String.Replace is not case insensitive, so what is the best way to do it. There are several ways, if I think of the quickest one it is using regex.

using System.Text.RegularExpressions;string 
string str = "The quick brown Fox jumps over the lazy dog";
string strReplace = "cat";
string result = Regex.Replace(str, "fox", strReplace, RegexOptions.IgnoreCase);

-Rujith