How do I catch this objectdisposedexception

It works fine for me, regardless of if I create a form instance and try that:

private void Button1_Click(object sender, EventArgs e)
    {
    FrmMain form = new FrmMain();
    try
        {
        form.Close();
        form.Show();
        }
    catch (ObjectDisposedException)
        {
        form.Show();
        }
    }

Or I use the existing form:

private void Button1_Click(object sender, EventArgs e)
    {
    try
        {
        Close();
        Show();
        }
    catch (ObjectDisposedException)
        {
        Show();
        }
    }

Either way, the debugger shows the catch working fine.
I think you are confused, because you will get an identical exception inside the catch handler when you try to Show the form for a second time. Try this:

private void Button1_Click(object sender, EventArgs e)
    {
    FrmMain form = new FrmMain();
    try
        {
        form.Close();
        form.Show();
        }
    catch (ObjectDisposedException ex)
        {
        MessageBox.Show(ex.Message);
        }
    }

It should show you the error.