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If you point the ControlToCompare attribute of the CompareValidator at a HiddenField you’ll get an error that looks something like this:

Control ‘hiddenBalance’ referenced by the ControlToCompare property of ‘CompareValidator1′ cannot be validated.

With a small amount of “googling” I found two useful articles:

• Validating against the value in a hidden input
• MS KB Article 310082

Between these two article I pieced together a solution.

The Solution

1. Create a new web project and add it to your solution. I called my project “CompareValidatorHelper”.
2. Right click on the project and select Add and the Class…. I also named my class “CompareValidatorHelper”.
3. Paste the following C# code into the class:

   
using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.ComponentModel;
namespace CompareValidatorHelper
{
[DefaultProperty("Value"), ValidationProperty("Value"), ToolboxData("<{0}:VHiddenField runat=server></{0}:VHiddenField>")]
public class VHiddenField : System.Web.UI.WebControls.HiddenField
{
}
}


4. Build the project and add a reference to it in your web application project.
5. Register the inherited control at the top of your page:
   <%@ Register TagPrefix="custom" Namespace="CompareValidatorHelper" Assembly="CompareValidatorHelper" %>
6. Add the HiddenField control to your page:
   <custom:VHiddenField ID="hiddenBalance" runat="server" Value='<%# Eval("Balance") %>' />
7. Finally, set the ControlToCompare attribute of your CompareValidator equal to the ID attribute of your HiddenField:
   <asp:CompareValidator ID="CompareValidator1" Display="Dynamic" runat="server" Type="Double" Operator="LessThanEqual" ErrorMessage="Overpayments are not allowed." ControlToCompare="hiddenBalance" ControlToValidate="txtPaymentAmount">*</asp:CompareValidator>