mysql - Mysql 类中未处理的异常

Question

我正在尝试创建一个 vb.net 类来管理 MySQL 数据库连接，并且我有一个 Connect 方法，该方法应该将连接字符串分配给 mysqlconnection 变量，但我总是收到错误:

An unhandled exception of type 'System.NullReferenceException' occurred in Projecto_Aula.exe Additional information: Object reference not set to an instance of an object.

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

所有这些都未设置(空/无):

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

您需要在使用 then 之前创建对象:

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

因此，在 Connect() 方法中创建一个新的 MySqlConnection 并将“myConnectionString”替换为有效的连接字符串。

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