Difference

public static string Difference(this string ? value, string str, bool ignoreCase) {
    if (value == null)
        return str;

    if (str == null)
        return value;

    string valueLower = value;
    string strLower = str;

    if (ignoreCase) {
        valueLower = valueLower.ToLowerInvariant();
        strLower = strLower.ToLowerInvariant();
    }

    // Find the common prefix length
    int prefixLength = 0;
    for (int i = 0; i < Math.Min(value.Length, str.Length); i++) {
        if (valueLower[i] != strLower[i]) {
            break;
        }

        prefixLength++;
    }

    // Return the remainder of the second string starting from the different part
    return str[prefixLength..];
}

string str1 = "hello world";
string str2 = "hello sunshine";
string difference = str1.Difference(str2, false); // Case-sensitive: difference will be " sunshine"

str1 = "MyFile.txt";
str2 = "MyFile.pdf";
difference = str1.Difference(str2, false); // Case-sensitive: difference will be ".pdf"