C# interface question

Question

Is there a cost in passing an object to a function that implements a particular interface where the function only accepts that interface? Like:

Change (IEnumerable<T> collection)

and I pass:

List<T>
LinkedList<T>
CustomCollection<T>

which all of them implements IEnumerable. But when you pass any of those to the Change method, are they cast to IEnumerable, thus there is a cast cost but also the issue of losing their unique methods, etc?

Answer 1

No, there is no cast involved since List<T> IS-A IEnumerable<T>. This is using polymorphism which does not require casting.

Edit: Here is an example:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        foo(new List<int>());
    }

    static void foo(IEnumerable<int> list) { }
}

The IL for Main is:

.method private hidebysig static void Main() cil managed
{
    .entrypoint
    .maxstack 8
    L_0000: nop 
    L_0001: newobj instance void [mscorlib]System.Collections.Generic.List`1<int32>::.ctor()
    L_0006: call void Program::foo(class [mscorlib]System.Collections.Generic.IEnumerable`1<int32>)
    L_000b: nop 
    L_000c: ret 
}

And as you can see there is no casting involved. The instance of List<T> is pushed onto the stack and them foo is called immediately after.