DropDownList的值没取到！？

Question

    下面这段代码中有三个DropDownList，分别是DropDownList_City、dropDownList_Road、DropDownList_Address，分别代表着城市、路段、门牌号。

    这段代码放在 protected void DropDownList_Address_SelectedIndexChanged(object sender, EventArgs e) 中正常运行，可是放在protected void Page_Load(object sender, EventArgs e)、 DropDownList_City_SelectedIndexChanged、 DropDownList_Road_SelectedIndexChanged中运行，会出现如图1所示错误。

    我想，这是因为页面刚打开，DropDownList的初始值没有被selected到。该如何修改呢？

---------------------------代码1--------------------------

public void BindLable_TypeOfPayment()
    {
        string strconn = "Data Source=.;Initial Catalog=长铁物业公司资产数据库;Integrated Security=True";
        SqlConnection cn = new SqlConnection(strconn);
        cn.Open();
        string typeofpaymentTop = "select top (1) TypeOfPayment from ContractTable where City='";
        string typeofpaymentSearch = DropDownList_City.SelectedItem.Value.ToString() + "' and Road ='" + DropDownList_Road.SelectedItem.Value.ToString() + "' and [Address] ='" + DropDownList_Address.SelectedItem.Value.ToString() + "' and ContractType ='租赁' Order by LeaseDateTo DESC";
        typeofpaymentTop = typeofpaymentTop + typeofpaymentSearch;
        SqlCommand cm = new SqlCommand(typeofpaymentTop, cn);
        SqlDataReader dr = cm.ExecuteReader();
        dr.Read();
        
        if (dr["TypeOfPayment"].ToString() == "")
            Label_TypeOfPayment.Text = "首次启用，无前期租赁方式";
        else
            Label_TypeOfPayment.Text = dr["TypeOfPayment"].ToString();
        
        cn.Close();

    }

---------------------------图1-----------------------------

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C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！

Answer 1

你好，你的估計是對的。從那錯誤信息來看，應該是那幾個DropDownList還有值，或還沒有SelectedItem。

你可以先用
If (null != DropDownList_City.SelectedItem)

如果可以通過才去取它的值吧。

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Answer 2

    我发现这个错误就是在于如果DropDownList如果没有经过点击选取，就不会有初始值，（尽管每个DropDownList显示出来了City、Road、Address），所以当页面一打开的时候，下面这段代码是不会显示结果的，必须等每个DropDownList重新点击选择后才能开始工作。

    有什么好办法可以把初始值也正确显示么？

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C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！

Answer 3

你的Dropdowlist请尝试为其中的某个ListView设置Selected=True：

<asp:DropDownList ID="id" runat="server">
      <asp:ListItem Text="a" Selected="True" Value="a"></asp:ListItem>
  </asp:DropDownList>

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Answer 4

    我的情况不一样啊，我的DropDownList的数据源是绑定了SQLDataSource的，没有ListItem啊！

<asp:DropDownList ID="DropDownList_City" runat="server" 
                    AutoPostBack="True" DataSourceID="SqlDataSource_City" DataTextField="City" 
                    DataValueField="City" Width="80px">
                </asp:DropDownList>


<asp:SqlDataSource ID="SqlDataSource_City" runat="server" 
                    ConnectionString="<%$ ConnectionStrings:长铁物业公司资产数据库ConnectionString %>" 
                    
                    SelectCommand="SELECT DISTINCT a.City FROM ContractTable AS a INNER JOIN PropertyTable AS b ON a.City + a.Road + a.Address = b.City + b.Road + b.Address WHERE (b.PropertyState = '1') order by city">
                </asp:SqlDataSource>

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C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！

Answer 5

你的下拉列表是在FormView里边还是独立于数据控件（在外边）的？

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Answer 6

    在外边的！就是最简单的网页，网页上放了一些SQLDataSource、DropDownList控件，没有FormView。

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C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！

Answer 7

在Page_Load中：

if(!IsPostBack)
{
      Dropdownlist1.SelectedIndex = 0;
}

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Answer 8

    根据您的提示做了修改，当DropDownList_City改变的时候其他的DropDownList也会有初始值了，可是这个初始值发生了错位，例如：长沙市 对应-车站南路，广州 对应 惠州，可是我Response了一下，发现界面中的DropDownList值和Response值不同步，在Response中显示出来的DropDownList_Road和DropDownList_Address的值，虽然会变，但会慢一步！！！如图，下面有代码！

