Validators & ICallbackEventHandler

[Nathan Sokalski]

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.

 

[bruce barker]

your approach will not work. ajax callbacks (which the
icallbackeventhander uses) are async, while the call to your validator
is synchronous. the valididator is called during the submit event, and
must return true or false on whether to cancel the postback. if your
validator makes an ajax call, the response event can not be fired until
after the original postback has started or been canceled.

you could fire an ajax event on lost focus, and update a flag the
validator looked at. if the postback happens before the ajax call
completes, then allow the postback and validate on the server.

-- bruce (sqlwork.com)