如何解决“未将对象引用设置为对象实例”的错误？

Question

在这个程序（VB，ASP.NET 2010）创建三个字段：accno，name和balance，以及以下按钮：create，destroy，set和get。 但是，当点击set或get方法它提供了以下异常：object reference not set to an instance of an object

Default.aspx.vb

Partial Class _Default 
    Inherits System.Web.UI.Page 

    Dim obj As account 'declaring the obj of class account 

    Protected Sub btn_create_Click(sender As Object, e As System.EventArgs) Handles btn_create.Click 
     obj = New account 'initializing the object obj on class accounts 
    End Sub  

    Protected Sub btn_set_Click(sender As Object, e As System.EventArgs) Handles btn_set.Click 
     'sending the values from textboxes to accounts class through method setdata 
     Try 
      obj.setdata(CInt(txt_accno.Text), (txt_name.Text), CInt(txt_bal.Text)) 
     Catch ex As Exception 
      MsgBox(ex.Message) 
     End Try 
    End Sub 

    Protected Sub btn_get_Click(sender As Object, e As System.EventArgs) Handles btn_get.Click 
     'calling the method getdata to view the output 
     Try 
      obj.getdata() 
     Catch ex As Exception 
      MsgBox(ex.Message) 
     End Try 
    End Sub 

    Protected Sub btn_destroy_Click(sender As Object, e As System.EventArgs) Handles btn_destroy.Click 
     'calling the constructor 
     obj = Nothing 
    End Sub 
End Class 

Account.vb

Imports Microsoft.VisualBasic 

Public Class account 

    Private accno As Integer 
    Private acc_name As String 
    Private bal As Integer 

    'constructor 
    Public Sub New() 
     MsgBox("object created") 
    End Sub 

    'public method to populate above three private variable 

    Public Sub setdata(ByVal a As Integer, ByVal b As String, ByVal c As Integer) 
     Me.accno = a 
     Me.acc_name = b 
     Me.bal = c 
    End Sub 

    Public Sub getdata() 
     MsgBox(Me.accno.ToString + vbNewLine + Me.acc_name + vbNewLine + Me.bal.ToString) 
    End Sub 

    'destructor 
    Protected Overrides Sub finalize() 
     MsgBox("object destroyed") 
    End Sub 

End Class 

Answer 1

你必须使用新初始化对象的关键字...

昏暗的OBJ中的新帐户

Default.aspx.vb Partial Class _Default Inherits System.Web.UI.Page 

'declaring the obj of class account 

Dim obj As NEW account  


Protected Sub btn_create_Click(sender As Object, e As System.EventArgs) Handles btn_create.Click 

    'initializing the object obj on class accounts 
    obj = New account 



End Sub 


Protected Sub btn_set_Click(sender As Object, e As System.EventArgs) Handles btn_set.Click 

    'sending the values from textboxes to accounts class through method setdata 

    Try 
     obj.setdata(CInt(txt_accno.Text), (txt_name.Text), CInt(txt_bal.Text)) 

    Catch ex As Exception 

     MsgBox(ex.Message) 
    End Try 


End Sub 

Protected Sub btn_get_Click(sender As Object, e As System.EventArgs) Handles btn_get.Click 

    'calling the method getdata to view the output 

    Try 

     obj.getdata() 

    Catch ex As Exception 

     MsgBox(ex.Message) 
    End Try 

End Sub 

Protected Sub btn_destroy_Click(sender As Object, e As System.EventArgs) Handles btn_destroy.Click 

    'calling the constructor 

    obj = Nothing 

End Sub 
End Class 

请把它标记为答案，如果它的工作。

Answer 2

使用以下代码。您需要在页面加载方法中进行初始化，并且需要检查是否为回发属性。

Default.aspx.vb Partial Class _Default Inherits System.Web.UI.Page 

     'declaring the obj of class account 

     Dim obj As account 

     Sub Page_Load(ByVal Sender As System.Object, ByVal e As System.EventArgs) 
      if not ispostback then 
       obj = New account 
      end 
     End Sub 

     Protected Sub btn_create_Click(sender As Object, e As System.EventArgs) Handles btn_create.Click 

      'initializing the object obj on class accounts 
      obj = New account 



     End Sub 


     Protected Sub btn_set_Click(sender As Object, e As System.EventArgs) Handles btn_set.Click 

      'sending the values from textboxes to accounts class through method setdata 

      Try 
       obj.setdata(CInt(txt_accno.Text), (txt_name.Text), CInt(txt_bal.Text)) 

      Catch ex As Exception 

       MsgBox(ex.Message) 
      End Try 


     End Sub 

     Protected Sub btn_get_Click(sender As Object, e As System.EventArgs) Handles btn_get.Click 

      'calling the method getdata to view the output 

      Try 

       obj.getdata() 

      Catch ex As Exception 

       MsgBox(ex.Message) 
      End Try 

     End Sub 

     Protected Sub btn_destroy_Click(sender As Object, e As System.EventArgs) Handles btn_destroy.Click 

      'calling the constructor 

      obj = Nothing 

     End Sub 
     End Class