public class TestForm : Form
        {

            const int SW_SHOWNOACTIVATE = 4;
            [System.Runtime.InteropServices.DllImport("user32.dll")]
            static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

            protected override void OnCreateControl()
            {
                base.OnCreateControl();
                // 在这里，可以增加其他标识来标识
                if(this.Tag != null)
                {
                    (this.Tag as Form).TopMost = false;
                    this.Tag = null;
                }

            }

            private void NewWindow(bool focused)
            {
                var f = new TestForm();

                if(focused) f.Show();
                else
                {
                    this.TopMost = true;
                    f.Tag = this;
                    ShowWindow(f.Handle, SW_SHOWNOACTIVATE);
                }
            }
        }