CustomValidator does not fire

Recently, I needed to ensure that a user fill in one of two TextBox controls on a form I was designing. I couldn't use two RequiredFieldValidator controls as that would mandate that both fields be completed. I therefore created a CustomValidator control and added it to the page. It did not work. There was a flaw in my thinking. The control did not ever fire when needed (ie. when both fields are blank), because the control does not, by default, validate blank fields. Thankfully, there is a way to change this. You can set the ValidateEmptyText to true:

<asp:CustomValidator ID="myValidator" runat="server" ErrorMessage="Error message" OnServerValidate="myValidator_ServerValidate ValidateEmptyText="true"></asp:CustomValidator>

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)

How to Set the Maximum Length of a MultiLine TextBox

The MaxLength property of a TextBox control has no effect when the TextMode is set to MultiLine. The reason for this is that a MultiLine TextBox is rendered as a <textarea> tag, which does not have a maximum length attribute:

<textarea name="txtComments" rows="2" cols="20" id="txtComments">

In contrast, a TextBox with its TextMode set to SingleLine is rendered as an <input> tag, which has the maxlength attribute:

<input name="txtComments" type="text" maxlength="5" id="txtComments" />

If you wish to restrict the amount of text in a MultiLine TextBox, the workaround is to use a RegularExpressionValidator control. In the following example, the validation expression allows between 0 and 5 alphanumeric characters to be entered:

string strMaxLength = "5";

revComments.ValidationExpression = "\\w{0," + strMaxLength + "}$";