Mysql & Vb.net '阅读器关闭时尝试读取无效'

Question

我在从 mysql 数据库收集数据并将其放入 DataView 控件时遇到此错误...这是我的代码：

    Private Function PopulateActivity()
    Dim loginStatement As String = "SELECT * FROM activity WHERE id = @userid"

    Dim cmd As MySqlCommand = New MySqlCommand(loginStatement, mainconn)
    cmd.Parameters.AddWithValue("@userid", LoggedInUser.ID)

    Dim drMyAcount As MySqlDataReader = cmd.ExecuteReader()

    Dim rowCount As Integer = 0
    Dim rowAmount As Integer = 0
    'gets count of rows returned from mysql query'
    Using dt As New DataTable
        dt.Load(drMyAcount)
        rowAmount = dt.Rows.Count
    End Using
    'adds an entry for each item returned from the mysql query'
    Do While rowCount < rowAmount
        drMyAcount.Read() 'HERE IS WHERE ERROR OCCURS'
        Dim tempDateTime As String = drMyAcount.Item("dateTime")
        Dim tempInfo As String = drMyAcount.Item("info")
        Dim tempBalChanges As String = drMyAcount.Item("balChange")
        Dim tempToFrom As String = drMyAcount.Item("toFrom")
        ActivityView.Rows.Add(tempDateTime, tempInfo, tempBalChanges, tempToFrom)
        rowCount = rowCount + 1
    Loop
    drMyAcount.Close()
    Return 0
    End Function

我不知道为什么会这样，但它给了我一个“阅读器关闭时尝试阅读无效”的错误：

drMyAccount.Read()

行...

我将不胜感激有关此主题的任何帮助！非常感谢...

Answer 1

取出 dt.Load() ，并在使用数据读取器之前计算行数。 DataReader 有一个内置属性.HasRows

if (drMyAcount.HasRows)            
    while (drMyAcount.Read())            
         Dim tempDateTime As String = drMyAcount.Item("dateTime")
         Dim tempInfo As String = drMyAcount.Item("info")
         Dim tempBalChanges As String = drMyAcount.Item("balChange")
         Dim tempToFrom As String = drMyAcount.Item("toFrom")
         ActivityView.Rows.Add(tempDateTime, tempInfo, tempBalChanges, tempToFrom)
         rowCount = rowCount + 1 //you can still count rows in the loop
    Loop

Answer 2

MSDN documentation 似乎没有指定，但显然DataTable.Load 方法在完成将数据加载到表中时会关闭给定的IDataReader。所以，一旦你调用dt.Load(drMyAcount)，drMyAcount 就会被关闭。