限制窗口重复开启

private void companyToolStripMenuItem1_Click(object sender, EventArgs e)
{
    Master.CompanyMasterForm cmpnymasterform = null;
    bool isFormOpen = IsAlreadyOpen(typeof(Master.CompanyMasterForm));
    if (isFormOpen == false)
    {
        cmpnymasterform = new Master.CompanyMasterForm();
        cmpnymasterform.StartPosition = FormStartPosition.CenterScreen;

        cmpnymasterform.MdiParent = this;
        cmpnymasterform.Show();
    }
}

private bool IsAlreadyOpen(Type formType)
{
    bool isOpen = false;
    foreach (Form f in Application.OpenForms)
    {
        if (f.GetType() == formType)
        {
            f.BringToFront();
            f.WindowState = FormWindowState.Normal;
            isOpen = true;
        }
    }
    return isOpen;
}