vb.net 收到错误“表达式不产生值
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原文地址: http://stackoverflow.com/questions/22124861/
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Getting the error "Expression does not produce a value
提问by Xero
I am relatively new to using visual basic and I am having a problem populating a list box from a database. The error that comes up is Expression does not produce a value.
我对使用 Visual Basic 比较陌生，并且在从数据库填充列表框时遇到问题。出现的错误是 Expression 没有产生值。
Here is the code from My form:
这是我的表单中的代码：
Private Sub frmMain_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'Populate Blu Ray and DVD listboxes
Dim objMovies As New clsMovies
objMovies.Select_BR_List()
objMovies.Select_Dvd_List()
For Each strBluRay As String In objMovies.Select_BR_List
lstBluRay.Items.Add(strBluRay)
Next
For Each strDVD As String In objMovies.Select_Dvd_List
lstDvd.Items.Add(strDVD)
Next
End Sub
And here's the code from the class:
这是课程中的代码：
Public Sub Select_Dvd_List()
Dim objConnection As New SqlCeConnection(mstrCN)
'Create SQL statement
mstrSQL = "Select * from Dvd"
'Instantiate command
Dim objCommand As New SqlCeCommand(mstrSQL, objConnection)
'open Database
objCommand.Connection.Open()
'Instantiate Data Reader
Dim objDataReader As SqlCeDataReader
'Execute SQL
objDataReader = objCommand.ExecuteReader()
'read Sql results
Do While (objDataReader.Read)
mlstDvd.Add(objDataReader.Item("dvdTitle").ToString)
Loop
'Close
objCommand.Dispose()
objDataReader.Close()
objDataReader.Dispose()
objConnection.Close()
objConnection.Dispose()
End Sub
回答by jmcilhinney
The issue is that you're trying to enumerate a Sub. In VB.NET, methods can either a Sub, which doesn't return a value, or a Function, which does return a value. Your Select_Dvd_Listmethod is a Sub, so it doesn't return a value. You have this code though:
问题是您正在尝试枚举一个 Sub。在 VB.NET 中，方法可以是不返回值的 Sub，也可以是返回值的 Function。您的Select_Dvd_List方法是一个 Sub，因此它不返回值。你有这个代码：
For Each strDVD As String In objMovies.Select_Dvd_List
That is trying to loop through the result of Select_Dvd_Listbut, as we've already established, Select_Dvd_Listhas no result. In that method, you are adding items to mlstDvdso surely that loop should be:
那是试图遍历Select_Dvd_Listbut的结果，正如我们已经确定的那样，Select_Dvd_List没有结果。在该方法中，您将项目添加到mlstDvd如此肯定的循环应该是：
For Each strDVD As String In objMovies.mlstDvd
