I am trying to create a UserControl that is a composite control for entering Dates. It involves 2 drop down controls (one for month and one for day) and a text box for year. It also has a CustomValidation control that I would like the user of the UserControl to be able to set the ClientValidationFunction property and have it triggered whenever the value in any of the 3 date part controls changes.

To hook up each of the 3 controls to the validation control, I use teh AJAX ValidatorHookupControlID method.

If I hard code the following code directly in my aspx page that houses the usercontrol, my Client Validation Function is successfulluy called when any of the 3 date part control values are changed:

<script type="text/javascript">
//<![CDATA[
debugger;
alert('hooking up');
ValidatorHookupControlID("ctl00_phPageContent_dtmPassportExpirationDate_txtYear", document.all("ctl00_phPageContent_dtmPassportExpirationDate_CustomValidator1"));
ValidatorHookupControlID("ctl00_phPageContent_dtmPassportExpirationDate_ddlDate", document.all("ctl00_phPageContent_dtmPassportExpirationDate_CustomValidator1"));
ValidatorHookupControlID("ctl00_phPageContent_dtmPassportExpirationDate_ddlMonth", document.all("ctl00_phPageContent_dtmPassportExpirationDate_CustomValidator1"));
//]]>

However, I would like my usercontrol to emit this Javascript, so I added the following Code.

Public Property ClientValidationFunction() As String
        Get
            Return CustomValidator1.ClientValidationFunction
        End Get
        Set(ByVal value As String)
            CustomValidator1.ClientValidationFunction = value
        End Set
    End Property

    Public Property EnableClientScript() As Boolean
        Get
            Return CustomValidator1.EnableClientScript
        End Get
        Set(ByVal value As Boolean)
            CustomValidator1.EnableClientScript = value           
        End Set
    End Property

    Protected Sub Page_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreRender

        If Me.ClientValidationFunction.Trim <> "" AndAlso
           Me.EnableClientScript Then
            RegisterValidationScript()
        End If
    End Sub

    Private Sub RegisterValidationScript()

        Dim Key As String = Me.ClientID & "_ClientValidate"

        If Not Page.ClientScript.IsClientScriptBlockRegistered(Key) Then
            Page.ClientScript.RegisterClientScriptBlock(GetType(String), Key, GetJavascript, True)
        End If

    End Sub

    Private Function GetJavascript() As String

        Dim Out As String = ""
        Const Quote As String = Chr(34)

        Out &= "debugger;" & vbCrLf
        Out &= "ValidatorHookupControlID(" & Quote & txtYear.ClientID & Quote & ", document.all(" & Quote & CustomValidator1.ClientID & Quote & "));" & vbCrLf
        Out &= "ValidatorHookupControlID(" & Quote & ddlDate.ClientID & Quote & ", document.all(" & Quote & CustomValidator1.ClientID & Quote & "));" & vbCrLf
        Out &= "ValidatorHookupControlID(" & Quote & ddlMonth.ClientID & Quote & ", document.all(" & Quote & CustomValidator1.ClientID & Quote & "));" & vbCrLf

        Return Out

    End Function

Unfortunately, when the ValidatorHookupControlID method is called, the second parameter (the one that locates the validation control) always evaluates to null and the method fails.

ValidatorHookupControlID("ctl00_phPageContent_DateSelector21_txtYear", document.all("ctl00_phPageContent_DateSelector21_CustomValidator1"));

Can you tell me how to correct this error?