Hello Keentoknow,
Thanks for your reply.

I have reproduced the issue. In design mode, there is no Web server, no
http request or http context. Thereby, it's not possible for your custom
control to get its parent page instance which exists in runtime. For this
reason, VS IDE creates default Page instance in designer mode. VS IDE
doesn't know your custom control hosted in page which derives from
MyPageBase class. This resulted the error message "Cannot cast object to
type 'MyPageBase'". But in runtime, asp.net runtime can get all the
information from http context. Thus, the new control works perfect.

For the above reason, I'm afraid to say there is no way to get MyProperty
of MyPagebase instance in Custom Web Control's render method at designer
time. In general, what we do is to generate some static information html
content in designer mode. You can check DesignMode property. The value is
true means it's in design mode. On the other side, the false value means
it's in runtime.

if (this.DesignMode == true)
{
output.Write("this design time property");
}
else
{
if (((MyPageBase)this.Page).MyProperty == true)
  output.Write("this MyPagebase property");
}

Another way is that you may create a ControlDesigner class for extend the
design-mode behavior of a Web server control
http://msdn2.microsoft.com/en-us/library/system.web.ui.design.controldesigne
r(VS.80).aspx
[ControlDesigner Class]

Hope this helps. Please feel free to let me know if there is anything
unclear. We are glad to assist you.

Have a great day,
Best regards,

Wen Yuan
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