Case Sensitive Replace

       static public string Replace(string original, string pattern, string replacement, StringComparison comparisonType)
        {
            return Replace(original, pattern, replacement, comparisonType, -1);
        }

        static public string Replace(string original, string pattern, string replacement, StringComparison comparisonType, int stringBuilderInitialSize)
        {
            if (original == null)
            {
                return null;
            }

            if (String.IsNullOrEmpty(pattern))
            {
                return original;
            }


            int posCurrent = 0;
            int lenPattern = pattern.Length;
            int idxNext = original.IndexOf(pattern, comparisonType);
            StringBuilder result = new StringBuilder(stringBuilderInitialSize < 0 ? Math.Min(4096, original.Length) : stringBuilderInitialSize);

            while (idxNext >= 0)
            {
                result.Append(original, posCurrent, idxNext - posCurrent);
                result.Append(replacement);

                posCurrent = idxNext + lenPattern;

                idxNext = original.IndexOf(pattern, posCurrent, comparisonType);
            }

            result.Append(original, posCurrent, original.Length - posCurrent);

            return result.ToString();
        }

// ============================================================ Example 2
private static string ReplaceEx(string original, string pattern, string replacement)
{
    int count, position0, position1;
    count = position0 = position1 = 0;
    string upperString = original.ToUpper();
    string upperPattern = pattern.ToUpper();
    int inc = (original.Length/pattern.Length) * (replacement.Length-pattern.Length);


    char [] chars = new char[original.Length + Math.Max(0, inc)];
    while((position1 = upperString.IndexOf(upperPattern, position0)) != -1)
    {
        for ( int i=position0 ; i < position1 ; ++i )
            chars[count++] = original[i];
        for ( int i=0 ; i < replacement.Length ; ++i )
            chars[count++] = replacement[i];
        position0 = position1+pattern.Length;
    }
    if ( position0 == 0 ) return original;
    for ( int i=position0 ; i < original.Length ; ++i )
        chars[count++] = original[i];
    return new string(chars, 0, count);
}
//=============================================================
// Souce: Internet