foreach(Form f in Application.OpenForms)
{
    if(f.Focused)
        return f;
}

MyDialog.ShowDialog(activedForm);

[DllImport("user32.dll")]
private static extern IntPtr GetForegroundWindow();

public Form GetFocusedForm()
{
    IntPtr fgw = GetForegroundWindow();
    foreach (Form f in Application.OpenForms)
    {
        if (f.Handle == fgw)
            return f;
    }
    return null;
}