在page_Load中动态填充dropdownlist [英] dynamically populating dropdownlist in page_Load

问题描述

我有2个下拉列表-

<asp:DropDownList ID="stateList" runat="server" Width ="170px"

                CssClass ="text" AutoPostBack="True"  OnSelectedIndexChanged ="stateList_SelectedIndexChanged">
            <asp:ListItem Value ="Andhra Pradesh">Andhra Pradesh</asp:ListItem>
            <asp:ListItem Value ="Arunachal Pradesh">Arunachal Pradesh</asp:ListItem>
<asp:ListItem Value ="Assam">Assam</asp:ListItem>
<asp:ListItem Value="Bihar">Bihar</asp:ListItem>
<asp:ListItem Value ="Chattisgarh">Chhattisgarh</asp:ListItem>
<asp:ListItem Value ="Goa">Goa</asp:ListItem>
<asp:ListItem Value="Gujarat">Gujarat</asp:ListItem>
<asp:ListItem Value ="Haryana">Haryana</asp:ListItem>

</asp:DropDownList>

</asp:DropDownList>

<asp:DropDownList ID="cityList" runat="server" Width ="135px" CssClass ="text" >
            </asp:DropDownList>

数据库中包含州和城市值.在Page_Load中，需要填充cityList，并且需要选择指定的城市.现在，正在填充cityList，但是当我给出
时

State and City values are there in the database. In the Page_Load the cityList needs to be populated and the specified city needs to be selected. Now, the cityList is being populated ,but when i give

cityList .Items .FindByText( cityname).Selected =true ;

它返回未设置为对象实例的对象引用.此代码有什么问题?
我已经在里面给出了这段代码

it returns object reference not set to an instance of an object. What is wrong with this code?
I have given this code inside

if(!IsPostBack )
   {}

在此先感谢

默认情况下未选择任何内容.我已通过以下方式填充cityList

Thanks in advance

Nothing is selected by default. I have populated the cityList in the following way

cityList.Items.Add("Agra");
            cityList.Items.Add("Aligarh");

推荐答案

您是否尝试过使用DEBBUGER?在该行上放置一个断点，并检查将变为NULL的情况.当您尝试访问空对象的某些属性时，将发生对象引用错误.

可能是:cityList.Items.FindByText( cityname)为NULL，并且继续尝试使用它的Selected属性.
此外，如果没有的话，这行代码是postback =>.首次加载页面.页面加载时默认情况下是否选择任何内容?我对此表示怀疑，这很可能就是没有得到选定值而导致错误的原因.

Did you tried to use DEBBUGER? Put a break point on this line and check what comes to be NULL. Object reference error happens when you try to access some property of a null object.

Might be: cityList.Items.FindByText( cityname) is NULL and you keep on trying a Selected property of it.
Further, this line of code is if not postback => page load for first time. Is anything selected by default when page loads? I doubt, so most probably thats the reason of not getting any value as selected and thus an error.

cityList.ClearSelection();
cityList .Items .FindByText(cityname).Selected = true;

尝试这个希望能奏效的

谢谢，

阿米特(Amit)

cityList.ClearSelection();
cityList .Items .FindByText( cityname).Selected =true ;

try this hope this will work

thanks,

amit

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