installer.msi windows dialogbox

I have Two winform1-frmcheckupdate 2-FrmUserLogin

simple I run our application ,if updated installer ".msi" is available then show a message box with MessageBoxButtons.YesNo to update existing ".msi".

if i click "YES" then updation start successfully,

But if i click "NO" then I want to show dialog as "FrmUserLogin " form and closed form as frmcheckupdate.

I have try as below code -

frmchkupdate fu = new frmchkupdate();
fu.Close();
fu.Hide();
//checkForUpdate.StopThread();
// fu.Dispose();

FrmUserLogin fl = new FrmUserLogin();
fl.ShowDialog();

plz tell me why frmchkupdate is not closed/hide.