Duck Typing in CSharp

List Initializer

Any type implemented IEnumerable and has a Add(T item): void method can use list initializer syntax.

• Add(T item): void can be implemented as extension method
• ICollection is more generally used to have list initializer enabled, because it's an extension of IEnumerable with Add method.

_ = new FooCollection<int>() { 1, 2, 3 };

class FooCollection<T> : IEnumerable<T> {
    public void Add(T item) { Console.WriteLine(item); } 
    public IEnumerator<T> GetEnumerator() {
        throw new NotImplementedException();
    }
    IEnumerator IEnumerable.GetEnumerator() {
        return GetEnumerator();
    }
}
// or implement it as extension method
static class Ext {
    public static void Add<T>(this FooCollection<T> self, T item) { } 
}

Deconstruction

var (a, b) = new Foo();

class Foo { };

static class FooExtension {
    public static void Deconstruct(this Foo self, out int a, out int b) { a = b = 1; } 
}

Iteration

Any type with a GetEnumerator implementation can be iterated by foreach statement.

• can be implemented as an extension method

using System.Collections;

foreach (var _ in new FooIterable()) { } 

class FooIterable { };

static class FooExtension {
    public static IEnumerator GetEnumerator(this FooIterable _) { 
        for (int i = 0; i < 100; i++) 
            yield return i;
    } 
}