Sql server 保存在数据库中的BER？是：）需要许多字符来发表评论。这非常容易受到sql注入的影响。第15行；cmd.Connection=con给出错误：Client.exe中发生“System.NullReferenceException”类型的未处理异常。其他信息

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保存在数据库中的BER？是：）需要许多字符来发表评论。这非常容易受到sql注入的影响。第15行；cmd.Connection=con给出错误：Client.exe中发生“System.NullReferenceException”类型的未处理异常。其他信息：对象引用未设置为对象的实例。它还警告我：警告1变量“cmd”在赋值之前使用。运行时可能会出现空引用异常。请删除它…不应该存在。SorryPurning：“已经有一个与此命令关联的打开的DataReader，必须先关闭。”（System.InvalidOperationException）异常消息=“已经有一个与此命令关联的打开的DataReader，必须先关闭。”，异常类型=“System.InvalidOperationException”，异常WinRT Data=第24行没有任何内容：reader=cmd.ExecuteReader（）System.Data.dll中发生类型为“System.InvalidOperationException”的未处理异常。其他信息：已存在与此命令关联的打开的DataReader，必须关闭该DataReaderfirst@H4rdstyler尝试在下一次查询之前关闭连接。。。请参阅我的UpdateAnks以获取建议和更正：）工作起来像个符咒：）

Private Sub GetAssociationButton_Click(sender As Object, e As EventArgs) Handles GetAssociationButton.Click
        Dim con As New SqlConnection
        Dim query as String
        Dim cmd As SqlCommand
        Dim reader As SqlDataReader
        Dim totalHeating As Integer = 0
        Dim apt_no As Integer = 1

        con.ConnectionString = "Server=10.176.165.29,1433;Database=EnergyDB;User=Clerk;Pwd=12345"
        con.Open()
        query = "SELECT MAX(apartment_no) as maxAptNo FROM HeatMeasurement WHERE association_no = '" & AssociationField.Text & "' and year(reading_date) = '" & YearField.Text & "'")
        cmd=new SqlCommand(query,con)
        reader = cmd.ExecuteReader()
        reader.read()
        Dim maxAptNo as integer = reader("maxAptNo")
        con.Close()

        con.Open()
        query = "SELECT apartment_no, reading FROM HeatMeasurement WHERE association_no = '" & AssociationField.Text & "' and year(reading_date) = '" & YearField.Text & "'")
        cmd=new SqlCommand(query,con)
        reader = cmd.ExecuteReader()

        If reader.HasRows Then
            Do Until apt_no = maxAptNo
                While reader.Read()
                    If Convert.ToInt32(reader("apartment_no") = apt_no Then
                        totalHeating += CINT(reader("reading"))
                    End If
                End While                    
                Console.WriteLine("Apartment " & apt_no & " totals: " & totalHeating)       apt_no+=1
                totalHeating=0
            Loop
        Else
            Console.WriteLine("No data found for the year " & YearField.Text)
        End If
        con.Close()
    End Sub