Problem with RenderControl

[M Amin]

Hello everyone

I am trying to create dynamic RadComboBox in my Ajax request, and then sending back as html, for this i am using RenderControl for the  RadComboBox control. but I get this error when it executes the RenderControl method ...

new ajax_error('System.InvalidOperationException','Page cannot be null. Please ensure that this operation is being performed in the context of an ASP.NET request.',0)

Here is the code snippet 

Dim sb As New StringBuilder

Dim sw As New System.IO.StringWriter(sb)

Dim hw As New HtmlTextWriter(sw)

Dim ddlCountries As New Telerik.Web.UI.RadComboBox

ddlCountries.ID = "ddlCountries"

Dim loc As New Frokht.Location

Dim dr As Data.SqlClient.SqlDataReader

dr = loc.GetAllCountries()

ddlCountries.DataSource = dr

ddlCountries.DataTextField = "Name"

ddlCountries.DataValueField = "Code"

ddlCountries.DataBind()  ddlCountries.RenderControl(hw)




Please help




Thanks in advance







[Simon]

Hi M Amin,

This error raises because the RadComboBox is not added to the Page control hierarchy.

You can add the control directly to the Form or some other container (in the page), e.g.

Form.Controls.Add(ddlCountries)

All the best,
Simon
the Telerik team

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[M Amin]

Hi Simon

Thanks for your help, but now i am getting another error.

Script control 'countries' is not a registered script control. Script controls must be registered using RegisterScriptControl() before calling RegisterScriptDescriptors().

i tried ScriptManager1.RegisterScriptControl(ddlCountries) before RenderControl, but it then gave me error Script controls may not be registered before PreRender.

Please help.

[Simon]

Hello M Amin,

This can happen if you are calling RenderControl too early in the page life cycle. 

The best way to approach this is to only add the control in the Init phase or later if a certain postback condition should be met and leave the control to the ASP.NET framework. It will call the control's respective methods and will register it whenever necessary.

Kind regards,
Simon
the Telerik team

Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking system and vote to affect the priority of the items.

[M Amin]

I am trying to create a dynamic control in an AJAX request, I did not understand "if a certain postback condition should be met and leave the control to the ASP.NET framework. It will call the control's respective methods and will register it whenever necessary." Can you please explain
Thanks

[Simon]

Hi M Amin,

What I meant was that you only need to add the Control to the page control hierarchy. In the render phase, the Render methods of all controls will be called automatically as well as their scripts registered. In other words you do not need to these things this manually.

Finally I suggest you try a different approach: add the RadComboBox to the page initially invisible and toggle its Visible property instead.

Best wishes,
Simon
the Telerik team

Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking system and vote to affect the priority of the items.