```
public partial class MainMenu : Form
{
    public MainMenu()
    {
        InitializeComponent();
    }
    windowOne One;
    windowTwo Two;
    private void OneToolStripMenuItem_Click(object sender, EventArgs e)
    {
        if (One != null)
        {
            One.WindowState = FormWindowState.Normal;
            One.Focus();
        }
        else
        {
            One = new windowOne();
            One.MdiParent = this;
            One.FormClosed += (o, ea) => One = null;
            One.Show();
        }
    }

    private void TwoToolStripMenuItem_Click(object sender, EventArgs e)
    {
        if (Two != null)
        {
            TwoWindowState = Two.Normal;
            Two.Focus();
        }
        else
        {
            Two = new windowTwo();
            Two.MdiParent = this;
            Two.FormClosed += (o, ea) => Two = null;
            Two.Show();
        }
    }

```

Im beginner in c#,
Im working on window application, i wish when windowOne is open user couldnot open windowTwo, im using above codes to avoid opening windowOne or windowTwo agian..