Question

我已经在VB.NET中实现了用户验证的代码。当我在表单的文本框中输入用户名和密码，然后单击“提交”按钮时，即使我为其编写了代码，也不会显示任何消息框。在try-catch块中是否存在一些问题，或者我错过了一些代码行？

有人能指出这段代码中有什么问题吗？

       Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As  System.EventArgs) Handles Button1.Click

    If TextBox1.Text = "" Or TextBox2.Text = " " Then
        MsgBox("Enter a user id and password")
    End If
    TextBox1.Text = userid
    TextBox2.Text = password

    Try
        myconnection = New SqlConnection("server=PARTH-PC\SQLEXPRESS;uid=sa;pwd=demo;database=fc")
        'you need to provide password for sql server
        myconnection.Open()

        mycommand = New SqlCommand("select * from student where user id='" &  TextBox1.Text & "' and password='" & TextBox2.Text & "')", myconnection)
        dr = mycommand.ExecuteReader()

    Catch ex As Exception
    Finally



        If (dr IsNot Nothing) Then

            If (dr.Read()) Then

                MsgBox("User is authenticated")
                Form2.Show()



            Else
                MsgBox("Please enter correct username and password")
            End If


        End If

    End Try

    myconnection.Close()
      End Sub
    End Class

Answer 1

我的猜测是你没有Option Strict On，而且你的Try / Catch块的Try部分正在创建dr。当你到达Finally部分时，它超出了范围。您也可以通过在catch块中没有throw语句来吞下您可能遇到的任何错误。

尝试：

Dim myconnection as SqlConnection
Dim mycommand as SqlCommand
Dim dr as SqlDataReader
Try 
    myconnection = New SqlConnection("server=PARTH-PC\SQLEXPRESS;uid=sa;pwd=demo;database=fc") 
        'you need to provide password for sql server 
    myconnection.Open() 

    mycommand = New SqlCommand("select * from student where user id='" &  TextBox1.Text & "' and password='" & TextBox2.Text & "')", myconnection) 
    dr = mycommand.ExecuteReader() 

Catch ex As Exception 
    Throw
Finally 
    If (dr IsNot Nothing) Then 
        If (dr.Read()) Then 
            MsgBox("User is authenticated") 
            Form2.Show() 
        Else 
            MsgBox("Please enter correct username and password") 
        End If 
    End If 

End Try 

myconnection.Close()

修改：Option Strict和Option Explicit声明的其他链接

http://www.readmespot.com/question/o/222370/option-strict-on-and--net-for-vb6-programmers

杰夫阿特伍德的编码恐怖article

Answer 2

这样：

TextBox1.Text = userid
TextBox2.Text = password

看起来不对。除此之外，你可能没有在阅读器上获得任何记录（因为那条线）..这就是为什么你没有得到任何结果。无论如何在finally块上使用它是浪费开销。

你的SQL也错了，它有a）超过需要的

Answer 3

使用Trim()和Length方法或String.IsNullOrWhiteSpace()（。net framework 4）检查空字符串或零长度字符串。

 If TextBox1.Text.Trim().Length = 0 Or TextBox2.Text.Trim().Length = 0 Then
   MsgBox("Enter a user id and password")
   Return 'Terminate this method
 End If

错误的作业，

 Dim userid=TextBox1.Text
 Dim password=TextBox2.Text

另一个问题是使用硬编码的sql语句。

myconnection = New SqlConnection("server=PARTH-PC\SQLEXPRESS;uid=sa;pwd=demo;database=fc")
mycommand = New SqlCommand("select * from student where [user id]=@userid and [password]=@password",myconnection)
mycommand.Parameters.Add("@userid",SqlDbType.VarChar,30).Value = userid
mycommand.Parameters.Add("@password",SqlDbType.VarChar,30).Value = password

myconnection.Open()
dr = mycommand.ExecuteReader()
Dim isFound as boolean  = false
if dr.Read() Then
   isFound=true
End If
dr.Close()
myConnection.Close()

if IsFound Then
   MsgBox("User is authenticated")
   Form2.Show()
Else
   MsgBox("Please enter correct username and password")
End If

VB.NET中的用户验证

3 个答案: