验证字符串用户输入

Question

我的问题是，我试图让用户只能输入“a，b，c或d”。如果他们没有输入这四个字母中的一个，而只是用户只能输入这些字母，我宁愿让它出错。我只能找到与数字数据类似的资源（使用try catch）。任何网站或提示将是伟大的。

   If String.Compare(TextBox2.Text, "a", True) = 0 AndAlso String.Compare(TextBox21.Text, "a", True) = 0 Then 
       'MessageBox.Show("A") 
       totCorrect = totCorrect + corAns 
      ElseIf String.Compare(TextBox2.Text, "b", True) = 0 AndAlso String.Compare(TextBox21.Text, "b", True) = 0 Then 
       'MessageBox.Show("B") 
       totCorrect = totCorrect + corAns 
      ElseIf String.Compare(TextBox2.Text, "c", True) = 0 AndAlso String.Compare(TextBox21.Text, "c", True) = 0 Then 
       'MessageBox.Show("C") 
       totCorrect = totCorrect + corAns 
      ElseIf String.Compare(TextBox2.Text, "d", True) = 0 AndAlso String.Compare(TextBox21.Text, "d", True) = 0 Then 
       'MessageBox.Show("D") 
       totCorrect = totCorrect + corAns 
      Else 
       totWrong = totWrong + wrgAns 
       Label13.Visible = True 
      End If 

Answer 1

这应该做的伎俩

Private Sub TextBox1_KeyPress(sender As Object, e As KeyPressEventArgs) Handles TextBox1.KeyPress 

    Dim allowableChar As New List(Of Char) From {"a"c, "b"c, "c"c, "d"c} 

    Call allowableChar.AddRange(allowableChar.Select(Function(c) Convert.ToChar(c.ToString().ToUpper())).ToList()) 

    If Not (allowableChar.Contains(e.KeyChar) OrElse e.KeyChar = Convert.ToChar(Keys.Delete) OrElse e.KeyChar = Convert.ToChar(Keys.Back)) Then 
     e.Handled = True 
    End If 

End Sub 

的Call allowableChar.AddRange(...)增加了大写字母的名单。就像现在一样，这会在每次执行方法时生成一个新列表，这有点浪费......如果你使用这段代码，我建议你将可允许的字符列表作为一个类级变量并仅填充它一次在你的表单的构造函数中。

要只允许一次每种类型的字符，使用此：

Private Sub TextBox1_KeyPress(sender As Object, e As KeyPressEventArgs) Handles TextBox1.KeyPress 

    Dim allowableChar As New List(Of Char) From {"a"c, "b"c, "c"c, "d"c} 

    Call allowableChar.AddRange(allowableChar.Select(Function(c) Convert.ToChar(c.ToString().ToUpper())).ToList()) 

    If Not (allowableChar.Contains(e.KeyChar) OrElse e.KeyChar = Convert.ToChar(Keys.Delete) OrElse e.KeyChar = Convert.ToChar(Keys.Back)) Then 
     e.Handled = True 
    Else 
     If Me.TextBox1.Text.Count(Function(c) c = e.KeyChar) >= 1 Then 
      e.Handled = True 
     End If 
    End If 

End Sub