C# Language Specification

Interfaces

An interface defines a contract that can be implemented by classes and structs. An interface can contain methods, properties, events, and indexers. An interface does not provide implementations of the members it defines—it merely specifies the members that must be supplied by classes or structs that implement the interface.

Interfaces may employ multiple inheritance. In the following example, the interface IComboBox inherits from both ITextBox and IListBox.

interface IControl
{
void Paint();
}

interface ITextBox: IControl

{
void SetText(string text);
}

interface IListBox: IControl

{
void SetItems(string[] items);
}

interface IComboBox: ITextBox, IListBox {}

Classes and structs can implement multiple interfaces. In the following example, the class EditBox implements both IControl and IDataBound.

interface IDataBound

{
void Bind(Binder b);
}

public class EditBox: IControl, IDataBound

{
public void Paint() {...}

public void Bind(Binder b) {...}

}

When a class or struct implements a particular interface, instances of that class or struct can be implicitly converted to that interface type. For example

EditBox editBox = new EditBox();
IControl control = editBox;
IDataBound dataBound = editBox;

In cases where an instance is not statically known to implement a particular interface, dynamic type casts can be used. For example, the following statements use dynamic type casts to obtain an object’s IControl and IDataBound interface implementations. Because the actual type of the object is EditBox, the casts succeed.

object obj = new EditBox();
IControl control = (IControl)obj;
IDataBound dataBound = (IDataBound)obj;

In the previous EditBox class, the Paint method from the IControl interface and the Bind method from the IDataBound interface are implemented using public members. C# also supports explicit interface member implementations, using which the class or struct can avoid making the members public. An explicit interface member implementation is written using the fully qualified interface member name. For example, the EditBox class could implement the IControl.Paint and IDataBound.Bind methods using explicit interface member implementations as follows.

public class EditBox: IControl, IDataBound
{
void IControl.Paint() {...}

void IDataBound.Bind(Binder b) {...}

}

Explicit interface members can only be accessed via the interface type. For example, the implementation of IControl.Paint provided by the previous EditBox class can only be invoked by first converting the EditBox reference to the IControl interface type.

EditBox editBox = new EditBox();
editBox.Paint(); // Error, no such method
IControl control = editBox;
control.Paint(); // Ok

10.