string s = "";
s = s + myChar.ToString(); // Where myChar is the char to be added
//Insert method returns a copy, it does not alter the string since strings are immutable
string str = "abc";
str = str.Insert(2, "XYZ"); //str == "abXYZc"
string oldString = "1+1=";
string newString = oldString+"2"; //newString = "1+1=2"