gistfile1.cs

/// <summary>
        /// Validates the VISA credit card.
        /// </summary>
        /// <param name="creditCardNumber">The credit card number.</param>
        /// <returns></returns>
        public static bool validateVISACreditCard(string creditCardNumber)
        {
            // this is a short credit card, the algorithm is unknown so return true
            //ISRACARD
            if (creditCardNumber.Length == 8 || creditCardNumber.Length == 9)
                return true;

            // this is a normal credit card (VISA or ISRACARD) so continue checking, if the lenght is invalid return false;
            if (creditCardNumber.Length > 19 || creditCardNumber.Length < 11)
                return false;

            //VISA
            // seperate all the numbers into an array of chars
            char[] numbers = creditCardNumber.ToCharArray();
            //declare the weight number var
            int weightNumber = 0;
            //the kength of the array
            int length = numbers.Length;
            // the current index of the array
            int currIndex = length - 1;

            List<CreditCardNumbers> creditCardNumbersList = new List<CreditCardNumbers>();

            // while the loop is shorter then the length of the char array
            // we are using this way beacue we need the credit card number from right to left and not the other way around
            while (currIndex >= 0)
            {
                //if the current index is an even number then the weightNumber is 2
                // else is 1
                weightNumber = NumberHelpers.isOddNumber(currIndex) ? 2 : 1;

                //add the number and the weight number to a list
                creditCardNumbersList.Add(new CreditCardNumbers(Convert.ToInt32(numbers[currIndex].ToString()), weightNumber));
                
                //subtract 1 off the index
                currIndex--;
            }
            // reset the counter of the index
            currIndex = length - 1;
            
            // var for the summing of all the weight numbers after the precious calculation
            int weightNumbersSum = 0;

            foreach (CreditCardNumbers item in creditCardNumbersList)
            {
                // weight number var
                weightNumber = item.weightNumber;
                // credit number var
                int creditNumber = item.number;
                // this is the calculated number of the weight number times the credit number
                int calculatedNumber = weightNumber * creditNumber;
                // seperate the calculated number into an array
                char[] calculatedWeightNumbers = calculatedNumber.ToString().ToCharArray();
                // if the length of the result is more then 2 numbers, add the first to the second
                // if the result 12
                // 1 + 2 ==> 3 --> this is the final expected result
                if (calculatedWeightNumbers.Length > 1)
                    calculatedNumber = (Convert.ToInt32(calculatedWeightNumbers[0].ToString()) + Convert.ToInt32(calculatedWeightNumbers[1].ToString()));
                
                //sum up the calculated numbers
                weightNumbersSum += calculatedNumber;
            }
            
            // if the final result is divided by 10, the credit card number is valid
            if (weightNumbersSum % 10 == 0)
                return true;
            else
                return false;
        }

I wrote up a little article about ISRACARD validation:
http://povolotski.me/2013/09/24/isracard-credit-card-number-validation-2/