connectionstring属性尚未初始化。可能是什么问题 [英] The connectionstring property has not been initialized. What could be the issue

问题描述

Imports System.Data.OleDb

Public Class MyAthleteEvents

    Public Shared Function GetConnection() As OleDbConnection
       
        Dim connectionString As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\lebom\Desktop\ICT3611Assignment\AthletesEvents.accdb")

        Return New OleDbConnection()
    End Function

End Class

Imports System.Data.OleDb

Public Class MyeventDB

    Public Shared Function GetEvent(eventID As Integer) As Myevent
        Dim myevent As New Myevent
        Dim connection As OleDbConnection = MyAthleteEvents.GetConnection
        Dim selectStatement As String =
            "SELECT EventID, Title, Date, Fee, Location, Distance " &
            "FROM Events " &
            "WHERE EventID = @EventID"
        Dim selectCommand As New OleDbCommand(selectStatement, connection)
        selectCommand.Parameters.AddWithValue("@EventID", eventID)
        Try
            connection.Open()
            Dim reader As OleDbDataReader _
                = selectCommand.ExecuteReader(CommandBehavior.SingleRow)
            If reader.Read Then
                myevent.EventID = CInt(reader("EventID"))
                myevent.Title = reader("Title").ToString
                myevent.DDate = reader("Date").ToString
                myevent.Fee = reader("Fee").ToString
                myevent.Location = reader("Location").ToString
                myevent.Distance = reader("Distance").ToString
            Else
                myevent = Nothing
            End If
            reader.Close()
        Catch ex As Exception
            Throw ex
        Finally
            connection.Close()
        End Try
        Return myevent
    End Function

    Public Shared Function AddEvent(myevent As Myevent) As Integer
        Dim connection As OleDbConnection = MyAthleteEvents.GetConnection
        Dim insertStatement As String =
            "INSERT Events " &
            "(Title, Date, Fee, Location, Distance) " &
            "VALUES (@Title, @Date, @Fee, @Location, @Distance)"
        Dim insertCommand As New OleDbCommand(insertStatement, connection)
        insertCommand.Parameters.AddWithValue("@Title", myevent.Title)
        insertCommand.Parameters.AddWithValue("@Date", myevent.DDate)
        insertCommand.Parameters.AddWithValue("@Fee", myevent.Fee)
        insertCommand.Parameters.AddWithValue("@Location", myevent.Location)
        insertCommand.Parameters.AddWithValue("@Distance", myevent.Distance)
        Try
            connection.Open()
            insertCommand.ExecuteNonQuery()
            Dim selectStatement As String _
                = "SELECT IDENT_CURRENT('Events') FROM Events"
            Dim selectCommand As New OleDbCommand(selectStatement, connection)
            Dim eventID As Integer = CInt(selectCommand.ExecuteScalar)
            Return eventID
        Catch ex As Exception
            Throw ex
        Finally
            connection.Close()
        End Try
    End Function

    Public Shared Function UpdateEvent(oldEvent As Myevent,
            newEvent As Myevent) As Boolean
        Dim connection As OleDbConnection = MyAthleteEvents.GetConnection
        Dim updateStatement As String =
            "UPDATE Events SET " &
            "Title = @NewTitle, " &
            "Date = @NewDate, " &
            "Fee = @NewFee, " &
            "Location = @NewLocation, " &
            "Distance = @NewDistance " &
            "WHERE EventID = @OldEventID " &
            "AND Title = @OldTitle " &
            "AND Date = @OldDate " &
            "AND Fee = @OldFee " & "AND Location = @OldLocation " &
            "AND Distance = @OldDistance"
        Dim updateCommand As New OleDbCommand(updateStatement, connection)
        updateCommand.Parameters.AddWithValue("@NewTitle", newEvent.Title)
        updateCommand.Parameters.AddWithValue("@NewDate", newEvent.DDate)
        updateCommand.Parameters.AddWithValue("@NewFee", newEvent.Fee)
        updateCommand.Parameters.AddWithValue("@NewLocation", newEvent.Location)
        updateCommand.Parameters.AddWithValue("@NewDistance", newEvent.Distance)
        updateCommand.Parameters.AddWithValue("@OldEventID", oldEvent.EventID)
        updateCommand.Parameters.AddWithValue("@OldTitle", oldEvent.Title)
        updateCommand.Parameters.AddWithValue("@OldDate", oldEvent.DDate)
        updateCommand.Parameters.AddWithValue("@OldFee", oldEvent.Fee)
        updateCommand.Parameters.AddWithValue("@OldLocation", oldEvent.Location)
        updateCommand.Parameters.AddWithValue("@OldDistance", oldEvent.Distance)

        Try
            connection.Open()
            Dim count As Integer = updateCommand.ExecuteNonQuery
            If count > 0 Then
                Return True
            Else
                Return False
            End If
        Catch ex As Exception
            Throw ex
        Finally
            connection.Close()
        End Try
    End Function

    Public Shared Function DeleteEvent(myevent As Myevent) As Boolean
        Dim connection As OleDbConnection = MyAthleteEvents.GetConnection
        Dim deleteStatement As String =
            "DELETE FROM Events " &
            "WHERE EventID = @EventID " &
            "AND Title = @Title " &
            "AND Date = @Date " &
            "AND Fee = @Fee " &
            "AND Location = @Location " &
            "AND Distance = @Distance"
        Dim deleteCommand As New OleDbCommand(deleteStatement, connection)
        deleteCommand.Parameters.AddWithValue("@EventID", myevent.EventID)
        deleteCommand.Parameters.AddWithValue("@Title", myevent.Title)
        deleteCommand.Parameters.AddWithValue("@Date", myevent.DDate)
        deleteCommand.Parameters.AddWithValue("@Fee", myevent.Fee)
        deleteCommand.Parameters.AddWithValue("@Location", myevent.Location)
        deleteCommand.Parameters.AddWithValue("@Distance", myevent.Distance)

        Try
            connection.Open()
            Dim count As Integer = deleteCommand.ExecuteNonQuery
            If count > 0 Then
                Return True
            Else
                Return False
            End If
        Catch ex As Exception
            Throw ex
        Finally
            connection.Close()
        End Try
    End Function

End Class

我尝试了什么：



我试图调试和更改代码，但仍然无法通过

What I have tried:

I tried to debug and change code but still cant pull through

推荐答案

在你的

GetConnection

它说的功能：

function it says:

Return New OleDbConnection()

返回未初始化的连接。



应该是：

That returns an uninitialized connection.

It should read :

Return connectionString

connectionstring属性尚未初始化的原因在于：

The reason of "The connectionstring property has not been initialized" error lies here:

Dim connection As OleDbConnection = MyAthleteEvents.GetConnection

MyAthleteEvents 是一个必须用 New 关键字初始化的类。 />


这应解决您的问题：

MyAthleteEvents is a class which must be initialized with New keyword.

This should resolve your issue:

Dim mae As MyAthleteEvents = new MyAthleteEvents()
Dim connection As OleDbConnection = mae.GetConnection

但是我会避免这种编程习惯。而不是它，请阅读 DAL [ ^ ]。

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