如何删除此错误，即“对象引用未设置为对象的实例”？ [英] How can I remove this error, i.e "Object reference is not set to an instance of an object"?

问题描述

Imports System.Data.SqlClient
Public Class frmCustomerEdit
    Dim con As SqlConnection
    Private Sub frmCustomerEdit_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'TODO: This line of code loads data into the 'TestdbDataSet.Customersdb' table. You can move, or remove it, as needed.
        Me.CustomersdbTableAdapter.Fill(Me.TestdbDataSet.Customersdb)
        'TODO: This line of code loads data into the 'TestdbDataSet.Customersdb' table. You can move, or remove it, as needed.
        Me.CustomersdbTableAdapter.Fill(Me.TestdbDataSet.Customersdb)
        Try
            con.ConnectionString = "Data Source=SuperComp-PC;Initial Catalog=Testdb;Integrated Security=True"
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

        Dim sDate As Date = Me.DateTimePicker1.Value
        Dim eDate As Date = Me.DateTimePicker2.Value
        Dim abc As String = " Update Customersdb set CName='" & TextBox2.Text & "', Address='" & RichTextBox1.Text & "', Delivery_Charge='" & TextBox3.Text & "', Start_Date='" & sDate & "', End_Date='" & eDate & "' Where Customer_ID='" & TextBox1.Text & "';"
        Dim da As New SqlDataAdapter(abc, con)
        Dim ds As New DataSet
        da.Fill(ds)
        MsgBox("UPDATE SUCESSFUL !")
       
    End Sub

    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
        Me.Close()
        frmCustomer.Show()

    End Sub
End Class

推荐答案

好吧，两件事：

1）将解决眼前的问题。你声明 con ，但你永远不会给它一个值！在某些时候，您需要将con设置为SqlConnection的实例，因此请将其替换为：

Well, two things:
1) Will fix the immediate problem. You declare con, but you never give it a value! At some point you need to set con to an instance of an SqlConnection, so replace this:

con.ConnectionString = "Data Source=SuperComp-PC;Initial Catalog=Testdb;Integrated Security=True"

这样：

With this:

con = New SqlConnection("Data Source=SuperComp-PC;Initial Catalog=Testdb;Integrated Security=True")

2）不要连接字符串来构建SQL命令。它让您对意外或故意的SQL注入攻击持开放态度，这可能会破坏您的整个数据库。请改用参数化查询。

2) Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.

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