Asp 下拉列表不返回選定的值。 （下拉列表是數據綁定的）

Question

我創建了一個下拉列表，它從表列加載數據。 現在我想在Index_change_event上選擇 dropdownlist 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;
    }
}

Answer 1

從DropDownList1_SelectedIndexChanged事件中刪除if (!IsPostBack)和if (!IsPostBack)應該在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();
     }
}

Answer 2

您只是在檢查 !IsPostBack。 由於事件是從回發中觸發的，因此永遠不會運行。 此外，請注意不要重新綁定數據源並因此更改 page_load 上的選定值。