How to setup a CompareValidator based on based on a different web control

<asp: CompareValidator runat="server" ID="RequiredFieldValidator5" 
      ControlToValidate="TextBox13" 
      ValidationExpression="RadioButtonList5.Text == 'No'">
    Please Answer Question
</asp:RequiredFieldValidator>`

I would like to setup a CompareValidator for a text box, based upon the response from a RadioButton.

I have a web page where the user is supposed to answer several questions. If the user answers no to question 5 (RadioButtonList5 Yes/No) then in question 6 they are supposed to enter a date.

I know the ValidationExpression is incorrect. I just need help figuring out how to set it up properly.

Since user input determines if the compare validator is enabled or not, then you need to use JavaScript/jQuery code, like this:

// This will enable the validator
ValidatorEnable(document.getElementById("RequiredFieldValidator5"), true);

// This will disable the validator
ValidatorEnable(document.getElementById("RequiredFieldValidator5"), false);

So you will need to run the above code in a change handler for the radio button list, like this:

$("#<%=RadioButtonList5.ClientID%> input").change(function() {
    if($(this).val() == "Yes") {
        // Enable or disable compare validator
    }
    if($(this).val() == "No") {
        // Enable or disable compare validator
    }
});