Frontend:
<telerik:RadGrid ID="RadGrid" runat="server" RenderMode="Auto" AllowPaging="True"  OnItemDataBound="RadGrid_ItemDataBound" OnNeedDataSource="RadGrid_NeedDataSource" 
    OnItemCommand="RadGrid_ItemCommand" OnItemCreated="RadGrid_ItemCreated" OnDeleteCommand="RadGrid_ItemDeleted" AllowSorting="true" AllowFilteringByColumn="true" 
    FilterType="HeaderContext" EnableHeaderContextMenu="True" EnableHeaderContextFilterMenu="True">   



<telerik:GridBoundColumn DataField="asset_SN" FilterControlAltText="Filter asset_SN column" HeaderText="Serial Number" SortExpression="asset_SN" UniqueName="asset_SN"></telerik:GridBoundColumn>


Backend: 
Dim assetSN As String = TryCast(editedItem.FindControl("asset_SN"), TextBox).Text

<telerik:GridTemplateColumn HeaderText="Column1" SortExpression="Column1" DataField="Column1" UniqueName="Column1" ColumnGroupName="Column1Data" AllowFiltering="false">
<EditItemTemplate>
<telerik:RadTextBox ID="txtColumn1" runat="server" RenderMode="Lightweight" MaxLength="6" Width="200px" Text='<%# Bind("Column1") %>' />
<asp:RequiredFieldValidator ID="reqColumn1" runat="server" ControlToValidate="txtColumn1" CssClass="standard-text" ErrorMessage="RequiredFieldValidator" Text="This field is required" />
</EditItemTemplate>
<ItemTemplate>
 <%#DataBinder.Eval(Container.DataItem, "Column1", "{0:@}")%>
</ItemTemplate>
</telerik:GridTemplateColumn>

Imports System.Data
Imports System.Data.SqlClient
Imports System.Drawing
Imports System.Web.UI
Imports System.Configuration
Imports System.Web.UI.WebControls
Imports Telerik.Web.UI

Partial Class RadGridEditCustomValidatorVB
    Inherits Page

    ' Page Load event
    Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
        If Not IsPostBack Then
            RadGrid.Rebind()
        End If
    End Sub

    ' Bind the grid to the data source
    Protected Sub RadGrid_NeedDataSource(sender As Object, e As GridNeedDataSourceEventArgs)
        RadGrid.DataSource = GetGridSource()
    End Sub

    ' Get data from the database
    Private Function GetGridSource() As DataTable
        Dim dataTable As New DataTable()

        ' Get the connection string from the web.config file
        Dim connectionString As String = ConfigurationManager.ConnectionStrings("Data_WarehouseConnectionString_PRODUCTION").ConnectionString
        Using connection As New SqlConnection(connectionString)
            Dim query As String = "SELECT [ID], [Asset_type], [PortalID], [asset_SN], [asset_model], [Maint_Date], [Fill_qty], [asset_location], [Active_indicator], [Asset_Notes] FROM [data_warehouse].[dbo].[ATracker]"
            Using command As New SqlCommand(query, connection)
                Using adapter As New SqlDataAdapter(command)
                    adapter.Fill(dataTable)
                End Using
            End Using
        End Using

        Return dataTable
    End Function

    Protected Sub RadGrid_ItemCreated(sender As Object, e As GridItemEventArgs)
        If e.Item.IsInEditMode Then
            Dim item As GridEditableItem = TryCast(e.Item, GridEditableItem)

            ' Add validator for Asset_type (Non-nullable)
            Dim assetTypeEditor As GridTextBoxColumnEditor = CType(item.EditManager.GetColumnEditor("Asset_type"), GridTextBoxColumnEditor)
            AddRequiredFieldValidator(assetTypeEditor.TextBoxControl, "Asset Type")

            ' Add validator for PortalID (Non-nullable)
            Dim portalIDEditor As GridTextBoxColumnEditor = CType(item.EditManager.GetColumnEditor("PortalID"), GridTextBoxColumnEditor)
            AddRequiredFieldValidator(portalIDEditor.TextBoxControl, "PortalID")

            ' Add validator for asset_sn (Non-nullable)
            Dim assetSNEditor As GridTextBoxColumnEditor = CType(item.EditManager.GetColumnEditor("asset_SN"), GridTextBoxColumnEditor)
            AddRequiredFieldValidator(assetSNEditor.TextBoxControl, "Asset Serial Number")

