How to prevent or block closing a WinForms window?

Question

How can I prevent window closing by showing a MessageBox? (Technology:WinForms with C#)

When the close event occurs I want the

Answer 1

A special twist might be to always prevent just the user closing the form:

private void Frm_FormClosing(object sender, FormClosingEventArgs e)
{
    e.Cancel = (e.CloseReason == CloseReason.UserClosing); 
    // disable user closing the form, but no one else
}