Including a Day Suffix when formatting a Date in C#

ToStringWithSuffix.cs

namespace System {
        /// <summary>
?        /// Convert a day of the month into one with a suffix attached
        /// </summary>
        /// <requires>
        /// A string representing the correct suffix for a given day of the month 
        /// </requires>
        public static string ToStringWithSuffix(this DateTime dt, string format) {   
            // The format parameter MUST contain [$suffix] within it, which will be replaced.   
            int day = dt.Day; string suffix = "";   
            // Exception for the 11th, 12th, & 13th   
            // (which would otherwise end up as 11st, 12nd, 13rd)   
            if (day % 100 >= 11 && day % 100 <= 13) {   
                suffix = "th";   
            }else{
                switch (day % 10) {   
                    case 1: 
                        suffix = "st";   
                        break;   
                    case 2:
                        suffix = "nd";   
                        break;   
                    case 3:   
                        suffix = "rd";   
                        break;  
                    default:   
                        suffix = "th";   
                        break;   
                }
            }
            // Convert the date to the format required, then add the suffix.   
        return dt.ToString(format).replace("[$suffix]",suffix);
    }
}