            ' Add validator for asset_model (Non-Nullable)
            Dim assetModelEditor As GridTextBoxColumnEditor = CType(item.EditManager.GetColumnEditor("asset_model"), GridTextBoxColumnEditor)
            AddRequiredFieldValidator(assetModelEditor.TextBoxControl, "Asset Model")

            ' Add validator for asset_location (Nullable)
            Dim assetLocEditor As GridTextBoxColumnEditor = CType(item.EditManager.GetColumnEditor("asset_location"), GridTextBoxColumnEditor)
            AddRequiredFieldValidator(assetLocEditor.TextBoxControl, "Location")

            ' Add validator for Fill_qty (Nullable but must be numeric if provided)
            Dim fillQtyEditor As GridTextBoxColumnEditor = CType(item.EditManager.GetColumnEditor("Fill_qty"), GridTextBoxColumnEditor)
            AddNumericValidator(fillQtyEditor.TextBoxControl, "Fill Quantity")

            Dim buttonName As String = If(TypeOf item Is IGridInsertItem, "PerformInsertButton", "UpdateButton")
            TryCast(item.FindControl(buttonName), Button).ValidationGroup = "gridFormValidation"
        End If
    End Sub

    Private Sub AddRequiredFieldValidator(textBox As TextBox, fieldName As String)
        Dim validator As New CustomValidator()
        validator.ID = "CustomValidator_" & fieldName.Replace(" ", "_")
        validator.ControlToValidate = textBox.ID
        validator.ValidateEmptyText = True
        validator.ErrorMessage = String.Format("* {0} is required", fieldName)
        validator.ForeColor = Color.OrangeRed
        validator.ClientValidationFunction = "requiredFieldValidation"
        validator.ValidationGroup = "gridFormValidation"
        AddHandler validator.ServerValidate, AddressOf RequiredField_ServerValidate
        textBox.Parent.Controls.Add(validator)
    End Sub

    Private Sub AddNumericValidator(textBox As TextBox, fieldName As String)
        Dim validator As New CustomValidator()
        validator.ID = "CustomValidator_" & fieldName.Replace(" ", "_")
        validator.ControlToValidate = textBox.ID
        validator.ValidateEmptyText = False
        validator.ErrorMessage = String.Format("* {0} must be numeric", fieldName)
        validator.ForeColor = Color.OrangeRed
        validator.ClientValidationFunction = "numericFieldValidation"
        validator.ValidationGroup = "gridFormValidation"
        AddHandler validator.ServerValidate, AddressOf NumericField_ServerValidate
        textBox.Parent.Controls.Add(validator)
    End Sub

    Protected Sub RequiredField_ServerValidate(source As Object, args As ServerValidateEventArgs)
        args.IsValid = Not String.IsNullOrWhiteSpace(args.Value)
    End Sub

    Protected Sub NumericField_ServerValidate(source As Object, args As ServerValidateEventArgs)
        Dim result As Decimal
        args.IsValid = Decimal.TryParse(args.Value.Trim(), result)
    End Sub

    Protected Sub RadGrid_ItemCommand(sender As Object, e As GridCommandEventArgs)
        If e.CommandName = RadGrid.UpdateCommandName Or e.CommandName = RadGrid.PerformInsertCommandName Then
            If Page.IsValid Then
                Dim editableItem As GridEditableItem = TryCast(e.Item, GridEditableItem)

                'Runs Validation Code from RequiredField_ServerValidate and NumericField_ServerValidate
                Dim txtAssetType As TextBox = CType(editableItem("Asset_type").Controls(0), TextBox)
                Dim txtPortalID As TextBox = CType(editableItem("PortalID").Controls(0), TextBox)
                Dim txtSerialNum As TextBox = CType(editableItem("asset_SN").Controls(0), TextBox)
                Dim txtFillQty As TextBox = CType(editableItem("Fill_qty").Controls(0), TextBox)
                Dim rdpMaintDate As RadDatePicker = CType(editableItem("Maint_Date").Controls(0), RadDatePicker)
                Dim txtLocation As TextBox = CType(editableItem("asset_location").Controls(0), TextBox)
                Dim txtModel As TextBox = CType(editableItem("asset_model").Controls(0), TextBox)
                Dim chkActive As CheckBox = CType(editableItem("Active_indicator").Controls(0), CheckBox)
                Dim txtNotes As TextBox = CType(editableItem("Asset_Notes").Controls(0), TextBox)

