Question

当我运行代码时，我收到一条错误消息：对象引用未设置为对象的实例。我想创建一个代码来验证数据库中的凭据。如果用户未输入有效信息，则会显示错误消息。这是我的代码：

    'Declare variables
    Dim pwd, username As String
    Dim dbpwd, dbUsername As String

    'Get credentials variables
    username = Me.username.Text
    pwd = Me.TextBox2.Text


    Dim objConn As MySqlConnection
    Dim objDataset As New DataSet
    Dim objDataAdapter As MySqlDataAdapter
    Dim sqlConn As String


    If username <> "" And pwd <> "" Then

        objConn = New MySqlConnection("server=localhost;userid=root;password= ;database=mayombe_mdcs")
        objConn.Open()
        sqlConn = "select agent_id, Password from password where agent_id = " & username & ""


        Try
        objDataAdapter = New MySqlDataAdapter(sqlConn, objConn)
        objDataAdapter.Fill(objDataset)

        ' intRowNumber = sqlR
        dbUsername = objDataset.Tables("password").Rows(1).Item(2)

        ' dbpwd = objDataset.Tables("password").Rows(1).Item(1)
        'WriteLine (dbUsername )
        'Force users to enter credentiasl

        objConn.Close()
        'Force user to enter true credentials
        If pwd = dbpwd And username = dbUsername Then
           open form

            Me.Close()

        End If
        Catch ex As Exception
         strMsg As String
        Prompt message that tells the user that credentials entered are not correct.
        strMsg = String.Format("One of the following is incorrect: {0}* Username entered {0}* Password entered.", Environment.NewLine)
        MessageBox.Show(strMsg, "Warning")
        End Try

Answer 1

您的代码中存在一些问题。

首先，如果agent_id是varchar字段，则需要在where子句中使用的值周围使用单引号，但最好避免此问题并使用parameterized query。

其次，如果你找到了什么，那么你应该使用索引0引用第一行，使用索引1引用第二列。你的代码假设数组的索引从索引1开始，但在NET世界中不是这样。。数组总是从索引0开始。

我会尝试重写你的代码

objDataset = new Dataset()
sqlConn = "select agent_id, Password from password where agent_id = @usr"
using objConn = New MySqlConnection(....)
    objConn.Open()
    Try
    objDataAdapter = New MySqlDataAdapter(sqlConn, objConn)
    objDataAdapter.SelectCommand.Parameters.AddWithValue("@usr", username)
    objDataAdapter.Fill(objDataset)
    if objDataset.Tables(0).Rows.Count > 0 Then
       dbUsername = objDataset.Tables(0).Rows(0).Item(1).ToString
    End If
End Using

无法从MySql数据库中检索数据

1 个答案: