Validation ASP.Net Tutorial

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<%@ Page Language="C#" %>
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

    void valProductCode_ServerValidate(Object source, ServerValidateEventArgs args)
    {
        if (args.Value.Length == 4)
            args.IsValid = true;
        else
            args.IsValid = false;
    }



    Show Validate Empty Text


    
    

            id="lblProductCode"
        Text="Product Code:"
        AssociatedControlID="txtProductCode"
        Runat="server" />
    
            id="txtProductCode"
        Runat="server" />
            id="valProductCode"
        ControlToValidate="txtProductCode"
        Text="(Invalid product code)"
        ValidateEmptyText="true"
        OnServerValidate="valProductCode_ServerValidate"
        Runat="server" />
            id="btnSubmit"
        Text="Submit"
        Runat="server" />