                Dim objConnection As SqlConnection
                Dim strSQL As String


                objConnection = New SqlConnection(ConfigurationManager.ConnectionStrings("Data_WarehouseConnectionString_PRODUCTION").ConnectionString)

                'Testing DB connection
                'objConnection = New SqlConnection(ConfigurationManager.ConnectionStrings("masterConnectionString").ConnectionString)

                'Determine if we are updating or inserting
                If e.CommandName = RadGrid.UpdateCommandName Then
                    strSQL = "UPDATE [data_warehouse].[dbo].[ATracker] SET [Asset_type] = @asset_type, [PortalID] = @PortalID, [asset_SN] = @AssetSN, [Fill_qty] = @FillQty, [Maint_Date] = @MaintDate, [asset_location] = @Location, [asset_model] = @Model, [Active_indicator] = @Active, [Asset_Notes] = @Notes WHERE [ID] = @ID"
                Else
                    strSQL = "INSERT INTO [data_warehouse].[dbo].[ATracker] ([Asset_type], [PortalID], [asset_SN], [Fill_qty], [Maint_Date], [asset_location], [asset_model], [Active_indicator], [Asset_Notes]) VALUES (@asset_type, @PortalID, @AssetSN, @FillQty, @MaintDate, @Location, @Model, @Active, @Notes)"
                End If

                'Add parameters to the SQL command
                Dim cmd As New SqlCommand(strSQL, objConnection)
                cmd.Parameters.Add("@asset_type", SqlDbType.VarChar, 20).Value = txtAssetType.Text
                cmd.Parameters.Add("@ID", SqlDbType.Int).Value = If(e.CommandName = RadGrid.UpdateCommandName, editableItem.GetDataKeyValue("ID"), DBNull.Value)
                cmd.Parameters.Add("@PortalID", SqlDbType.VarChar, 10).Value = txtPortalID.Text
                cmd.Parameters.Add("@AssetSN", SqlDbType.VarChar, 20).Value = txtSerialNum.Text
                cmd.Parameters.Add("@FillQty", SqlDbType.Int).Value = If(String.IsNullOrEmpty(txtFillQty.Text), DBNull.Value, Convert.ToInt32(txtFillQty.Text))
                cmd.Parameters.Add("@MaintDate", SqlDbType.Date).Value = If(rdpMaintDate.SelectedDate Is Nothing, DBNull.Value, rdpMaintDate.SelectedDate)
                cmd.Parameters.Add("@Location", SqlDbType.VarChar, 20).Value = txtLocation.Text
                cmd.Parameters.Add("@Model", SqlDbType.VarChar, 20).Value = txtModel.Text
                cmd.Parameters.Add("@Active", SqlDbType.Bit).Value = chkActive.Checked
                cmd.Parameters.Add("@Notes", SqlDbType.VarChar, -1).Value = txtNotes.Text

                Try
                    objConnection.Open()
                    cmd.ExecuteNonQuery()
                Catch ex As Exception
                    System.Console.WriteLine(ex.ToString())
                Finally
                    objConnection.Close()
                End Try

                RadGrid.Rebind()
            Else
                e.Canceled = True
            End If
        End If
    End Sub


    'CANT GET THIS TO TRIGGER
    Protected Sub RadGrid_ItemDeleted(sender As Object, e As GridCommandEventArgs)
        If e.CommandName = RadGrid.DeleteCommandName Then
            Dim item As GridDataItem = TryCast(e.Item, GridDataItem)

            If item IsNot Nothing Then
                ' Find the hidden field in the grid data item where ID is bound
                Dim atID As Integer = Convert.ToInt32(item.OwnerTableView.DataKeyValues(item.ItemIndex)("ID"))

                Dim objConnection As SqlConnection
                Dim strSQL As String

                objConnection = New SqlConnection(ConfigurationManager.ConnectionStrings("Data_WarehouseConnectionString_PRODUCTION").ConnectionString)

                ' SQL command to delete the record where ID matches
                strSQL = "DELETE FROM [data_warehouse].[dbo].[ATracker] WHERE [ID] = @ID"

                Dim cmd As New SqlCommand(strSQL, objConnection)
                cmd.Parameters.Add("@ID", SqlDbType.Int).Value = atID

                Try
                    objConnection.Open()
                    cmd.ExecuteNonQuery()
                Catch ex As Exception
                    System.Console.WriteLine(ex.ToString())
                Finally
                    objConnection.Close()
                End Try
            End If

