Question

我在System.NullReferenceException中出错了mysql select＆＃34; COMMAND.CommandText =＆＃34; select from from users where user = @user and pass = @ pass＆＃34;＆＃34;点击按钮。

        MysqlConn = New MySqlConnection(ConfigurationManager.ConnectionStrings("Mydb.My.MySettings.Mydb").ToString)
    Dim READER As MySqlDataReader
    Me.Cursor = Cursors.WaitCursor
    MysqlConn.Open()
    COMMAND.CommandText = "select * from Users where user= @user and pass= @pass"
    COMMAND.Parameters.AddWithValue("@user", If(String.IsNullOrEmpty(TextBox_Username.Text), DBNull.Value, TextBox_Username.Text))
    COMMAND.Parameters.AddWithValue("@pass", If(String.IsNullOrEmpty(TextBox_Password.Text), DBNull.Value, TextBox_Password.Text))
    COMMAND = New MySqlCommand(COMMAND.CommandText, MysqlConn)
    READER = COMMAND.ExecuteReader

有什么问题？

编辑：

这是完整的代码按钮：

MysqlConn = New MySqlConnection(ConfigurationManager.ConnectionStrings("Mydb.My.MySettings.Mydb").ConnectionString)
    Dim READER As MySqlDataReader
    Me.Cursor = Cursors.WaitCursor
    MysqlConn.Open()
    COMMAND.CommandText = "select * from Users where user= @user and pass= @pass"
    COMMAND.Parameters.AddWithValue("@user", If(String.IsNullOrEmpty(TextBox_Username.Text), DBNull.Value, TextBox_Username.Text))
    COMMAND.Parameters.AddWithValue("@pass", If(String.IsNullOrEmpty(TextBox_Password.Text), DBNull.Value, TextBox_Password.Text))
    READER = COMMAND.ExecuteReader
    Dim count As Integer
    count = 0
        While READER.Read
            count = count + 1
        End While
        If count = 1 Then
            MessageBox.Show("Usuario y Contraseña Correctos")
            Dim role As String = READER("tipo")
            If role = "Administrador" Then
                Inicio.Show()
            ElseIf role = "Calidad" Then
                Calidad.Show()
            ElseIf role = "Almacen" Then
                Almacen.Show()
            ElseIf role = "Oficina" Then
                Oficina.Show()
            ElseIf role = "Minas" Then
                Minas.Show()
            End If
            Me.Hide()
        ElseIf count > 1 Then
            MessageBox.Show("Usuario y Contraseña Duplicados")
        Else
            MessageBox.Show("Usuario y Contraseña Incorrectos")
        End If
        MysqlConn.Close()

    MysqlConn.Open()
    COMMAND.CommandText = "insert into Registro_Login (user,fecha,ip) values (@user,@fecha,@ip)"
    COMMAND.Parameters.AddWithValue("@user", If(String.IsNullOrEmpty(TextBox_Username.Text), DBNull.Value, TextBox_Username.Text))
    COMMAND.Parameters.AddWithValue("@fecha", If(String.IsNullOrEmpty(Label4.Text), DBNull.Value, Label4.Text))
    COMMAND.Parameters.AddWithValue("@ip", If(String.IsNullOrEmpty(Label3.Text), DBNull.Value, Label3.Text))
    READER = COMMAND.ExecuteReader
    MysqlConn.Close()
    Me.Cursor = Cursors.Default

但同样的问题

Answer 1

COMMAND在使用之前似乎没有定义。就像你定义了READER一样，定义COMMAND然后使用它。并且还改变下面的代码：

ConfigurationManager.ConnectionStrings("Mydb.My.MySettings.Mydb")

要

ConfigurationManager.ConnectionStrings("Mydb.My.MySettings.Mydb").ConnectionString

.ToString应该是.ToString（），因为它是一种方法。

我不确定您是否真的能够编译此代码或发布整个代码，如果您还没有这样做，建议解决方案。

Answer 2

您的命令没有设置连接。

替换这个：

 COMMAND.CommandText = "select * from Users where user= @user and pass= @pass"

与

 COMMAND = New MySqlCommand("select * from Users where user= @user and pass= @pass", MysqlConn)

VB Net + Mysql + System.NullReferenceException

2 个答案: