Quick port of the function used here to detect the credit card type: http://webstandardssherpa.com/reviews/auto-detecting-credit-card-type

GetCardType.cs

// Quick port of the function used here to detect the credit card type: 
// http://webstandardssherpa.com/reviews/auto-detecting-credit-card-type

public static string GetCardType(string Number)
{
    //start without knowing the credit card type
    var result = "unknown";

    //first check for MasterCard
    if (Regex.Match(Number, @"^5[1-5]").Success)
    {
        result = "mastercard";
    }

    //then check for Visa
    else if (Regex.Match(Number, @"^4").Success)
    {
        result = "visa";
    }

    //then check for AmEx
    else if (Regex.Match(Number, @"^3[47]").Success)
    {
        result = "amex";
    }

    return result;
}