这必须是微不足道的。我这样做是为了谋生，但无法弄清楚为什么会出现此异常：
System.NullReferenceException 未处理 Message=Object 引用未设置为对象的实例。
这是代码：
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
结束类
帮助？