AppendIfNotContains string helper method

I’ve created a helper function, that allows me to set extra CSS class name(space separated)
public static string AppendIfNotContains(string str, string sToAppend, string delimeter)
            {
                if (!str.Contains(sToAppend))
                {
                    str = AppendWithDelimeter(str, sToAppend, delimeter);
                }
                return str;
            }
public static string AppendWithDelimeter(string str, string sToAppend, string delimeter)
            {
                if ((!str.EndsWith(delimeter) & !String.IsNullOrEmpty(str)) & !String.IsNullOrEmpty(sToAppend))
                {
                    str = str + delimeter;
                }
                str = str + sToAppend;
                return str;
            }