下拉列表选择的值没有被选中

Question

这里发生的事情是，每当我从dropdownlist 中选择一个值时，页面就会得到回发，然后它又回到了“--- Select ---”强>。

我尝试了链接here，但没有成功

if (DtMainA.Rows.Count > 0)
{
    DataTable DtGoodType = new DataTable();
    ObjPriDaAdp          = new OracleDataAdapter("select mkey, vehicle_no from XXCUS.XXGID_AUDIT_ENTRY where mkey= '" + Hid_MKey.Value + "'", ObjPriCon);
    ObjPriDaAdp.Fill(DtGoodType);

    if (DtGoodType.Rows.Count > 0)
    {
        cmdVehicleNo.DataTextField = "vehicle_no";
        cmdVehicleNo.DataValueField = "mkey";
        cmdVehicleNo.DataSource = DtGoodType;
        cmdVehicleNo.DataBind();
        cmdVehicleNo.Items.Insert(0, new ListItem("--- Select ---", "0"));
   }
}

另见我的 aspx:-

<asp:DropDownList ID="cmdVehicleNo" runat="server" Style="width: 50%" AutoPostBack="true"
     OnSelectedIndexChanged="cmdVehicleNo_SelectedIndexChanged">
    <asp:ListItem Value="0" Selected="True" >--- Select ---</asp:ListItem>
</asp:DropDownList>

更新

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        if (ObjPriCon.State == System.Data.ConnectionState.Closed)
        {
            ObjPriCon.Open();
        }
        if (hidAttachid.Value == "")
        {
            hidAttachid.Value = "0";
        }

        ObjPriCmd = new OracleCommand("Select count(MKEY)cntrow from XXCUS.XXGID_AUDIT_ENTRY", ObjPriCon);
        FunFillData();
        ObjPriCon.Close();
        fillProject();
        fillGoodType();
    }
}


protected void CmbProject_SelectedIndexChanged(object sender, EventArgs e)
{
    //  ClientScript.RegisterStartupScript(this.GetType(), "SuccessScript", "window.location.href='FrmCentral_Monitoring_Entry_OUT.aspx?userid=" + Hid_userid.Value + "';", true);
    if (CmbProject.SelectedIndex > 0)
    {
        DtCombo = FillComboBox("select mkey, vehicle_no from XXCUS.XXGID_AUDIT_ENTRY where time_out IS NULL and Project_id='" + CmbProject.SelectedValue + "'");

        cmdVehicleNo.DataTextField = "vehicle_no";
        cmdVehicleNo.DataValueField = "mkey";
        cmdVehicleNo.DataSource = DtCombo;
        cmdVehicleNo.DataBind();
        cmdVehicleNo.Items.Insert(0, new ListItem("--- Select ---", "0"));
        if (DtCombo.Rows.Count == 1)
        {
            cmdVehicleNo.SelectedIndex = 1;
        }
        DtCombo.Clear();
    }
    Funclear();
}

Answer 1

当前代码的一个非常明显的问题是，每当您从下拉列表中选择任何项目时，它都会执行 PostBack 并触发 CmbProject_SelectedIndexChanged 事件。在这种情况下，我不知道您为什么再次填充下拉列表。理想情况下，您应该在初始页面加载时只绑定一次下拉列表，如下所示：-

if (!IsPostBack)
{
   BindDropDownList(); //Call this method
}

//Write a separate method to bind the dropdown
private void BindDropDownList()
{
    DataTable DtGoodType = new DataTable();
    ObjPriDaAdp = new OracleDataAdapter("select mkey, vehicle_no from XXCUS.XXGID_AUDIT_ENTRY where mkey= '" + Hid_MKey.Value + "'", ObjPriCon);
    ObjPriDaAdp.Fill(DtGoodType);

    if (DtGoodType.Rows.Count > 0)
    {
       cmdVehicleNo.DataTextField = "vehicle_no";
       cmdVehicleNo.DataValueField = "mkey";
       cmdVehicleNo.DataSource = DtGoodType;
       cmdVehicleNo.DataBind();
       cmdVehicleNo.Items.Insert(0, new ListItem("--- Select ---", "0"));
   }
}

最后，在选定的选项更改事件中，您应该只编写用户更改选择时要执行的操作的逻辑：-

protected void CmbProject_SelectedIndexChanged(object sender, EventArgs e)
{
   //Do Stuffs
}