Is it possible to make a parameter implement two interfaces?

Question

Is it possible to define a function that takes in a parameter that must implement two interfaces?

(The two interfaces are ones I just remembered off the top of my head; not the ones I want to use)

private void DoSomthing(IComparable, ICollection input)
{

}

Answer 1

You can inherit another interface from those two interfaces and make your parameter implement that interface.

Answer 2

You can:

1) Define an interface that inherits both required interfaces:

public interface ICombinedInterface : IComparable, ICollection {... }

private void DoSomething(ICombinedInterface input) {... }

2) Use generics:

private void DoSomething<T>(T input)
    where T : IComparable, ICollection
{...}

HTH, Kent

Answer 3

Well, yes, and no.

You can, as Steve has suggested, create another, third, interface which descends from the two you want, and use that for the parameter type.

However, this will also make it a requirement that the class being used implements that third interface as well.

In other words, this won't work:

public interface I1 { }
public interface I2 { }
public class C : I1, I2 { }

public interface I3 : I1, I2 { }
public class YourClass
{
    public void Test(I3 i) { }
}

...
YourClass yc = new YourClass();
C c = new C();
yc.Test(c); // won't work, C does not implement I3

However, you can trick the compiler into what you want by way of generics.

public class YourClass
{
    public void Test<T>(T i) where T: I1, I2 { }
}

Now it will work. I'm still not 100% sure that won't give you other issues though, I'd have to think about it.