代码错误 - dbCommand.ExecuteReader()

Question

谁能告诉我如何解决这个错误。 错误：NullReferenceException 未处理 - “未将对象引用设置为对象的实例。” 行： Dim reader As OleDbDataReader = dbCommand.ExecuteReader() 数据库：MS Access IDE：VB 2010 Express

Imports System.Data.OleDb
Imports System.Data.SqlClient
Imports System


Public Class Form1       
Dim dbConnection As OleDbConnection        
Dim dbCommand As OleDbCommand       
Dim strInsert As String       
Dim dbDataAdapter As OleDbDataAdapter        
Dim ConnectString As String = "Provider = Microsoft.Jet.OLEDB.4.0;" & "Data Source =atg.mdb"

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

End Sub


Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

 dbConnection = New OleDbConnection(ConnectString) 'here I use the new keyword to initialize connection  
 dbConnection.Open()
 Dim command As New OleDbCommand("SELECT XX FROM ABC", dbConnection) ' initialize command and pass query and open connection to its constructor  
 Dim reader As OleDbDataReader = dbCommand.ExecuteReader() 'ERROR IS HERE
 Do While (reader.Read()) ' loop here until reader finish reading and add every row to the list box  
        ListBox1.BeginUpdate()
        ListBox1.Items.Add(reader.Item("ABC"))             
        ListBox1.EndUpdate()
    Loop
    reader.Close()
    dbConnection.Close()
End Sub
End Class

Answer 1

在 dbCommand.ExecuteReader() 语句中找到 dbCommand 的位置。请改用 command.ExecuteReader() 。

您没有启动 dbCommand。您只是声明了它。您所做的是创建另一个名为 command 的变量并启动它。所以尝试使用这个

Answer 2

程序的一部分：

Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'MajDgv()
        Module1.connect()
        Me.fillcombo()
End Sub
Sub fillcombo()

        strsql = "select from Liste des responsables"
        Dim acscmd As New OleDb.OleDbCommand
        Dim acsdr As OleDbDataReader
        acscmd.CommandText = strsql
        acsdr = acscmd.ExecuteReader()""THE PROBLEM IS HERRE he sayed that ExecuteReader: Connection property has not been initialized."""""""
        While (acsdr.Read())
            ComboBox1.Items.Add(acsdr("Nom"))

        End While
        acscmd.Dispose()
        acsdr.Close()

    End Sub
End Class

我创建一个模块：

Imports System.Data.OleDb
Module Module1
    Public acsconn As New OleDb.OleDbConnection
    Public acsdr As OleDbDataReader
    Public strsql As String
    Sub connect()
        acsconn.ConnectionString = "Provider=Microsoft.ace.oledb.12.0;data source=|datadirectory|\Base1.accdb;"
        acsconn.Open()
    End Sub
End Module