c# string character replace

Question

I have a string where the third last character is sometimes a , If this is the case I want to replace it with a . The string could also have other

Answer 1

How about:

if (text[text.Length - 3] == ',')
{
    StringBuilder builder = new StringBuilder(text);
    builder[text.Length - 3] = '.';
    text = builder.ToString();
}

EDIT: I hope the above is just about the most efficient approach. You could try using a char array instead:

if (text[text.Length - 3] == ',')
{
    char[] chars = text.ToCharArray();
    chars[text.Length - 3] = '.';
    text = new string(chars);
}

Using Substring will work as well, but I don't think it's any more readable:

if (text[text.Length - 3] == ',')
{
    text = text.Substring(0, text.Length - 3) + "."
           + text.Substring(text.Length - 2);
}

EDIT: I've been assuming that in this situation you already know that text will be at least three characters length. If that's not the case, you'd obviously want a test for that as well.