private void tabbedView1_EndFloating(object sender, DocumentEventArgs e)
{
DockPanel panel =(DockPanel)e.Document.Control;
panel.ParentForm.FormClosed += ParentForm_FormClosed;
}
private void ParentForm_FormClosed(object sender, FormClosedEventArgs e)
{
DockPanel dock=(DockPanel)((FloatDocumentForm)sender).Document.Control;
dock.DockedAsTabbedDocument = true;
tabbedView1.AddDocument(dock);
tabbedView1.ActivateDocument(dock);
}