Visual Basic - 表单验证类 - ＆＃39; System.NullReferenceException＆＃39;

Question

我正在尝试使用Validator类验证用户表单，但会收到以下错误...

  

未处理的类型＆＃39; System.NullReferenceException＆＃39;发生在Motor Racing.exe中

     

附加信息：未将对象引用设置为对象的实例。

并突出显示以下代码行...

  

MessageBox.Show（textBox.Tag.ToString＆amp;＆＃34;是必填字段。＆＃34;，标题）

这里的完整代码......

Public Class Validator

  Public Shared Property Title As String = "Entry Error"

  Public Shared Function IsPresent(textBox As TextBox) _
        As Boolean
    If textBox.Text = "" Then
        MessageBox.Show(textBox.Tag.ToString & " is a required field.", Title)
        textBox.Select()
        Return False
    Else
        Return True
    End If
  End Function

  Public Shared Function validateCombo(txt As ComboBox) As Boolean
    If Not txt.Items.Contains(txt.Text.ToString) Then
        Return False
    Else
        Return True
    End If
  End Function

  Public Shared Function IsInteger(textBox As TextBox) _
        As Boolean
    Dim number As Integer = 0
    If Integer.TryParse(textBox.Text, number) Then
        Return True
    Else
        MessageBox.Show(textBox.Tag.ToString & " must be a integer value.", Title)
        textBox.Select()
        textBox.SelectAll()
        Return False
    End If
  End Function

End Class