how to show mainform after login

Hi,

Move the code out of your login form into the "Program.cs" file.
When your program first starts, you'll create and show the login form as a modal dialog.

As like the below one

static void Main()
{
    LoginForm frmLog = new LoginForm();
    if (frmLog.ShowDialog() == DialogResult.OK)
    {
        Application.Run(new Form2());
    }
    else
    {
        Application.Exit();
    }
}