回发后占位符丢失数据

Question

我正在将数据库表中的 ID 号列表读取到占位符文本框中，但是；如果我单击按钮，则数据将被删除。

 protected void btnSearch_Click(object sender, EventArgs e)
    {

     while (myReader.Read())
        {


            TextBox txt = new TextBox();
            txt.Text = (string)myReader["idNumber"];
            txt.ID = "txt" + i;
            txt.ReadOnly = true;
            ContentPlaceHolder1.Controls.Add(txt);
            ContentPlaceHolder1.Controls.Add(new LiteralControl("     "));

            i++;
        }
}

Answer 1

这是在 Web 表单中使用动态添加的控件时的常见问题（特别是如果您来自 winforms 背景）。 ASP.NET Web 窗体中的页面是无状态的，并且在每次回发时都会重新构建。因此，如果您在服务器事件期间将控件添加到页面，如果您希望它出现，您还必须在后续页面加载时将其添加到页面。您可以使用类似于以下内容的方法来完成此操作：

protected List<Control> ControlCache
{
    get => (List<Control>)(Session["cachedControlsForPageX"] = (Session["cachedControlsForPageX"] as List<Control>) ?? new List<Control>());
    set => Session["cachedControlsForPageX"] = value;
}

/* If you can't use C# 7's expression bodied property accessors, here's the equivalent in blocks:
protected List<Control> ControlCache
{
    get { return (List<Control>)(Session["cachedControlsForPageX"] = (Session["cachedControlsForPageX"] as List<Control>) ?? new List<Control>()); }
    set { Session["cachedControlsForPageX"] = value; }
}
*/

protected void Page_Load(object sender, EventArgs e)
{
    if (Page.IsPostBack)
    {
        foreach (var control in ControlCache)
        {
            ContentPlaceHolder1.Controls.Add(control);
            ContentPlaceHolder1.Controls.Add(new LiteralControl("&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;"));
        }
    }
    else
        ControlCache = null;
}


protected void btnSearch_Click(object sender, EventArgs e)
{
    while (myReader.Read())
    {
        TextBox txt = new TextBox();
        txt.Text = (string)myReader["idNumber"];
        txt.ID = "txt" + i;
        txt.ReadOnly = true;
        ContentPlaceHolder1.Controls.Add(txt);
        ContentPlaceHolder1.Controls.Add(new LiteralControl("&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;"));
        ControlCache.Add(txt);
        i++;
    }
}