CustomValidator Control

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`CustomValidator` Control

Sometimes none of the previously covered validation controls are sufficient to properly validate user input. For instance, none of the preceding examples would be up to the task of validating that a credit card number was valid, or better yet, checking to make sure that that account had sufficient funds for the pending transaction. For situations like these that require the use of special business rules to validate the user input, the CustomValidator control is the tool to use. Table 8.7 details the additional properties specific to the CustomValidator control.

Table 8.7. `CustomValidator` Class Additional Properties

Method	Type	Description
`ClientValidationFunction`	`String`	The client-side function to use for `Uplevel` browser client-side validation.

The CustomValidator control also exposes a method, ServerValidate , that must be handled by the form's server-side code and referenced when the control is used with the OnServerValidate="" attribute. It is possible to create very advanced CustomValidator validation logic, but for this example, we will just check to see if the value entered is a perfect square (an integer raised to the power of 2) or not. Listings 8.5 and 8.6 show the ASPX file and its codebehind used for this validation. For this example, only server-side validation is being implemented, which is frequently all that is possible with custom validation because they often rely on remote data stores to perform their tasks .

Listing 8.5 Adding custom validation to a text box (Example0905.aspx).

<%@ Page language="c#" Codebehind="Example0905.aspx.cs" Src="Example0905.aspx.cs"

AutoEventWireup="false" Inherits="validate.Example0905" %>
<HTML>
    <HEAD>
        <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
    </HEAD>
    <body>
        <form method="post" runat="server" ID="Form1">
            <asp:Label ID="errorMessage" Runat="server"></asp:Label>
            <table>
                <tr>
                    <td>
                        Enter a Perfect Square (the square of an integer):
                    </td>
                    <td>
                        <asp:textbox runat="Server" columns="30"
    id="square"></asp:textbox>
                    </td>
                    <td>
                        <asp:CustomValidator Runat="server"
    ID="square_custom" Display="Dynamic" OnServerValidate="validateSquare"
    EnableClientScript="False">
                        *
                        </asp:CustomValidator>
                    </td>
                </tr>
            </table>
            <asp:button id="save_button" runat="Server"
    text="Save"></asp:button>
        </form>
    </body>
</HTML>

Listing 8.6 Adding custom validation to a text box (Example0905.aspx.cs).

protected void validateSquare(object sender,
    System.Web.UI.WebControls.ServerValidateEventArgs e)
    {
        int _square;
        int _sqrt;

        try
        {
            _square = Int32.Parse(square.Text.ToString());
        }
        catch
        {
            e.IsValid = false;
            return;
        }
        try
        {
            _sqrt =
    Int32.Parse(Math.Sqrt(Double.Parse(_square.ToString())).ToString());
        }
        catch
        {
            e.IsValid = false;
            return;
        }
        e.IsValid = true;
    }

This example takes advantage of error handlers to determine whether or not the square root of the value entered into the "square" text box is an integer. If no error is raised when converting the value itself or its square root into an integer value, the value entered must be a perfect square. Otherwise, the value must not be a perfect square. It is interesting to note that the event handler validateSquare does not return a value, but instead sets the value of the ServerEventArgs object's IsValid property to true or false , as appropriate. Although this is a very simple example, custom validation controls can be as simple or as complex as your business needs dictate .

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