Asp Dropdownlist 不返回所选值。（下拉列表为数据绑定）答案

【问题标题】：Asp Dropdownlist does not return selected value. (Dropdownlist is Databound)Asp Dropdownlist 不返回所选值。（下拉列表为数据绑定）
【发布时间】：2015-05-05 07:23:05
【问题描述】：

我创建了一个下拉列表，它从表格列中加载其数据。现在我想在Index_change_event 上选择下拉列表的值。

protected void Page_Load(object sender, EventArgs e)
{
    string username = Session["username"].ToString();
    SqlConnection con = new SqlConnection("Data Source=DLINK\\SQLEXPRESS;User ID=sa;Password=logmein;Initial Catalog=AndroidAppDB");
    SqlCommand cmd = new SqlCommand();
    cmd.Connection = con;
    con.Open();
    string query = "select Fence_Name from Fence where Username='" + username + "'";
    SqlCommand command = new SqlCommand(query, con);
    DropDownList1.DataSource = command.ExecuteReader();
    DropDownList1.DataValueField = "Fence_Name";
    DropDownList1.DataTextField = "Fence_Name";
    DropDownList1.DataBind();
    con.Close();
    //arr = Session["arr"].ToString();        
}

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
       Label2.Text = DropDownList1.SelectedItem.Value;
    }
}

【问题讨论】：

标签： c# sql asp.net data-binding

【解决方案1】：

从DropDownList1_SelectedIndexChanged 事件中删除if (!IsPostBack) 并且if (!IsPostBack) 应该在Page_Load 事件上。

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    Label2.Text = DropDownList1.SelectedItem.Value;
}

protected void Page_Load(object sender, EventArgs e)
{
     if (!IsPostBack)
     {
          string username = Session["username"].ToString();
          SqlConnection con = new SqlConnection("Data Source=DLINK\\SQLEXPRESS;User ID=sa;Password=logmein;Initial Catalog=AndroidAppDB");
          SqlCommand cmd = new SqlCommand();
          cmd.Connection = con;
          con.Open();
          string query = "select Fence_Name from Fence where Username='" + username + "'";
          SqlCommand command = new SqlCommand(query, con);
         DropDownList1.DataSource = command.ExecuteReader();
         DropDownList1.DataValueField = "Fence_Name";
          DropDownList1.DataTextField = "Fence_Name";
        DropDownList1.DataBind();
         con.Close();
     }
}

【讨论】：

【解决方案2】：

您只是在检查 !IsPostBack。由于事件是从回发中触发的，因此永远不会运行。另外，请注意不要重新绑定数据源并因此更改 page_load 上的选定值。

【讨论】：