我用VB.NET编写的一个上位机应用软件，调试没问题，一运行就报错
我把程序简化到只有一个登陆界面，求大神帮我看看怎么了，跪谢！
登陆界面代码：
Public Class denglu
Dim dengluyanzhengq As TensionInfoDB
Dim username As String
Public Sub Btnqueren_Click(sender As Object, e As EventArgs) Handles btnqueren.Click
dengluyanzhengq = New TensionInfoDB()
username = New String(txtyonghuming.Text)
If (username <> "") Then
Dim details As UsersDetail
details = New UsersDetail()
details = dengluyanzhengq.Getmima(username)
If details.Password = " " Then
MsgBox("用户不存在！")
ElseIf txtmima.Text = details.Password Then
MsgBox("登录成功")
Else
MsgBox("密码错误！")
End If
Else
MsgBox("请输入用户名！")
End If
End Sub
End Class
TensionInfoDB（关于数据库的语句都放在里面了）代码：
Imports System.Data.SqlClient
Public Class TensionInfoDB
Public connectionString As String
Dim dengluyanzheng As UsersDetail
Dim reader As SqlDataReader
Dim con As SqlConnection
Dim cmd As SqlCommand
Public Sub New()
connectionString = System.Configuration.ConfigurationManager.ConnectionStrings("Tension").ConnectionString
End Sub
Public Function Getmima(ByVal username As String) As UsersDetail
dengluyanzheng = New UsersDetail()
con = New SqlConnection(connectionString)
cmd = New SqlCommand("", con)
cmd.CommandText = "SELECT * FROM Users WHERE UserName='" & username & "'"
Try
con.Open()
dengluyanzheng.Password = " "
reader = cmd.ExecuteReader()
While reader.Read
dengluyanzheng.UserName = reader("UserName")
dengluyanzheng.Password = reader("Password")
dengluyanzheng.Grade = reader("Grade")
End While
reader.Close()
Catch ex As SqlException
dengluyanzheng = Nothing
Throw New ApplicationException(ex.Message)
Finally
con.Close()
End Try
Return dengluyanzheng
End Function
End Class
UsersDetail类代码：
Public Class UsersDetail
Public Sub TensionDetails(ByVal ISBNUserName As String, ByVal Password As String, ByVal Grade As String)
Me.UserName = UserName
Me.Password = Password
Me.Grade = Grade
End Sub
Private pUserName As String
Public Property UserName()
Get
Return pUserName
End Get
Set(ByVal value)
pUserName = value
End Set
End Property
Private pPassword As String
Public Property Password()
Get
Return pPassword
End Get
Set(ByVal value)
pPassword = value
End Set
End Property
Private pGrade As String
Public Property Grade()
Get
Return pGrade
End Get
Set(ByVal value)
pGrade = value
End Set
End Property
End Class