Trim string from the end of a string in .NET - why is this missing?

Question

I need this all the time and am constantly frustrated that the Trim(), TrimStart() and TrimEnd() functions don\'t take strings as inputs. You call EndsWith() on a string, an

Answer 1

I recently needed a high performance way to remove single or multiple instances of a string from the start/end of a string. This implementation I came up with is O(n) on the length of the string, avoids expensive allocations, and does not call SubString at all by using a span.

No Substring hack! (Well, now that I edited my post).

    public static string Trim(this string source, string whatToTrim, int count = -1) 
        => Trim(source, whatToTrim, true, true, count);

    public static string TrimStart(this string source, string whatToTrim, int count = -1) 
        => Trim(source, whatToTrim, true, false, count);

    public static string TrimEnd(this string source, string whatToTrim, int count = -1) 
        => Trim(source, whatToTrim, false, true, count);

    public static string Trim(this string source, string whatToTrim, bool trimStart, bool trimEnd, int numberOfOccurrences)
    {
        // source.IsNotNull(nameof(source));  <-- guard method, define your own
        // whatToTrim.IsNotNull(nameof(whatToTrim));  <-- "

        if (numberOfOccurrences == 0 
            || (!trimStart && !trimEnd) 
            || whatToTrim.Length == 0 
            || source.Length < whatToTrim.Length)
            return source;

        int start = 0, end = source.Length - 1, trimlen = whatToTrim.Length;

        if (trimStart)
            for (int count = 0; start < source.Length; start += trimlen, count++)
            {
                if (numberOfOccurrences > 0 && count == numberOfOccurrences)
                    break;
                for (int i = 0; i < trimlen; i++)
                    if ((source[start + i] != whatToTrim[i] && i != trimlen) || source.Length - start < trimlen)
                        goto DONESTART;
            }

        DONESTART:
        if (trimEnd)
            for (int count = 0; end > -1; end -= trimlen, count++)
            {
                if (numberOfOccurrences != -1 && count == numberOfOccurrences)
                    break;

                for (int i = trimlen - 1; i > -1; --i)
                    if ((source[end - trimlen + i + 1] != whatToTrim[i] && i != 0) || end - start + 1 < trimlen)
                        goto DONEEND;
            }

        DONEEND:
        return source.AsSpan().Slice(start, end - start + 1).ToString();
    }