C# interface inheritance (can inherit how many other interfaces)

I understand that

a class can inherit multiple interfaces and

a abstract class can inherit from another class  and one or more interfaces

How bout interface inheritance,

Can an interface inherit from multiple interfaces?


Yes, it can. Look at ICollection<T>:

public interface ICollection<T> : IEnumerable<T>, IEnumerable

An interface is not inherited it is implemented, so it is not involved in the 'single inheritence' rule. Anything that can implement an interface can implement multiple interfaces.

The slightly confusing thing - and the answer to your question - is that an interface can implement other interfaces. Effectively the interface inherits all of the characteristics of every interface it implements:

interface IHasPosition
    float X { get; }
    float Y { get; }

interface IHasValue<T>
    T Value { get; }

interface IPositionValue<T> : IHasPosition, IHasValue<T>
{ }

Rather than being simply an empty interface, IPositionValue<T> has all three properties from the two interfaces it implements. When creating a class that implements IPositionValue<T>, the class automatically implements the interfaces that IPositionValue<T> implements:

class StringAtLocation : IPositionValue<string>
    public float X { get; set; }
    public float Y { get; set; }
    public string Value { get; set; }

static void Main()
    StringAtLocation foo = new StringAtLocation { X = 0, Y = 0, Value = "foo" };
    // All of the following are valid because of interface inheritence:
    IHasPosition ihp = foo;
    IHasValue<string> ihv = foo;
    IPositionValue<string> ipv = foo;       

And no, it's not documented in the interface keyword documentation or the Interfaces (C# Programming Guide) section on MSDN. Sadly.

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