VB.NET-文本框的更改.Text属性抛出空引用异常

这必须是微不足道的。我这样做是为生活和想不通为什么我收到此异常：VB.NET-文本框的更改.Text属性抛出空引用异常

System.NullReferenceException了未处理消息=对象引用不设置到对象的实例。

下面是代码：

Public Class frmMain 

Private Sub frmMain_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load 
    Dim counties() As String 

    counties = {"", "Huntsville, AL", "Madison, AL", "Rural Madison County, AL", "Taft, TN"} 
    Me.cbCounties.DataSource = counties 

    Me.lblStatus.Text = "[ Please select a county ]" 
    Me.lblStatus.Left = Me.ClientSize.Width \ 2 - Me.lblStatus.Width \ 2 



End Sub 

Private Sub cbCounties_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cbCounties.SelectedIndexChanged 
    Select Case cbCounties.SelectedIndex 
     Case 1 
      txtTaxRate.Text = "8.00%" 'Issue is these, when index is changed. 
     Case 2 
      txtTaxRate.Text = "8.50%" 
     Case 3 
      txtTaxRate.Text = "5.50" 
     Case 4 
      txtTaxRate.Text = "9.50%" 
     Case Else 
      txtTaxRate.Text = Nothing 
    End Select 

    Me.lblStatus.Text = "[ Please enter net amount ]" 
    Me.lblStatus.Left = Me.ClientSize.Width \ 2 - Me.lblStatus.Width \ 2 

End Sub

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帮助？

来源

2012-08-16 Josh McKearin

在哪里抛出的异常？ – 2012-08-16 04:36:10

查看代码中的评论。具体来说，它被引用到select case语句中，引用txtTaxRate TextBox – 2012-08-16 04:37:06

这是在你改变了几次之后？设置txtTaxRate = Nothing，稍后尝试将txtTaxRate.Text设置为其他内容会导致一些问题。尝试更改txtTaxRate = Nothing to txtTaxRate.Text =“” – 2012-08-16 04:38:15

这是你改变它几次后？

设置txtTaxRate = Nothing和稍后尝试将txtTaxRate.Text设置为别的会导致一些问题。

您正在设置文本框对象为空，并稍后尝试引用其属性之一。

尝试改变

txtTaxRate = Nothing

到

txtTaxRate.Text = ""

来源

2012-08-16 04:41:04

VB.NET-文本框的更改.Text属性抛出空引用异常

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