Заменить в строке вхождения строки - C#

string S1 = "God bless you (variants include God bless or bless you) is a common English expression, used to wish a person blessings in various situations, especially as a response to a sneeze, and also, when parting or writing a valediction.",
       S2 = "bless",
       S3 = "hate";
int choice = 1;
 
switch(choice)
{
    case 1:
        {
            int IndexFirst = S1.IndexOf(S2);
            S1 = S1.Remove(IndexFirst, S2.Length).Insert(IndexFirst, S3);
        } break;
    case 2: 
        {
            int IndexLast = S1.LastIndexOf(S2);
            S1 = S1.Remove(IndexLast, S2.Length).Insert(IndexLast, S3);
        } break;
    case 3: 
        { 
            S1 = S1.Replace(S2, S3); 
        } break;
}