vb.net 对象变量或未设置块变量，OleDbDataReader答案

【问题标题】：vb.net Object variable or With block variable not set , OleDbDataReadervb.net 对象变量或未设置块变量，OleDbDataReader
【发布时间】：2012-08-08 04:17:29
【问题描述】：

我正在尝试使用 vb.net 编写一个 mdb 到 CVS 转换器。我想使用 oledbdata 阅读器获取列标题名称，这是我目前的代码

 Public Cheadernames As Array
    Public firstRead As Boolean = False
    Public columnCount

    Dim connectionstring = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & strfilename
    Dim queryString = "SELECT * FROM products"
    Dim connection As New OleDbConnection(connectionstring)
    Dim command As New OleDbCommand(queryString, connection)
    connection.Open()
    Dim reader As OleDbDataReader = command.ExecuteReader()
    While reader.Read
        If firstRead Then
            columnCount = reader.FieldCount()
            firstRead = False
            For i = 0 To columnCount - 1
                Cheadernames(i) = reader.GetName(i) ' i am getting an error with this line
                Console.WriteLine(Cheadernames(i))
            Next
        End If

所以我试图将所有列标题名称放在一个数组中，但注释行给了我这个错误“对象变量或未设置块变量”

【问题讨论】：

您的问题是 Cheadernames 返回数组未初始化。您需要首先使用返回数组（应该初始化），然后返回该数组。看看dotnetperls.com/array-vbnet 和返回数组部分。

标签： vb.net

【解决方案1】：

您的 Cheadernames 变量是一个空数组，没有元素也没有类型。那是错误的：您几乎不应该将变量或方法参数声明为“As Array”。相反，你想要这样的东西：

Private HeaderHames() As String
Private firstRead As Boolean = True
Private columnCount As Integer

Dim connectionstring = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" & strfilename
Dim queryString = "SELECT * FROM products"
Using connection As New OleDbConnection(connectionstring), _ 
      command As New OleDbCommand(queryString, connection)

    connection.Open()
    Using reader As OleDbDataReader = command.ExecuteReader()
        While reader.Read
            If firstRead Then
                columnCount = reader.FieldCount()
                HeaderNames = New String(columnCount-1)
                firstRead = False
                For i = 0 To columnCount - 1
                    HeaderNames(i) = reader.GetName(i) 
                    Console.WriteLine(HeaderNames(i))
                Next
            End If
            '...
         End While
     End Using
 End Using

【讨论】：