返回接口的方法

IFoo SimpleFoo AdvancedFoo

List<T> IEnumerable<T>

IFoo foo = getFoo();
SimpleFoo simpleFoo = (SimpleFoo)foo;

as

IFoo foo = getFoo();
SimpleFoo simpleFoo = foo as SimpleFoo;
if (simpleFoo == null)
{
     // Too bad...
}
else
{
    // Now we can use simpleFoo.
}

public interface IFooBar
{
    String GetData();
}

public class Foo : IFooBar
{
    public String GetData() { return "Foo"; }
}

public class Bar : IFooBar
{
    public String GetData() { return "Bar"; }
}

public class DataManager
{
    public static IFooBar GetFooBar()
    {
        IFooBar foobar = ...
        return foobar;
     }
}

public class MainAppClass
{
     public void SomeMethod()
     {
         //You don't care what type of class you get here
         //you only care that the object you get back let's you
         //call GetData.
         IFooBar foobar = DataManager.GetFooBar();
         String data = foobar.GetData();
         //etc
     }
}

using System.Collections.Generic;

ICollection<int> getCollection() {
    return new LinkedList<int>();
}

C# interfaces tutorial