变量'ddl'在赋值之前使用 [英] variable 'ddl' is used before it has been assigned a value

问题描述

大家好，



有人可以告诉我如何或者我需要分配给varicale'ddl'，因为我在获取此错误后变得有点卡住'变量'ddl'在被赋值之前被使用'



我试图使用查询字符串从数据库填充我的下拉列表。

 如果 Request.QueryString（  userid）<>   然后 
 
  Dim  ddlResult  As  DataSet 
 ddlResult = GetUserName.ddlData（Request.QueryString（< span class =code-string>  userid）。ToString）
 
  Dim  ddl  As  DropDownList 
 
 对于 每次遇到在 ddlResult.Tables（ 0 ）。行
 
  Dim  ddlString  As   String  = encounter.Item（  description）。ToString 
 
  Dim  newListItem  As  ListItem 
 newListItem = 新 ListItem（  description）
 ddl.Items.Add（newListItem）
 
 下一步 
 
 结束 如果 

谢谢



我尝试添加更多代码，如下所示，但DDL没有出现在这页纸。我不能直接将DDL添加到页面，因为由于某种原因它停止按钮工作我用来打开页面。

 如果 Request.QueryString（  userid ）<>   然后 
 
  Dim  ddl  As  DropDownList = 新 DropDownList 
 
  Dim  ddlResult  As  DataSet 
 ddlResult = GetUserName。 ddlData（Request.QueryString（  userid）。ToString）
 
 < span class =code-keyword> Dim  dv  As  新 HtmlGenericControl（  div）
 dv.ID =   sysData 
 
 对于 每次遇到在 ddlResult.Tables（ 0 ）。行
 
  Dim  ddlString 作为 字符串 = encounter.Item（   description，  description）。ToString 
 
  Dim  newListItem  As  ListItem 
 newListItem = 新 ListItem（  description，   description）
 
 
 ddl.Items.Add（ newListItem）
 dv.Controls.Add（ddl）
 sysData.Controls.Add（dv）
 
  Next  
 
 结束 如果 

解决方案

< blockquote>实例化ddl

 Dim ddl As New DropDownList 

你所做的就是创建一个变量 DDL 。您需要先为该变量指定一个值，然后才能向其中添加项目。

例如

  Dim  ddl  As  DropDownList = this.Page.Master.FindControl（  YourDropDownList）

如果这是WinForm，那么

  Dim  ddl 作为 DropDownList = this.YourDropDownList 

（你需要替换YourDropDownList的页面/表单上的下拉列表的ID，或者您可以使用

  Dim  ddl 作为 DropDownList = 新 DropDownList（）

- 但是没有人会看到你的列表

更改

  Dim  ddl 作为 DropDownList 

to

  Dim  ddl 正如 新 DropDownList 

使用New关键字创建DropDownList的新实例



如果您已将DropDownList添加到您的aspx页面，如下所示

  <   asp：DropDownList     ID   =  DropDownList1    runat   =  server    /  >  

那么你不需要创建新的。通过ID访问它，示例代码

 对于 每个遇见在 ddlResult.Tables（ 0 ）。行
 
 < span class =code-keyword> Dim  ddlString  As   String  = encounter.Item（   description）。ToString 
 
  Dim  newListItem  As  ListItem 
 newListItem = 新 ListItem（  description）
 DropDownList1.Items.Add（newListItem）
 
 < span class =code-keyword>下一步 

Hi All,

Can someone please advise me how or what i need to assign to the varicale 'ddl' as i a little stuck after getting this error 'variable 'ddl' is used before it has been assigned a value'

I am trying to populate my dropdown with data from database using the querystring.

If Request.QueryString("userid") <> "" Then

         Dim ddlResult As DataSet
         ddlResult = GetUserName.ddlData(Request.QueryString("userid").ToString)

         Dim ddl As DropDownList

         For Each encounter In ddlResult.Tables(0).Rows

             Dim ddlString As String = encounter.Item("description").ToString

             Dim newListItem As ListItem
             newListItem = New ListItem("description")
             ddl.Items.Add(newListItem)

         Next

     End If

thank you

I have tried adding a little bit more code as shown below but the DDL is not showing up on the page. I cant add the DDL direct to the page because for some reason it stops the button from working i use to open the page.

If Request.QueryString("userid") <> "" Then

          Dim ddl As DropDownList = New DropDownList

          Dim ddlResult As DataSet
          ddlResult = GetUserName.ddlData(Request.QueryString("userid").ToString)

          Dim dv As New HtmlGenericControl("div")
          dv.ID = "sysData"

          For Each encounter In ddlResult.Tables(0).Rows

              Dim ddlString As String = encounter.Item("description", "description").ToString

              Dim newListItem As ListItem
              newListItem = New ListItem("description", "description")


              ddl.Items.Add(newListItem)
              dv.Controls.Add(ddl)
              sysData.Controls.Add(dv)

          Next

      End If

解决方案

Instantiate the ddl

Dim ddl As New DropDownList

All you have done is create a variable ddl. You need to assign a value to that variable before you can add items to it.
E.g.

Dim ddl As DropDownList = this.Page.Master.FindControl("YourDropDownList")

If this is WinForm then

Dim ddl As DropDownList = this.YourDropDownList

(you need to substitute the id of the drop down list on your page/form for "YourDropDownList") or you could use

Dim ddl As DropDownList = New DropDownList()

- but no-one is going to see your list

change

Dim ddl As DropDownList

to

Dim ddl As New DropDownList

use the New keyword to create a new instance of DropDownList

if you already added DropDownList to your aspx page like below

<asp:DropDownList ID="DropDownList1" runat="server" />

then you don't need to create new one. access it by ID, sample code

For Each encounter In ddlResult.Tables(0).Rows

          Dim ddlString As String = encounter.Item("description").ToString

          Dim newListItem As ListItem
          newListItem = New ListItem("description")
          DropDownList1.Items.Add(newListItem)

      Next

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