C# MDI Child changing size during run time?

Question

Essentially I have a MDI Parent window, which runs my MainMenu form as a MDI Child, from the Main Menu I can go open other forms. I had the MainMenu then resize the MDI Parent to the size of the MDI Child form. This works perfectly, however the problem is when I go back to the MainMenu form, for some reason the size property has changed to whichever form I had opened.

    private void MDI_MdiChildActivate(object sender, EventArgs e)
    {
        try
        {
            this.ClientSize = this.ActiveMdiChild.Size;
            this.CenterToScreen();
        }
        catch { }
    }

This is the code for resizing my MDI Parent.

I am not asking about how to get the size of a Parent or child form, my code works just fine one way. The issue is that the MainMenu size property changes during run time despite me never changing the size of the MainMenu. The MainMenu form is not the MDI Parent but a child also.

Answer 1

To make a Window as big as your Mdi parent, you should set the FormState of the Mdi Child to maximized instead.

Also, why do you modify the size when an MDI child is activated ? Why not just setting it to Maximized on creation of the MDI child, like this:

private void button1_Click(object sender, EventArgs e)
{
    Form2 f = new Form2();

    f.MdiParent = this;
    f.WindowState = FormWindowState.Maximized;

    f.Show();                
}

If you want to have only one MDI child visible at a time, then make sure you hide the others. You can do it like this for instance:

private Form2 _f2;
private Form3 _f3;

private void open1ToolStripMenuItem_Click(object sender, EventArgs e)
{
    if (_f2 == null)
    {
        _f2 = new Form2();
    }

    ShowForm(_f2);
}

private void open2ToolStripMenuItem_Click(object sender, EventArgs e)
{
    if (_f3 == null)
    {
        _f3 = new Form3();
    }
    ShowForm(_f3);
}

private void ShowForm(Form f)
{

    var otherWindows = this.MdiChildren.Where(child => child != f);

    foreach (var o in otherWindows)
    {
        o.Hide();
    }

    f.MdiParent = this;
    f.WindowState = FormWindowState.Maximized;

    f.Show();
}

Note that you can also choose to close the other forms, depending on what's on them and if you need to save, etc...