[Proposal] Add Add(IEnumerable items) for ICollection to support the collection initialisation syntax

[MihailsKuzmins]

Would it be possible to add a method Add(IEnumerable items) for the IList interface?
My problem is that I would like to use the collection initialisation syntax on this model (unfortunately, the IList cannot be changed to the implementation).

public sealed class Class {}

public sealed class SomeClass
{
    public IList<Class> Classes { get; }
}

// this is what I want
var items = Enumerable.Empty<Class>();

var item = new SomeClass
{
    Classes = { items } // does not compile
};

I have tried creating an extension method, but it seems that generic extension methods for interfaces are not accepted as valid candidates for the collection initialisation 😭

public static void Add<T>(this IList<T> @this, IEnumerable<T> items)
{
    foreach (var item int items)
        @this.add(item);
}

So I thought maybe it could be possible to add this method to IList<T> or even ICollection<T>?

[HaloFour]

API suggestions are better made on the Runtime repository. This repository is more specifically for the C# language itself, and the language wouldn't need any changes to call an Add instance or extension method if it were to exist.

[MihailsKuzmins]

@HaloFour, oh, didn't know that. Thank you. I will bear it in mind for the next time

[CyrusNajmabadi]

Transferring this to dotnet/runtime. Thanks!

[huoyaoyuan]

Default interface methods makes modifying interface technically non-breaking, but not used in practice yet.
Supporting AddRange in collection initializer may not be a good idea. Similar has been complained in dotnet/csharplang#4522