สอบถาม Code C# ครับ ในการกำหนด MDI Form ให้เปิดได้แค่ Form เดียว

private int numOfChilde = 0;
private void mnuCreateChild_Click(object sender,EventArgs e)
{
	if(numOfChild == 0)
	{
		childForm.Show();
		numOfChild++;
	}
}

 private void Form1_Click(object sender, EventArgs e)
        {
            foreach (Form currentform in this.MdiChildren)
            {
                if (currentform is Form1)
                {
                    currentform.MdiParent = this;
                    currentform.Show();
                    return;
                }
            }
            Form1 frm = new Form1();
            frm.WindowState = FormWindowState.Maximized;
            frm.MdiParent = this;
            frm.Show();
        }

Public Class MainMDI
    Public Sub ShowForm(Of FormType As {Windows.Forms.Form, New})()
        Try
            For Each ChildForm As Form In Me.MdiChildren
                If TypeOf ChildForm Is FormType Then
                    ChildForm.Focus()
                    Exit Sub
                End If
            Next
            Dim frm As New FormType
            frm.MdiParent = Me
            frm.Show()
        Catch ex As Exception
            ' แสดงข้อผิดพลาด
        End Try
    End Sub

    Private Sub miTestForm_Click(sender As System.Object, e As System.EventArgs) Handles miTestForm.Click
        Me.ShowForm(Of TestForm)()
    End Sub
End Class