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

As an extension method.

public static class StringBuilderExtension
{
    public static string SubsituteString(this string OriginalStr, int index, int length, string SubsituteStr)
    {
        return new StringBuilder(OriginalStr).Remove(index, length).Insert(index, SubsituteStr).ToString();
    }
}