            RadGrid.Rebind()
        End If
    End Sub


    'Show a dropdown list of the asset types in the database

    Protected Sub RadGrid_ItemDataBound(sender As Object, e As GridItemEventArgs)
        If e.Item.IsInEditMode Then
            Dim item As GridEditableItem = DirectCast(e.Item, GridEditableItem)

            If Not (TypeOf e.Item Is IGridInsertItem) Then
                Dim combo As RadComboBox = DirectCast(item.FindControl("Asset_type"), RadComboBox)

                Dim preselectedAsset As New RadComboBoxItem()
                preselectedAsset.Text = item("Asset_type").Text
                preselectedAsset.Value = item("Asset_type").Text
                combo.Items.Insert(0, preselectedAsset)
            End If
        End If
    End Sub



End Class

    <form id="form1" runat="server">
        <telerik:RadStyleSheetManager ID="RadStyleSheetManager2" runat="server"></telerik:RadStyleSheetManager>
        <telerik:RadScriptManager ID="RadScriptManager2" runat="server"></telerik:RadScriptManager>
        <telerik:RadAjaxManager ID="RadAjaxManager1" runat="server"></telerik:RadAjaxManager>
        <telerik:RadGrid ID="RadGrid" runat="server" AllowPaging="True" Width="800px"
            OnNeedDataSource="RadGrid_NeedDataSource" OnItemCommand="RadGrid_ItemCommand" onitemdatabound="RadGrid_ItemDataBound"
            OnItemCreated="RadGrid_ItemCreated" OnDeleteCommand="RadGrid_ItemDeleted" Skin="MetroTouch">
            <GroupingSettings CollapseAllTooltip="Collapse all groups"></GroupingSettings>
            <MasterTableView AutoGenerateColumns="False" CommandItemDisplay="Top" DataKeyNames="ID">
                <RowIndicatorColumn ShowNoSortIcon="False"></RowIndicatorColumn>
                <ExpandCollapseColumn ShowNoSortIcon="False"></ExpandCollapseColumn>
                <Columns>
                    <telerik:GridBoundColumn DataField="ID" DataType="System.Int32" FilterControlAltText="Filter ID column" HeaderText="ID" ReadOnly="True" SortExpression="ID" UniqueName="ID" Visible="False"></telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="Asset_type" FilterControlAltText="Filter Asset_type column" HeaderText="Asset_type" SortExpression="Asset_type" UniqueName="Asset_type" ></telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="PortalID" FilterControlAltText="Filter PortalID column" HeaderText="PortalID" SortExpression="PortalID" UniqueName="PortalID"></telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="asset_SN" FilterControlAltText="Filter asset_SN column" HeaderText="Serial Number" SortExpression="asset_SN" UniqueName="asset_SN"></telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="asset_model" FilterControlAltText="Filter asset_model column" HeaderText="Model" SortExpression="asset_model" UniqueName="asset_model"></telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="asset_location" FilterControlAltText="Filter asset_location column" HeaderText="Location" SortExpression="asset_location" UniqueName="asset_location"></telerik:GridBoundColumn>
                    <telerik:GridDateTimeColumn DataField="Maint_Date" FilterControlAltText="Filter Maint_Date column" HeaderText="Date of Service" SortExpression="Maint_Date" UniqueName="Maint_Date"></telerik:GridDateTimeColumn>
                    <telerik:GridBoundColumn DataField="Fill_qty" FilterControlAltText="Filter Fill_qty column" HeaderText="Fill Qty (Gallons)" SortExpression="Fill_qty" UniqueName="Fill_qty"></telerik:GridBoundColumn>
                    <telerik:GridCheckBoxColumn DataField="Active_indicator" DataType="System.Boolean" FilterControlAltText="Filter Active_indicator column" HeaderText="Active?" SortExpression="Active_indicator" UniqueName="Active_indicator"></telerik:GridCheckBoxColumn>
                    <telerik:GridBoundColumn DataField="Asset_Notes" FilterControlAltText="Filter Notes column" HeaderText="Notes" SortExpression="Asset_Notes" UniqueName="Asset_Notes"></telerik:GridBoundColumn>
                    <telerik:GridEditCommandColumn EditText="Edit" />
                    <telerik:GridButtonColumn ConfirmText="Are you sure you want to delete this record?" ConfirmDialogType="RadWindow" ConfirmTitle="Delete" CommandName="Delete" Text="Delete" />
                                  

                </Columns>
               
            </MasterTableView>
        </telerik:RadGrid>
    </form>