You can use numerous different instances of RadComboBox inside your DataGrid.

For complex load-on-demand scenarios - for example load-on-demand combobox inside an asp:datagrid EditTemplate, you will need to use the ExternalCallBackPage property of RadComboBox. The callback ItemsRequested event fires only when then combobox is initially visible in the page life-cycle (Page.IsPostBack == false). This is often not the case when the combobox has been added dynamically to the page or when the combo is inside a template that it not initially visible (EditTemplate in asp:datagrid for example). This is where ExternalCallBackPage comes to help.

ExternalCallBackPage is a normal ASP.NET aspx page that contains only one instance of RadComboBox. The ID of the combobox should match the ID of the combobox initiating the request. You can then wire the ItemsRequested event handler of the external combobox and write your custom logic, the same way you would do in the original page. Finally, set the ExternalCallBackPage property of the original combobox to the name of the callback (streamer) page. The path is relative to the current path of the parent page (e.g. no path needed if the streamer page is in the same folder - just the name of the file).

One additional benefit of ExternalCallBackPages is that methods in the original page lifecycle (Page_Init, Page_Load, etc) do not get executed - the execution is directly transferred to the external streamer page.