How do I validate a UPC or EAN code?

Question

I need a C# .NET function to evaluate whether a typed or scanned barcode is a valid Global Trade Item Number (UPC or EAN).

The last digit of a bar code number is a computer Check Digit which makes sure the bar code is correctly composed. GTIN Check Digit Calculator

Answer 1

public static bool IsValidGtin(string code) {     if (code != (new Regex("[^0-9]")).Replace(code, ""))     {         // is not numeric         return false;     }     // pad with zeros to lengthen to 14 digits     switch (code.Length)     {         case 8:             code = "000000" + code;             break;         case 12:             code = "00" + code;             break;         case 13:             code = "0" + code;             break;         case 14:             break;         default:             // wrong number of digits             return false;     }     // calculate check digit     int[] a = new int[13];     a[0] = int.Parse(code[0].ToString()) * 3;     a[1] = int.Parse(code[1].ToString());     a[2] = int.Parse(code[2].ToString()) * 3;     a[3] = int.Parse(code[3].ToString());     a[4] = int.Parse(code[4].ToString()) * 3;     a[5] = int.Parse(code[5].ToString());     a[6] = int.Parse(code[6].ToString()) * 3;     a[7] = int.Parse(code[7].ToString());     a[8] = int.Parse(code[8].ToString()) * 3;     a[9] = int.Parse(code[9].ToString());     a[10] = int.Parse(code[10].ToString()) * 3;     a[11] = int.Parse(code[11].ToString());     a[12] = int.Parse(code[12].ToString()) * 3;     int sum = a[0] + a[1] + a[2] + a[3] + a[4] + a[5] + a[6] + a[7] + a[8] + a[9] + a[10] + a[11] + a[12];     int check = (10 - (sum % 10)) % 10;     // evaluate check digit     int last = int.Parse(code[13].ToString());     return check == last; }