Web Forms :: Parent-child Web Form: How To Close Dialog Box And Return Values To Parent Form?

In my website I have two forms: parent form and child form(dialog box). Their expected behaviour is like this: if clicked on 'show' button on parent form, a dialog form opens that displays a gridview. In dialog form, if clicked on 'select' button it closes
itself and returns value in selected row back to parent form.

To achieve this I write following code (.cs) : in parent form:

void ShowBtn_Click(object sender, EventArgs e)

{

StringBuilder jScript = new StringBuilder(); [code]....

Now problem is: the code in parent .cs works fine, it opens the dialog properly. But when clicked on 'select', instead of returning back to parent, it opens the same dialog again in new window. This newly open window says "Done but error on page" at left bottom.