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Please follow the step-by-step below in order to implement you required scenario.

1. Create grid, assign DataSource, DataMember and RetrieveStructure as shown in the CustomVirtualLoad.aspx tutorial file. The tutorial file is available at WebGrid Tutorial project inside the V3.5 folder.
2. In the attached sample file, the value of VirtualPageSize property is set to “10”.
3. Add an ASP.NET HiddenField control, HiddenField1, into the page. This control is used to store the total loaded rows of WebGrid.

   <asp:HiddenField ID="HiddenField1" runat="server" />

4. Add OnBeforeRequest client-side event of WebGrid. This event specifies the client-side function to be invoked when a request action to server is about to be made. Within this event, the value of HiddenField1 control is set based on the value of the action parameter.

   function WebGrid1_OnBeforeRequest(controlId, action) {
       var WebGrid1 = ISGetObject(controlId);
       var hf = document.getElementById("HiddenField1");
               
       if (action == "More") {
           if (hf.value != "")
               hf.value = parseInt(hf.value) + 10;
           else
               hf.value = WebGrid1.TotalLoadedRows + 10;
       }
               
       return true;
   }

5. Invoke the InitializePostBack server side event.

   protected void WebGrid1_InitializePostBack(object sender, PostbackEventArgs e)
   {
       if (HiddenField1.Value != "")
       {
           WebGrid1.LayoutSettings.VirtualPageSize = int.Parse(HiddenField1.Value);
           WebGrid1.VirtualLoadArgs.RequestedRows = int.Parse(HiddenField1.Value);
       }
   
       if (e.Action == "More")
       {
           WebGrid1.RebindDataSource();
           ISNet.WebUI.FunctionParameter[] total = new ISNet.WebUI.FunctionParameter[1];
           total[0] = new ISNet.WebUI.FunctionParameter(WebGrid1.VirtualLoadArgs.TotalDataSourceRows.ToString(), "string");
           WebGrid1.ClientAction.InvokeScript("SetTotal", total);
       }
   
       else if (e.Action == "ColumnSort")
       {
           WebGrid1.RebindDataSource();
           ISNet.WebUI.FunctionParameter[] total = new ISNet.WebUI.FunctionParameter[1];
           total [0] = new ISNet.WebUI.FunctionParameter(WebGrid1.VirtualLoadArgs.TotalDataSourceRows.ToString(),"string");
           WebGrid1.ClientAction.InvokeScript("SetTotal",total);
       }
   }

6. Attach a client-side event on OnAfterResponse event. The event is used to set the value of TotalLoadedRows property of WebGrid based on the value stored in HiddenField1 control.

   function WebGrid1_OnAfterResponseProcess(controlId, actionName, lastRequestObject, xmlResponseObject) {
       var WebGrid1 = ISGetObject(controlId);
       var hf = document.getElementById("HiddenField1");
       var table = WebGrid1.RootTable;
   
       WebGrid1.TotalLoadedRows = hf.value;
   
       return true;
   }

7. Add SetTotal JavaScript function. This function is used to synchronize the loaded indicator shown in the status bar of WebGrid.

   function SetTotal(total) {
       var WebGrid1 = ISGetObject("WebGrid1");
       var hf = document.getElementById("HiddenField1");
   
       if (hf.value != "")
           WebGrid1.GetElement(WG40.STATUSBAR, WG40.HTMLROW).childNodes[3].innerText = "Loaded " + hf.value + " of " + total;
   }

A simple sample is enclosed as attachment. Please have the sample tested on your end and let us know whether this helps or not.