How to disable the parent form when a child form is active?

Have you tried using Form.ShowDialog() instead of Form.Show()?

ShowDialog shows your window as modal, which means you cannot interact with the parent form until it closes.

Are you calling ShowDialog() or just Show() on your child form from the parent form?

ShowDialog will "block" the user from interacting with the form which is passed as a parameter to ShowDialog.

Within the parent you might call something like:

MyChildForm childForm = new MyChildForm();

childForm.ShowDialog(this);

where this is the parent form.

What you could do, is to make sure to pass the parent form as the owner when showing the child form:

  Form newForm = new ChildForm();
  newForm.Show(this);

Then, in the child form, set up event handlers for the Activated and Deactivate events:

private void Form_Activated(object sender, System.EventArgs e)
{
    if (this.Owner != null)
    {
        this.Owner.Enabled = false; 
    }
}

private void Form_Deactivate(object sender, System.EventArgs e)
{
    if (this.Owner != null)
    {
        this.Owner.Enabled = true;
    }
}

However, this will result in a truly wierd behaviour; while you will not be able to go back and interact with the parent form immediately, activating any other application will enable it, and then the user can interact with it.

If you want to make the child form modal, use ShowDialog instead:

  Form newForm = new ChildForm();
  newForm.ShowDialog(this);

You can do that with the following:

Form3 formshow = new Form3();

formshow.ShowDialog();

If you are just trying to simulate a Form.ShowDialog call but WITHOUT blocking anything (kinda like a Simulated Dialog Form) you can try using Form.Show() and as soon as you show the simulated dialog form then immediately disable all other windows using something like...

private void DisableAllWindows()
{
foreach (Form f in Application.OpenForms)
if (f.Name != this.Name)f.Enabled = false;
else f.Focus();
}

Then when you close your "pseudo-dialog form" be sure to call....

private void EnableAllWindows()
{
foreach (Form f in Application.OpenForms) f.Enabled = true;
}