How to replace part of string by position?

Question

I have this string: ABCDEFGHIJ

I need to replace from position 4 to position 5 with the string ZX

It will look like this: ABC

Answer 1

If you care about performance, then the thing you want to avoid here are allocations. And if you're on .Net Core 2.1+ (or the, as yet unreleased, .Net Standard 2.1), then you can, by using the string.Create method:

public static string ReplaceAt(this string str, int index, int length, string replace)
{
    return string.Create(str.Length - length + replace.Length, (str, index, length, replace),
        (span, state) =>
        {
            state.str.AsSpan().Slice(0, state.index).CopyTo(span);
            state.replace.AsSpan().CopyTo(span.Slice(state.index));
            state.str.AsSpan().Slice(state.index + state.length).CopyTo(span.Slice(state.index + state.replace.Length));
        });
}

This approach is harder to understand than the alternatives, but it's the only one that will allocate only one object per call: the newly created string.