CustomValidator not working well

Question

I have the following piece of asp:

<asp:ValidationSummary ID="RegisterUserValidationSummary" runat="server" CssClass="failureNotification" 
        ValidationGroup="RegisterUserValidationGroup"/>

...

<asp:Label ID="UserNameLabel" runat="server" AssociatedControlID="UserNameTB">Username:</asp:Label>
<asp:TextBox ID="UserNameTB" runat="server" CssClass="textEntry"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" ControlToValidate="UserNameTB" 
      ValidationExpression="[a-zA-Z]{3,8}" ErrorMessage="Username must be between 3 to 8 chars" runat="server"
      CssClass="failureNotification" ToolTip="Username must be between 3 to 8 chars" ValidationGroup="RegisterUserValidationGroup">
    *</asp:RegularExpressionValidator>
<asp:CustomValidator ID="NoUserValidator" ControlToValidate="UsernameTB" runat="server" ErrorMessage="User Taken!" CssClass="failureNotification" 
      ValidationGroup="RegisterUserValidationGroup"  OnServerValidate="UserValidate">*</asp:CustomValidator>

And then the function:

protected void UserValidate(object source, ServerValidateEventArgs args)
    {
        SqlDataSource1.SelectCommand = "SELECT ClientID FROM [Clients] WHERE Username= '" + UserNameTB.Text + "'";
        DataView dv = (DataView)SqlDataSource1.Select(DataSourceSelectArguments.Empty);
        if (dv.Table.Rows.Count != 0)
            args.IsValid = false;
        else
            args.IsValid = true;
    }

Button:

<asp:Button ID="CreateUserButton" runat="server" CommandName="MoveNext" Text="Create User" 
       ValidationGroup="RegisterUserValidationGroup" 
       onclick="CreateUserButton_Click"/>

Problem is that even though the custom validator function is called and sets .IsValid to false, the button logic still runs!

Answer 1

In your onclick function for the button, add a check to see if the page is valid

protected void CreateUserButton_Click(object sender, EventArgs e)
{
    if (Page.IsValid)
    { 
        // Create the user
    }
}

That should do it. This is because your custom validator is set up to validate on the server, during the postback. What happens is that the code first runs the validator code UserValidate, where you set the IsValid flag. Next in the postback stack is the button's onclick function. This function will run regardless of the result in the validator function, so this is where you need to check the value of the IsValid flag. This is the behavior when you validate the custom validation control on the server side.

An alternative is to validate on the client side. If you look at the page source code generated by your browser, you'll see that Javascript is added for the RegularExpressionValidator. It's behavior is known, and handled on the client side, so no post back is required to evaluate the expression and validate the page (it's all handled by javascript). The custom validator function is not known, so a postback is required unless you define a client-side validation script yourself.

Here's a link to more information on MSDN.