines all call the same method.
//sample.Paint(); // Compiler error.
control.Paint();  // Calls IControl.Paint on SampleClass.
surface.Paint();  // Calls ISurface.Paint on SampleClass.

// Output:
// IControl.Paint
// ISurface.Paint

Explicit implementation is also used to resolve cases where two interfaces each declare different members of the same name such as a property and a method. To implement both interfaces, a class has to use explicit implementation either for the property P, or the method P, or both, to avoid a compiler error. For example:

interface ILeft
{
    int P { get;}
}
interface IRight
{
    int P();
}

class Middle : ILeft, IRight
{
    public int P() { return 0; }
    int ILeft.P { get { return 0; } }
}

An explicit interface implementation doesn't have an access modifier since it isn't accessible as a member of the type it's defined in. Instead, it's only accessible when called through an instance of the interface. If you specify an access modifier for an explicit interface implementation, you get compiler error CS0106. For more information, see interface (C# Reference).

You can define an implementation for members declared in an interface. If a class inherits a method implementation from an interface, that method is only accessible through a reference of the interface type. The inherited member doesn't appear as part of the public interface. The following sample defines a default implementation for an interface method:

public interface IControl
{
    void Paint() => Console.WriteLine("Default Paint method");
}
public class SampleClass : IControl
{
    // Paint() is inherited from IControl.
}

The following sample invokes the default implementation:

var sample = new SampleClass();
//sample.Paint();// "Paint" isn't accessible.
var control = sample as IControl;
control.Paint();

Any class that implements the IControl interface can override the default Paint method, either as a public method, or as an explicit interface implementation.

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Explicit Interface Implementation (C# Programming Guide)

See also

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