C#动态添加控件到窗体不显示

在FormLoad里面添加控件时，用this.Controls.Add()方法向窗体里面动态添加控件，调试，一直不显示，后来发现原来窗体上有一个groupbox覆盖了整个窗体，其实控件已经添加上去了，只是被遮盖了。

后面改为this.groupBox1.Controls.Add()，把控件添加到groupbox里面就不会被遮盖了

 1 private void UniqueForm_Load(object sender, EventArgs e)
 2         {
 3             if (pFlag == 1)
 4             {
 5                 Label lbllevel = new Label();
 6                 lbllevel.Text = "请设置级数：";
 7                 lbllevel.Location = new System.Drawing.Point(38, 95);
 8                 lbllevel.AutoSize = true;
 9                 lbllevel.Visible = true;
10                 txtLevel = new TextBox();
11                 txtLevel.Name = "txtLevel";
12                 txtLevel.Location = new System.Drawing.Point(121, 92);
13                 txtLevel.Height = 21;
14                 txtLevel.Width = 150;
15                 txtLevel.Visible = true;
16                 this.groupBox1.Controls.Add(lbllevel);
17                 this.groupBox1.Controls.Add(txtLevel);
18             }
19 }

C#动态添加控件到窗体不显示