使用 TryParse 方法从字符串" "转换为类型"Double"无效

Question

我在 VB.Net 中有一个简单的加法程序，我正在尝试测试文本框是否只接受数字而不是任何字母。我需要使用 TryParse 方法，但我无法弄清楚我仍然收到此错误。请帮助

Public Class perrySolutionForm
Dim numberOne As Double
Dim numberTwo As Double
Public Function sum(ByRef numberOne As Double, ByRef numberTwo As Double)
    sum = Val(numberOne) + Val(numberTwo)
End Function
Public Function difference(ByRef numberOne As Double, numberTwo As Double)     
difference = Val(numberOne) - Val(numberTwo)
End Function
Private Sub sumButton_Click(sender As Object, e As EventArgs) Handles sumButton.Click
    If numberOneInput.Text = "" Then
        MessageBox.Show("Both fields must be filled out.")
        If Double.TryParse(numberOneInput.Text, numberOne) Then
            MessageBox.Show("Success")
            'numberOne has a Double value
        Else
            MessageBox.Show("Failure")
            'numberOne = Nothing
        End If
    Else
        outputLabel.Text = sum(numberOne, numberTwo)
    End If
End Sub
Private Sub numberOneInput_TextChanged(sender As Object, e As EventArgs) Handles numberOneInput.TextChanged
    numberOne = numberOneInput.Text
End Sub
Private Sub numberTwoInput_TextChanged(sender As Object, e As EventArgs) Handles numberTwoInput.TextChanged
    numberTwo = numberTwoInput.Text
End Sub

Answer 1

以下是您使用 TryParse 的方式：

If Double.TryParse(numberOneInput.Text, numberOne) Then
  'code for a success
  'numberOne has a Double value
Else
  'code for a failure
  'numberOne = Nothing
End If

如果文本为空，则结束代码块：

If String.IsNullOrWhiteSpace(numberOneInput.Text) Then 
   MessageBox.Show("Value is incorrect format")
   Exit Sub
End If