    总之DropDownList显示的值是对的，而Response出来的值错位了，慢了一拍！

--------------------------------------图1---------------------------------------

----------------------------------代码----------------------------------

protected void DropDownList_City_SelectedIndexChanged(object sender, EventArgs e)
    {
        DropDownList_Road.SelectedIndex = 0;
        DropDownList_Address.SelectedIndex = 0;
        string strconn = "Data Source=.;Initial Catalog=长铁物业公司资产数据库;Integrated Security=True";
        SqlConnection cn = new SqlConnection(strconn);
        cn.Open();
        string typeofpaymentTop = "select top (1) ContractCompany from ContractTable where City='";
        string typeofpaymentSearch = DropDownList_City.SelectedItem.Value.ToString() + "' and Road ='" + DropDownList_Road.SelectedItem .Value.ToString() + "' and [Address] ='" + DropDownList_Address.SelectedItem.Value.ToString() + "' and ContractType ='租赁' Order by LeaseDateTo DESC";
        typeofpaymentTop = typeofpaymentTop + typeofpaymentSearch;
        SqlCommand cm = new SqlCommand(typeofpaymentTop, cn);
        SqlDataReader dr = cm.ExecuteReader();
        dr.Read();
        Response.Write(typeofpaymentTop);

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C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！

Answer 9

那你在Page_Load中设法同步这些数值（同时给三个Dropdownlist的SelectedIndex都赋值为0）。

或者把三个Dropdownlist的AutoPostBack=True，应该会自动更新的。

---

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Answer 10

那你在Page_Load中设法同步这些数值（同时给三个Dropdownlist的SelectedIndex都赋值为0）。

或者把三个Dropdownlist的AutoPostBack=True，应该会自动更新的。

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    我是这样做的啊，可是就是不行。感觉我代码并没有错，而是程序执行时，可能先运行了前台再运行后台，造成的错位！

    是不是不应该在SelectedIndex事件中写啊，是不是要在其他的事件中写？

---

C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！

Answer 11

你前台写了啥？

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Answer 12

没写啥啊！就是下面的代码：（两部分,3个dropDownList是通过3个sqldatasource联动的）

<asp:DropDownList 
                    ID="DropDownList_City" runat="server" 
                    AutoPostBack="True" DataSourceID="SqlDataSource_City" DataTextField="City" 
                    DataValueField="City" Width="80px" 
                    onselectedindexchanged="DropDownList_City_SelectedIndexChanged" 
                    onload="DropDownList_City_Load">
                </asp:DropDownList>
            &nbsp;&nbsp;&nbsp;
                所在路段：<asp:DropDownList ID="DropDownList_Road" runat="server" 
                    AutoPostBack="True" DataSourceID="SqlDataSource_Road" DataTextField="Road" 
                    DataValueField="Road" Width="80px" 
                    onselectedindexchanged="DropDownList_Road_SelectedIndexChanged" >
                </asp:DropDownList>
&nbsp;
            </td>
            <td align="left">
                门牌号：<asp:DropDownList ID="DropDownList_Address" runat="server" AutoPostBack="True" 
                    DataSourceID="SqlDataSource_Address" DataTextField="Address" 
                    DataValueField="Address" 
                    onselectedindexchanged="DropDownList_Address_SelectedIndexChanged">
                </asp:DropDownList>

----------------------------------------------sqldatasource----------------------------------

<asp:SqlDataSource ID="SqlDataSource_City" runat="server" 
                    ConnectionString="<%$ ConnectionStrings:长铁物业公司资产数据库ConnectionString %>" 
                    
                    SelectCommand="SELECT DISTINCT a.City FROM ContractTable AS a INNER JOIN PropertyTable AS b ON a.City + a.Road + a.Address = b.City + b.Road + b.Address WHERE (b.PropertyState = '1') order by city">
                </asp:SqlDataSource>
            </td>
            <td width="600">
                <asp:SqlDataSource ID="SqlDataSource_Road" runat="server" 
                    ConnectionString="<%$ ConnectionStrings:长铁物业公司资产数据库ConnectionString %>" 
                    
                    SelectCommand="SELECT DISTINCT [Road] FROM [PropertyTable] WHERE ([City] = @City) and propertystate='1'">
                    <SelectParameters>
                        <asp:ControlParameter ControlID="DropDownList_City" Name="City" 
                            PropertyName="SelectedValue" Type="String" />
                    </SelectParameters>
                </asp:SqlDataSource>
            </td>
            <td>
                <asp:SqlDataSource ID="SqlDataSource_Address" runat="server" 
                    ConnectionString="<%$ ConnectionStrings:长铁物业公司资产数据库ConnectionString %>" 
                    
                    SelectCommand="SELECT [Address] FROM [PropertyTable] WHERE (([City] = @City) AND ([Road] = @Road)) and propertystate='1'">
                    <SelectParameters>
                        <asp:ControlParameter ControlID="DropDownList_City" Name="City" 
                            PropertyName="SelectedValue" Type="String" />
                        <asp:ControlParameter ControlID="DropDownList_Road" Name="Road" 
                            PropertyName="SelectedValue" Type="String" />
                    </SelectParameters>
                </asp:SqlDataSource>

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C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！

Answer 13

    给sql中union一个“未选择”，整个世界清静了，规避了这个问题！

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C# 菜鸟中的雏鸟！提的问题也许很幼稚，但我是认真的。希望看在党国的面子上拉兄弟一把！