C# 单击按钮时，下拉列表将丢失其值

我有两个下拉列表。第一个是州，第二个是城市。当我选择该州时，其各自的城市将在下一个下拉列表中填充。我有一个选项可以通过点击按钮上的文本框添加更多城市，同时从城市下拉列表中选择另一个选项

问题是，每当我从文本框添加城市时，它就会被插入到相应的状态中，dropdownlist也会丢失其值。我尝试过调试，但找不到确切的问题

请参阅代码以供参考：

protected void btnAddDropDown_Click1(object sender, EventArgs e)
{
    using (SqlConnection con = new SqlConnection(constring))
    {
        con.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.CommandText = "Add_CityforLocation";
        cmd.Parameters.Add("@ID", SqlDbType.VarChar).Value = 0;
        cmd.Parameters.Add("@CountryName", SqlDbType.VarChar).Value = "India";
        cmd.Parameters.Add("@CityName", SqlDbType.VarChar).Value = txtOtherCity.Text.Trim();
        cmd.Parameters.Add("@StateName", SqlDbType.VarChar).Value = ddllocation1.SelectedItem.ToString();
        cmd.Connection = con;
        try
        {
            cmd.ExecuteNonQuery();
            BindContrydropdown();
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message);//You Can Haave Messagebox here
        }
        finally
        {
            con.Close();
        }
    }
}

protected void BindContrydropdown()
{
    //conenction path for database
    //string connection = WebConfigurationManager.ConnectionStrings["myconn"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constring))
    {
        con.Open();
        SqlCommand cmd = new SqlCommand("Select Id,CityName From Career.Location", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        ddllocation1.DataSource = ds;
        ddllocation1.DataTextField = "CityName";
        ddllocation1.DataValueField = "Id";
        ddllocation1.DataBind();
        ddllocation1.Items.Insert(0, new ListItem("--Select--", "0"));
        ddllocation1.Items.Insert(1, new ListItem("--OTHER--", "0"));
        con.Close();
    }
}

此外，请参见HTML以了解相同的内容：-

 <tr>
        <td class="td">Location/State</td>
        <td>
            <asp:DropDownList CssClass="txtfld-popup" ID="ddllocation1" OnSelectedIndexChanged="ddllocation1_SelectedIndexChanged" runat="server" AutoPostBack="true"></asp:DropDownList>
            <asp:RequiredFieldValidator CssClass="error_msg" ID="RequiredFieldValidator1" ControlToValidate="ddllocation1" runat="server" ErrorMessage="Please enter location" InitialValue="--Select--" SetFocusOnError="true"></asp:RequiredFieldValidator>
        </td>
    </tr>
    <tr>
        <td class="td">Location/City</td>
        <td>
            <asp:DropDownList CssClass="txtfld-popup" ID="ddlLocation" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlLocation_SelectedIndexChanged"></asp:DropDownList>
            <asp:RequiredFieldValidator CssClass="error_msg" ID="reqLocation" ControlToValidate="ddlLocation" runat="server" ErrorMessage="Please enter location" InitialValue="--Select--" SetFocusOnError="true"></asp:RequiredFieldValidator>
        </td>
    </tr>
    <tr>
        <td>
            <asp:TextBox ID="txtOtherCity" runat="server" Visible="false" CssClass="txtfld-popup"></asp:TextBox>
            &nbsp;&nbsp;&nbsp;
                    <asp:Button ID="btnAddDropDown" runat="server" Width="63" Text="Add" CausesValidation="false" OnClick="btnAddDropDown_Click1" />
        </td>
    </tr>

试试这个

try
{

    if(!IsPostBack)
    {
        cmd.ExecuteNonQuery();
        BindContrydropdown();
    }

}

---

您需要记住回发会清除值。现在值不会被插入到城市表中。为什么？还有别的吗。请看我的js小提琴以供参考。每当我将bindcontroldropdown添加到pageload时，它都会给出“System.Data.DataRowView”不包含名为“StateName”的属性。此外，该值未插入到CityDropDown中。很抱歉响应延迟。发生的情况是回发刷新值，请尝试将其放入updatepanel中，这样不会导致完全回发