ASP.NET Custom Validator of ISBN Numbers

International Standard Book Numbers (ISBNs), like other important numbers, such as credit card numbers, lend themselves to self-validation. The last digit of an ISBN is a check digit. A value found by applying a mathematical formula to other digits of the ISBN can be compared to this check digit. If the values relate to each other in a certain way, the ISBN is taken to be of a valid format.

To use an ASP.NET custom validator to apply the validation logic, proceed as follows. (The code validates ten-digit ISBN numbers.)

First, drag a CustomValidator control to the design surface of Visual Studio. Set its ControlToValidate to the ID of the control that is to be validated (eg. a TextBox). Set the ErrorMessage and Text properties as required. Double-click the CustomValidator control and add the following code to the ServerValidate event handler (this is example starter code: you will need to check its suitability against your particular requirements):

protected void cvISBN_ServerValidate( object source,
ServerValidateEventArgs args )
{
    string strInput = args.Value;

    if( strInput.Length != 10 )
    {
        args.IsValid = false;
    }
    else
    {
        int checksum;

        if( strInput[9] == 'X' )
        {
            checksum = 10;
        }
        else
        {
            checksum = (int)strInput[9] - 48;
        }

        int total = 0;

        for( int i = 0; i < 9; i++ )
        {
            total += ( i + 1 ) * ( (int)strInput[i] - 48 );
        }

        args.IsValid = ( total % 11 == checksum );
    }
}

To perform client-side validation too, you can write a similar function in JavaScript and attach it to the CustomValidator’s ClientValidationFunction property. The function should have the following signature:

function myvalidationfunction(source, args)