TheoryData is a collection type, it implements the old Pattern to initialize it (Add) so it would be convinient if it would support the C#12's collection expression syntax as well.

Currently:

    public static TheoryData<string> NumberStrings()
        => new() { string.Empty, "no number", "-12", "0"};

Proposed:

public static TheoryData<string> NumberStrings()
    => [string.Empty, "no number", "-12", "0"];

The implementation would require the implementation of a simple Collection Builder.

The other theoretical advantage to the old pattern is the support of immutable collections, but we don't have that in this case.

What do you think?