public partial class _Default : System.Web.UI.Page 
{
    protected Control ctl;
    string str1;
    protected void Page_Load(object sender, EventArgs e)
    {
        if(IsPostBack)   
        {
            if (Session["System"] != null)
                str1 = Session["System"].ToString();
            if (ViewState["isLoadControl"] != null && ((bool)ViewState["isLoadControl"]))//再次加载
              {
                this.ctl = this.LoadControl(str1);//加载控件
                  this.ctl.ID = "TEST";
                this.PlaceHolder1.Controls.Add(this.ctl);   //在页面上呈现   
              }
             
         }   

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        BlindHolder("test/test_2.ascx");
    }

    public void BlindHolder(string str)
    {
        PlaceHolder1.Controls.Clear();
        if (this.PlaceHolder1.Controls.Count == 0)   //第一次加载 
        {
            this.ctl = this.LoadControl(str);
            this.ctl.ID = "TEST";
            this.PlaceHolder1.Controls.Add(this.ctl);
        }      
        ViewState["isLoadControl"] = true;
        Session["System"] = str;
    }
}

public partial class test_test_2 : System.Web.UI.UserControl
{
    string str;
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Label1.Text = "TEST2测试成功！";
    }
}