c# - Why does a collection initializer expression require IEnumerable to be implemented?

Question

Why does this generate a compiler error:

class X { public void Add(string str) { Console.WriteLine(str); } }

static class Program
{
    static void Main()
    {
        // error CS1922: Cannot initialize type 'X' with a collection initializer
        // because it does not implement 'System.Collections.IEnumerable'
        var x = new X { "string" };
    }
}

but this doesn’t:

class X : IEnumerable
{
    public void Add(string str) { Console.WriteLine(str); }
    IEnumerator IEnumerable.GetEnumerator()
    {
        // Try to blow up horribly!
        throw new NotImplementedException();
    }
}

static class Program
{
    static void Main()
    {
        // prints “string” and doesn’t throw
        var x = new X { "string" };
    }
}

What is the reason for restricting collection initializers — which are syntactic sugar for a call to an Add method — to classes that implement an interface which doesn’t have an Add method and which isn’t used?

Answer 1

An object initializer doesn't; a collection initializer does. It's so that it's applied to classes which really represent collections, rather than just arbitrary ones which have an Add method. I have to admit that every so often I've "implemented" IEnumerable explicitly, just to allow collection initializers - but thrown a NotImplementedException from GetEnumerator().

Note that early in C# 3's development, collection initializers had to implement ICollection<T>, but that was found to be too restrictive. Mads Torgersen blogged about this change, and the reason behind requiring IEnumerable, back in 2006.