Question

我正在尝试创建一个vb.net类来管理MySQL数据库连接，我有一个方法Connect，它将连接字符串分配给mysqlconnection变量，但我总是得到错误：

未处理的类型＆＃39; System.NullReferenceException＆＃39; 发生在Projecto_Aula.exe中的附加信息：对象引用没有设置为对象的实例。

DatabaseManager.vb

Imports MySql.Data.MySqlClient

Public Class DatabaseManager
    Private connetionString As String = vbNullString
    Private sqlReader As MySqlDataReader
    Private sqlAdapter As MySqlDataAdapter
    Private sqlCommand As MySqlCommand
    Private sqlConnection As MySqlConnection

    Sub New(ByVal host As String, ByVal user As String, ByVal password As String, Optional ByVal database As String = "requisicoes")
        connetionString = "server=" & host & "; uid=" & user & "; pwd=" & password & "; database=" & database
        MsgBox(connetionString)
    End Sub

    Public Sub Connect()

        If (sqlConnection.State = ConnectionState.Open) Then ''throws exception when run
            MsgBox("Already connected to the database.")
            Return
        End If

        sqlConnection.ConnectionString = connetionString.ToString()

        If (connetionString = vbNullString) Then
            MsgBox("Invalid Connection String!")
            Application.Exit()
        End If

        Try
            sqlConnection.Open()
        Catch ex As MySqlException
            MsgBox("Error connecting: " & ex.ToString())
            Application.Exit()
        End Try

    End Sub
End Class

MainMenu.vb

Public Class Login

    Public databaseManager As New DatabaseManager("localhost", "root", "", "requisicoes")

    Private Sub Form1_Load_1(sender As System.Object, e As System.EventArgs) Handles MyBase.Load

        databaseManager.Connect() 'throws exception when run

    End Sub
End Class

Answer 1

所有这些都没有设置（null / Nothing）：

Private sqlReader As MySqlDataReader
Private sqlAdapter As MySqlDataAdapter
Private sqlCommand As MySqlCommand
Private sqlConnection As MySqlConnection

您需要在使用之前创建对象：

Me.sqlReader = New MySqlDataReader
Me.sqlAdapter = New MySqlDataAdapter
Me.sqlCommand = New MySqlCommand
Me.sqlConnection = New MySqlConnection

所以在Connect()方法中创建一个新的MySqlConnection并用有效的连接字符串替换“myConnectionString”。

Public Sub Connect()

    Me.sqlConnection = New MySqlConnection("myConnectionString")


    Try
        Me.sqlConnection.Open()
        MsgBox(Me.sqlConnection.State.ToString())
    Catch ex As MySqlException
        MsgBox("Error connecting: " & ex.ToString())
        Application.Exit()
    End Try

End Sub

Answer 2

使用此提及代码

更改以下代码行

 sqlConnection.ConnectionString = connetionString.ToString()

更改为：

     sqlConnection.ConnectionString = Convert.ToString(connetionString)

Mysql类中未处理的异常

2 个答案: