Question

while the method we use in Substitution control should return strings, so how is it possible to use a donut caching in web forms on a server control which should be rendered server side?
for example Loginview control?

Answer 1

UPDATE
This is now a fully working example. There a few things happening here:

1. Use the call back of a substitution control to render the output of the usercontrol you need.
2. Use a custom page class that overrides the VerifyRenderingInServerForm and EnableEventValidation to load the control in order to prevent errors from being thrown when the usercontrol contains server controls that require a form tag or event validation.

Here’s the markup:

<asp:Substitution runat="server" methodname="GetCustomersByCountry" />

Here’s the callback

public string GetCustomersByCountry(string country)
{
   CustomerCollection customers = DataContext.GetCustomersByCountry(country);

    if (customers.Count > 0)
        //RenderView returns the rendered HTML in the context of the callback
        return ViewManager.RenderView("customers.ascx", customers);
    else
        return ViewManager.RenderView("nocustomersfound.ascx");
}

Here’s the helper class to render the user control

public class ViewManager
{
    private class PageForRenderingUserControl : Page
    {
        public override void VerifyRenderingInServerForm(Control control)
        { /* Do nothing */ }

        public override bool EnableEventValidation
        {
            get { return false; }
            set { /* Do nothing */}
        }
    }

    public static string RenderView(string path, object data)
    {
        PageForRenderingUserControl pageHolder = new PageForUserControlRendering();
        UserControl viewControl = (UserControl) pageHolder.LoadControl(path);

        if (data != null)
        {
            Type viewControlType = viewControl.GetType();
            FieldInfo field = viewControlType.GetField("Data");
            if (field != null)
            {
                field.SetValue(viewControl, data);
            }
            else
            {
                throw new Exception("ViewFile: " + path + "has no data property");
            }
        }

        pageHolder.Controls.Add(viewControl);
        StringWriter result = new StringWriter();
        HttpContext.Current.Server.Execute(pageHolder, result, false);
        return result.ToString();
    }
}

See these related questions:

• Turn off page-level caching in a
  user control
• UserControl’s RenderControl is
  asking for a form tag in (C#
  .NET)