Can a form tell if there are any modal windows open?

Question

How, being inside the main form of my WinForm app can I tell if there are any modal windows/dialogs open that belong to the main form?

Answer 1

This is an illustration why this answer is correct and the assumptions made in this answer are sometimes wrong.

if (MyForm.CanFocus == false && MyForm.Visible == true)
{
    // we are modal            
}

This works for modal shown sub Forms and for MessageBoxes.

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Some demo code:

private void modalTest_Click(object sender, EventArgs e)
{
    // Timer which fires 100ms after the first message box is shown
    System.Windows.Forms.Timer canFocusTimer = new System.Windows.Forms.Timer();
    canFocusTimer.Tick += CanFocusTimer_Tick;
    canFocusTimer.Interval = 100;
    canFocusTimer.Start();
    
    // First MessageBox shows that CanFocus == true
    MessageBox.Show($"First MessageBox: { nameof(this.CanFocus) } == { this.CanFocus }");
}

private void CanFocusTimer_Tick(object sender, EventArgs e)
{
    (sender as System.Windows.Forms.Timer).Stop();

    // Even though there is already a MessageBox shown the second gets
    // displayed. But now CanFocus == false
    MessageBox.Show($"Second MessageBox: { nameof(this.CanFocus) } == { this.CanFocus }");
}

Result: