GridView must be placed inside a form tag with runat=server

Control 'grvtype' of type 'GridView' must be placed inside a form tag with runat=server

Here I will explain how to solve the problem Control 'grvtype' of type 'GridView' must be placed inside a form tag with runat=server during if u want to render

Grid view control in HtmlTextWriter
pnlResults.RenderControl(new HtmlTextWriter(new System.IO.StringWriter(sb)));

Control 'grvtype' of type 'GridView' must be placed inside a form tag with runat=server

This error occurs whenever I am trying to export gridview data to excel or word or csv because compiler thinks that the control is not added to the form.  

To solve this problem I have added one overriding function VerifyRenderingInServerForm event in code behind it solves my problem. 

public override void VerifyRenderingInServerForm(Control control) { /* Verifies that the control is rendered */ }

By Setting adding this function in code behind that problem has solved and code runs successfully.