如何处理以下代码中未处理的Nullreferenceexception [英] How Do I Handle Nullreferenceexception Unhandled In The Following Code

问题描述

  Imports  System.Data.SqlClient 
  Imports  System.Linq 
  Imports  System.Data 
 
 
 
 
 公开 类表单
 
 私有  Sub  Button1_Click（ ByVal  sender  As  System 。对象， ByVal  e 作为系统。 EventArgs）句柄 Button1.Click 
  Dim  conn 正如 SqlConnection 
  Dim  cmd 作为 SqlCommand 
  Dim  rd  As  SqlDataReader 
 conn.ConnectionString =   Data Source = localhost; Initial Catalog = user; Integrated security = true 
 cmd.Connection = conn 
 conn.Open（）
 cmd.CommandText =  从用户中选择登录名，密码login ='& TextBox1.Text&  'and password ='& TextBox2.Text&  ' 
 rd = cmd.ExecuteReader 
 如果 rd.HasRows 那么 
 Welcome.Show（）
 否则 
 MsgBox（ 无效的登录名或密码）
 结束 如果 
 
 结束  Sub  
 结束 类 

解决方案

您声明 conn 和 cmd 作为变量但您没有使用SqlConnection / SqlCommand实例初始化它们。您需要使用关键字 New 创建实例，或使用 ProviderFactory [ ^ ] -class获取它们的实例。



后者（使用ProviderFactory）因为那时候，如果你也使用抽象的Db ****** - 基类（如DbConnection，DbCommand，..）或接口（IDbConnection，IDbCommand，..）而不是具体和SQL-依赖于服务器的类（SqlConnection，SqlCommand，..），您将使您的代码独立于具体的数据库系统，并且能够轻松地或至少更轻松地从SQL Server更改为其他数据库系统。



此外，您不应该使用字符串连接来构建SQL语句。您的代码容易受到SQL注入的影响，而且可读性较差。请改用参数： SqlCommand.Parameters Property （System.Data.SqlClient） [ ^ ]

Dim conn As SqlConnection 语句， conn 仍为空（对于 cmd 和 rd ）。你需要通过调用它们的构造函数给 conn 和 cmd 一个值。

  Dim  conn  As   New  SqlConnection（）
  Dim  cmd  As  新 SqlCommand（）

注意：您不必为 rd 执行此操作，因为 SqlDataReader 没有构造函数，无论如何你从 ExecuteReader 获得了读者。




你必须开始初始化你的对象......

  Dim  conn  As  SqlConnection 

此行为SqlConnection类型的对象创建占位符，但没有创建真实对象，因此占位符是空的 - null !!!

  Dim  conn 作为  新  SqlConnection 

有关详细信息： SqlConnection构造函数（字符串）（系统。 Data.SqlClient） [ ^

Imports System.Data.SqlClient
Imports System.Linq
Imports System.Data




Public Class Form

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim conn As SqlConnection
        Dim cmd As SqlCommand
        Dim rd As SqlDataReader
        conn.ConnectionString = "Data Source=localhost;Initial Catalog=user;Integrated security=true"
        cmd.Connection = conn
        conn.Open()
        cmd.CommandText = "Select login,password from user where login='" & TextBox1.Text & "'and password='" & TextBox2.Text & "'"
        rd = cmd.ExecuteReader
        If rd.HasRows Then
            Welcome.Show()
        Else
            MsgBox("Invalid Login or Password")
        End If

    End Sub
End Class

解决方案

You declared conn and cmd as variables but you didn't initialize them with instances of SqlConnection / SqlCommand. You need to either create an instance with the keyword New or use the ProviderFactory[^]-class to get an instance of them.

The latter (using the ProviderFactory) is recommended because then, if you also use either the abstract Db******-base-classes (like DbConnection, DbCommand, ..) or the interfaces (IDbConnection, IDbCommand, ..) instead of the concrete and SQL-Server-dependent classes (SqlConnection, SqlCommand, ..) you'll make your code independent of a concrete database system and will be able to change from SQL Server to some other DB system easily or at least more easily.

Also, you shouldn't use string concatenation for building SQL-statements. Your code becomes susceptible to SQL-injection and less readable. Use Parameters instead: SqlCommand.Parameters Property (System.Data.SqlClient)[^]

After the Dim conn As SqlConnection statement, conn is still null (same for cmd and rd). You need to give conn and cmd a value by calling their constructor.

Dim conn As New SqlConnection()
Dim cmd As New SqlCommand()

Note: you do not have to do this for rd, because SqlDataReader has no constructor and you get the reader from ExecuteReader anyway.

You have to start initializing your objects...

Dim conn As SqlConnection

This line creates a 'placeholder' for an object of type SqlConnection, but no real object created so the 'placeholder' is empty - null!!!

Dim conn As New SqlConnection

For more info: SqlConnection Constructor (String) (System.Data.SqlClient)[^]

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