C# Language Tutorial => Using an extension method for collection...

Example

Collection initialization syntax can be used when instantiating any class which implements IEnumerable and has a method named Add which takes a single parameter.

In previous versions, this Add method had to be an instance method on the class being initialized. In C#6, it can also be an extension method.

public class CollectionWithAdd : IEnumerable
{
    public void Add<T>(T item)
    {
        Console.WriteLine("Item added with instance add method: " + item);
    }

    public IEnumerator GetEnumerator()
    {
        // Some implementation here
    }
}

public class CollectionWithoutAdd : IEnumerable
{
    public IEnumerator GetEnumerator()
    {
        // Some implementation here
    }
}

public static class Extensions
{
    public static void Add<T>(this CollectionWithoutAdd collection, T item)
    {
        Console.WriteLine("Item added with extension add method: " + item);
    }
}

public class Program
{
    public static void Main()
    {
        var collection1 = new CollectionWithAdd{1,2,3}; // Valid in all C# versions
        var collection2 = new CollectionWithoutAdd{4,5,6}; // Valid only since C# 6
    }
}

This will output:

Item added with instance add method: 1
Item added with instance add method: 2
Item added with instance add method: 3
Item added with extension add method: 4
Item added with extension add method: 5
Item added with extension add method: 6

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