form can not disappear after code visible=false

[C# Programming]

Hello Experts!!

i am making visible false after validating my login form and open MDI form.But i got problem is login form cant disappear.It shows in taskbar.what to do?
i am using following code for it

frmLogin login = new frmLogin();
login.Visible = false; //THIS CONDITION
MDIPROJECT mdi = new MDIPROJECT();
mdi.Show();