I have been attempting to create a Validator that checks whether a value
entered into a TextBox is unique from the values in a specified field in a
database. Doing this validation is quite simple when using postbacks, but I
would like to do it using client callbacks by implementing the
ICallbackEventHandler interface (if you want to do postbacks, just use the
CustomValidator). Here is the code I have written for my first attempt:

Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Data.OleDb
Public Class UVVOleDb : Inherits BaseValidator : Implements
ICallbackEventHandler
   Dim connstring As String = ""
   Dim connstringkey As String = ""
   Dim dbfield As String = ""
   Dim dbtable As String = ""
   Dim dbtype As DBTypes = DBTypes.Character

   Public Property ConnectionString() As String
       Get
           Return Me.connstring
       End Get
       Set(ByVal value As String)
           If Me.connstringkey = "" Then Me.connstring = value
       End Set
   End Property
   Public Property ConnectionStringKey() As String
       Get
           Return Me.connstringkey
       End Get
       Set(ByVal value As String)
           Me.connstring =
System.Configuration.ConfigurationManager.AppSettings(value)
           Me.connstringkey = value
       End Set
   End Property
   Public Property FieldName() As String
       Get
           Return Me.dbfield
       End Get
       Set(ByVal value As String)
           Me.dbfield = value
       End Set
   End Property
   Public Property TableName() As String
       Get
           Return Me.dbtable
       End Get
       Set(ByVal value As String)
           Me.dbtable = value
       End Set
   End Property
   Public Property FieldType() As DBTypes
       Get
           Return Me.dbtype
       End Get
       Set(ByVal value As DBTypes)
           Me.dbtype = value
       End Set
   End Property

   Protected Overrides Function EvaluateIsValid() As Boolean
       Dim validationvalue As String =
Me.GetControlValidationValue(Me.ControlToValidate)
       Select Case Me.dbtype
           Case DBTypes.Character
               Return Me.DoValidation("'" & validationvalue.Replace("'",
"''") & "'")
           Case DBTypes.DateTime
               Return Me.DoValidation(String.Concat("#",
CDate(validationvalue).ToShortDateString(), "#"))
           Case DBTypes.NoQuotes
               Return Me.DoValidation(validationvalue)
       End Select
   End Function

   Private Function DoValidation(ByVal currvalue As String) As Boolean
       Dim myconnection As New OleDbConnection(Me.connstring)
       Dim cmdunique As New OleDbCommand(String.Format("SELECT {0} FROM {1}
WHERE {0}={2}", Me.dbfield, Me.dbtable, currvalue), myconnection)
       Dim uniquereader As OleDbDataReader
       myconnection.Open()
       uniquereader = cmdunique.ExecuteReader()
       Return Not uniquereader.HasRows()
       myconnection.Close()
   End Function

   Private Sub Page_PreRender(ByVal sender As Object, ByVal e As
System.EventArgs) Handles Me.PreRender
       If Not
Me.Page.ClientScript.IsClientScriptBlockRegistered(String.Concat(Me.ClientID,
"UVVevalfunc")) Then
Me.Page.ClientScript.RegisterClientScriptBlock(Me.GetType(),
String.Concat(Me.ClientID, "UVVevalfunc"), String.Format("function
{0}UVVevalfunc(val){{{1};return tempisvalid;tempvalid=null;}}", Me.ClientID,
Me.Page.ClientScript.GetCallbackEventReference(Me,
"document.getElementById(val.controltovalidate).value", "SetIsUnique",
Nothing)), True)
       If Not
Me.Page.ClientScript.IsClientScriptBlockRegistered("SetIsUnique") Then
Me.Page.ClientScript.RegisterClientScriptBlock(Me.GetType(), "SetIsUnique",
"function SetIsUnique(isvalid){tempisvalid=isvalid;return true;}", True)
   End Sub

   Public Function GetCallbackResult() As String Implements
System.Web.UI.ICallbackEventHandler.GetCallbackResult
       Return CStr(IIf(Me.IsValid, "1", "0"))
   End Function

   Public Sub RaiseCallbackEvent(ByVal eventArgument As String) Implements
System.Web.UI.ICallbackEventHandler.RaiseCallbackEvent
       Me.IsValid = Me.EvaluateIsValid()
   End Sub

   Protected Overrides Sub AddAttributesToRender(ByVal writer As
System.Web.UI.HtmlTextWriter)
       MyBase.AddAttributesToRender(writer)
       writer.AddAttribute("evaluationfunction", String.Concat(Me.ClientID,
"UVVevalfunc"))
   End Sub
End Class

Public Enum DBTypes As Byte
   Character 'Include single quotes, used for text types such as VARCHAR,
CHAR, etc.
   DateTime 'Use the appropriate format for DATETIME for the selected
provider
   NoQuotes 'Do not modify, used for numeric and other types that do not
require formatting
End Enum

The validator gives the correct validation, but it always does a postback,
which pretty much removes the purpose of implementing ICallbackEventHandler.
I believe the reason for the problem is that when the validation is
triggered on the client side, the function specified by the validator's
evaluationfunction is called. However, the basic procedure for the callback
is to initiate the callback using a JavaScript statement, which I placed
inside the function specified by evaluationfunction. When returning from the
callback, another JavaScript function is called and is passed the result of
the callback. I had this function assign the result to a variable. The
evaluationfunction function then, supposedly, returns that variable to
complete the process. However, because this is not working, I have a theory
that because the client callback occurs while inside the evaluationfunction,
the value that it is supposed to return is not getting returned, causing
some sort of problem during the attempt to do client-side validation that
forces the page to use a postback. I may not have explained this perfectly,
but if you look at my code, you should be able to figure out what I was
attempting to do. Any ideas for a solution or workaround? Thanks.