选择Option Change DropdownList C#ASP.NET Fire Event

Question

我在asp.net中有一个下拉列表当我点击一个选项时,我应该选择值,然后将其传递给数据库,然后使用查询结果填充第二个下拉列表.

当我点击第一个下拉菜单时,我似乎无法"触发"此事件.以下是我所拥有的:

ASPX代码

<td class="style3">
                <asp:DropDownList ID="Currencies" runat="server" 
                    onselectedindexchanged="Currencies_SelectedIndexChanged">
                </asp:DropDownList>
            </td>
            <td>
                &nbsp;</td>
        </tr>
        <tr>
            <td class="style2">
                Payment Mode</td>
            <td class="style3">
                <asp:DropDownList ID="PaymentModes" runat="server">
                </asp:DropDownList>
            </td> 

CodeBehind代码C#

String conn = WebConfigurationManager.ConnectionStrings["pvconn"].ToString();
    protected void Page_Load(object sender, EventArgs e)
    {
        populatecurrencylist();

    }

    public void populatecurrencylist() 
    {
        SqlCommand sql = new SqlCommand("SELECT * FROM CURRENCIES_TBL ORDER BY Currency_Initials",new SqlConnection(conn));
        sql.Connection.Open();
        SqlDataReader listcurrencies;
        listcurrencies = sql.ExecuteReader();
        Currencies.DataSource = listcurrencies;
        Currencies.DataTextField = "Currency_Initials";
        Currencies.DataValueField = "Currency_Group_ID";
        Currencies.DataBind();
        sql.Connection.Close();
        sql.Connection.Dispose();
    }

    protected void TextBox1_TextChanged(object sender, EventArgs e)
    {

    }
    protected void Currencies_SelectedIndexChanged(object sender, EventArgs e)
    {
        var currid = Currencies.SelectedValue;
        HttpContext.Current.Response.Write(currid);
        //int currid = 0;
        try
        {
            SqlCommand sql = new SqlCommand("SELECT * FROM PAYMENT_MODES_TBL WHERE Currency_ID = @currencyid", new SqlConnection(conn));
            SqlParameter param0 = new SqlParameter();
            param0.ParameterName = "@currencyid";
            param0.SqlDbType = System.Data.SqlDbType.Int;
            param0.Value = currid;

            sql.Parameters.Add(param0);
            sql.Connection.Open();
            SqlDataReader listpaymodes;
            listpaymodes = sql.ExecuteReader();
            PaymentModes.DataSource = listpaymodes;
            PaymentModes.DataTextField = "Payment_Mode";
            PaymentModes.DataValueField = "Paying_Account_Code";
            PaymentModes.DataBind();
            sql.Connection.Close();
            sql.Connection.Dispose();
        }
        catch(Exception s)
        {
            HttpContext.Current.Response.Write("Error Occured " + s.Message);
        }            
    } 

我可以毫无问题地填充第一个下拉列表.第二个似乎不起作用.ASP.NET中的新功能.我来自PHP背景,可以使用jquery ajax轻松实现,但我想学习C#.

任何帮助赞赏.

编辑

所有的答案都表明我将货币下拉列表设为AutoPostBack = true 我已经完成了:

<asp:DropDownList ID="Currencies" runat="server" AutoPostBack="True" 
                    onselectedindexchanged="Currencies_SelectedIndexChanged">
                </asp:DropDownList> 

但它似乎仍无法奏效.另外,页面重新加载,我的选择菜单选项重置为第一个选项.

Answer 1

更改

<asp:DropDownList ID="Currencies" runat="server" 
                    onselectedindexchanged="Currencies_SelectedIndexChanged">
                </asp:DropDownList>

至

<asp:DropDownList ID="Currencies" runat="server" AutoPostBack="True"
                    onselectedindexchanged="Currencies_SelectedIndexChanged">
                </asp:DropDownList>

UPDATE

更新后更新您的问题.

改变这个:

protected void Page_Load(object sender, EventArgs e)
    {
        populatecurrencylist();

    }

对此:

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

    }