DropDownlist PostBack 丢失了选择的值

Question

我有一个从文本文件加载所有列表项的下拉列表，它包含打印机名称、描述、IP 地址和连接字符串 ID 的列表。所以我向用户显示打印机名称描述，当用户选择打印机时，我传递的值是 ip 地址或连接字符串，具体取决于情况。

 protected void Page_Load(object sender, EventArgs e)
  {
   LoadPrinterList();
  }
protected void LoadPrinterList()
 {
 string CSVFilePathName = 
 System.Configuration.ConfigurationManager.AppSettings["FilePath"];
 string[] Lines = File.ReadAllLines(CSVFilePathName);
 string[] Fields;
 Fields = Lines[0].Split(new char[] { ',' });
 int Cols = Fields.GetLength(0);
 DataTable dt = new DataTable();
 //1st row must be column names; 
 //force lower case to ensure matching later on.
 for (int i = 0; i < Cols; i++)
 dt.Columns.Add(Fields[i].ToLower(), typeof(string));
 dt.Columns.Add("nameanddescription", 
 typeof(string), "name +'-'+ description");
 dt.Columns.Add("ipandconnectionstring", 
 typeof(string), "ip +'-'+ ConncetionStringID");
 DataRow Row;
 for (int i = 1; i < Lines.GetLength(0); i++)
 {
  Fields = Lines[i].Split(new char[] { ',' });
  Row = dt.NewRow();
  for (int f = 0; f < Cols; f++)
  Row[f] = Fields[f];
  dt.Rows.Add(Row);
  }
   string hostname = Request.UserHostName.Substring(0, 3);
   string[] name = Printerlist.SelectedValue.Split('-');
   //string plant = name[0].ToString();
   //string plantvalue = plant.Substring(0, 3);
   //if(hostname == plantvalue)
   //{
   Printerlist.DataTextField = "nameanddescription";
   Printerlist.DataValueField = "ipandconnectionstring";
   //}
   Printerlist.DataSource = dt;
   Printerlist.DataBind();
}

What the user sees first as an option for the printer list

当用户单击下拉菜单时，他们会看到：

when drop down is clicked

所以现在用户选择了一台打印机，所以选择的索引 > 0 所以基于此我在后面的代码中执行以下操作。

protected void Printerlist_SelectedIndexChanged(object sender, EventArgs e)
{
  if (!Page.IsPostBack)
   {
   }
  else
  {

   if (Printerlist.SelectedItem.Text.Length > 0)
    {
    TxtItem.Focus();
    }
   else
    {
    TxtItem.Text = string.Empty;
    TxtQty.Text = string.Empty;
    DropDownList2.SelectedIndex = 0;
    lbldesc.Visible = false;
    //TxtBase.Text = string.Empty;
    //TxtBase1.Text = string.Empty;
    bestbeforewillbe.Text = string.Empty;
    TxtBestBeforeMonths.Text = string.Empty;
    TxtRotcode.Text = string.Empty;
    zplcode.Text = string.Empty;
    string message = "The selected printer is
    not a local printer";
    System.Text.StringBuilder sb = 
    new System.Text.StringBuilder();
    sb.Append("<script type = 'text/javascript'>");
    sb.Append("window.onload=function(){");
    sb.Append("alert('");
    sb.Append(message);
    sb.Append("')};");
    sb.Append("</script>");
   ClientScript.RegisterClientScriptBlock
   (this.GetType(), "alert", sb.ToString());
  }

 }

}

这是我的 aspx 页面中的下拉列表值

<asp:DropDownList ID="Printerlist" runat="server"
 Height="16px" Width="268px" AutoPostBack="True" OnSelectedIndexChanged="Printerlist_SelectedIndexChanged" OnTextChanged="Printerlist_TextChanged" ViewStateMode="Enabled"></asp:DropDownList>

我有 EnableViewState=true；但这也无济于事。

请帮助我似乎无法弄清楚，每次用户选择一个值时都会有一个回发，回发后“-”被选为打印机值。

Answer 1

只需在页面加载时检查 ISPOSTBACK

protected void Page_Load(object sender, EventArgs e)
{
  if(!IsPostBack) 
   LoadPrinterList();
}

Answer 2

您需要明确检查是否发生了 PostBack，否则只要您的 Page_Load 方法被触发，它就会用它们的初始数据重新填充您的控件（即您正在丢失您的更改那里）。

您只需添加一个 if 语句，并且仅在初始加载时才执行您的 LoadPrinterList() 方法：

protected void Page_Load(object sender, EventArgs e)
{
   if(!IsPostBack)
   {
        // Only perform this on the initial load
        LoadPrinterList();
   }
}

此外，您不需要在 Printerlist_SelectedIndexChanged 事件中进行类似的检查，因为该事件将从页面内触发（即在它已经加载之后），因此该语句永远不应该评估为真。

DropDownlist PostBack 丢失了选择的值

DropDownlist PostBack Looses Selected Value

c#

asp.net

postback

text-files