Question

I have two forms. I show my second form with this code:

Form2 f = new Form2();
f.ShowDialog();

I need to understand when focus returns to main form. I tried Activate event but that’s not it.

Answer 1

The call to ShowDialog() is blocking the parent form.
When your code exit from ShowDialog() your current form become active again,

For example:

using(Form2 f = new Form2())
{
    // At the moment of the call of ShowDialog, the Net Framework start its work to
    // pass the focus to the first control in the Form2 instance
    if(DialogResult.OK == f.ShowDialog())
    {
        // Form confirmed, do your stuff
    }
}

// At the end of the using block the parent form is again the active form with the focus on
// the button that has started this process (or the last control that had focus)