winform实现父窗体里面只能打开一个子窗体

使用Application.OpenForms解决

if(Application.OpenForms["Form1"]==null)
{
    Form1 frm =new Form1();
   frm.Show();
   frm.MdiParent=this;
} else{
    Application.OpenForms["Form1"].Show();
}

//在mdi窗体中显示一个子窗体
        //调用方法 Form form1 = new Form(); open（form1）;
        private void open(Form form)
        {
            if (!form.Created)
            {
                foreach (var item in this.MdiChildren)
                {
                    item.Close();
                }
                form = (Form)Activator.CreateInstance(form.GetType());
                form.MdiParent = this;
                form.Show();


            }
            else
            {
                form.Focus();
            }

        }

//将form 窗体中显示中mdi窗体中
            Form2 form = new Form2();
            form.MdiParent = this;
            form.Show();