Question

我有一个使用Visual Studio 2013用Visual Basic编写的单位转换器。程序运行正常，直到用户输入零（0）或以小数（。）开头。我收到此错误消息：从字符串“”到“Decimal”类型的转换无效。如何让程序接受零点和/或小数点作为用户的初始输入而不会导致程序崩溃？代码如下。

Private Function GetLength1(ByVal decLengthUnit1 As Decimal) As Decimal

    Dim decResult1 As Decimal

    If cboUnitType.SelectedItem = "Length" Then

        ' converts kilometer to...
        If cbo1.SelectedItem = "Kilometer" Then
            If cbo2.SelectedItem = "Kilometer" Then
                decResult1 = txtUnit1.Text
            ElseIf cbo2.SelectedItem = "Meter" Then
                decResult1 = (decLengthUnit1 * 1000)
            ElseIf cbo2.SelectedItem = "Centimeter" Then
                decResult1 = (decLengthUnit1 * 100000)
            ElseIf cbo2.SelectedItem = "Millimeter" Then
                decResult1 = (decLengthUnit1 * 1000000)
            ElseIf cbo2.SelectedItem = "Mile" Then
                decResult1 = (decLengthUnit1 * 0.621371191)
            ElseIf cbo2.SelectedItem = "Yard" Then
                decResult1 = (decLengthUnit1 * 1093.613297)
            ElseIf cbo2.SelectedItem = "Foot" Then
                decResult1 = (decLengthUnit1 * 3280.83989)
            ElseIf cbo2.SelectedItem = "Inch" Then
                decResult1 = (decLengthUnit1 * 39370.07868)
            End If
        End If
    End If

    If decResult1 < 0 Then
        txtUnit2.ForeColor = Color.Red
    ElseIf decResult1 >= 0 Then
        txtUnit2.ForeColor = Color.Green
    End If

    Return decResult1.ToString().Trim("0")
End Function


Private Sub txtUnit1_TextChanged(sender As Object, e As EventArgs) Handles txtUnit1.TextChanged

    If suppressTextBox1TextChanged = False Then

        ' convert string to numeric data type
        Decimal.TryParse(txtUnit1.Text, decUnit1)

        ' handle String.Empty, or negative sign
        If txtUnit1.Text = "" OrElse txtUnit1.Text = "-" Then
            txtUnit2.Text = ""

        ElseIf cboUnitType.SelectedItem = "Length" Then
            suppressTextBox2TextChanged = True
            txtUnit2.Text = GetLength1(decUnit1)
            suppressTextBox2TextChanged = False

        ElseIf cboUnitType.SelectedItem = "Pressure" Then


        ElseIf cboUnitType.SelectedItem = "Temperature" Then
            suppressTextBox2TextChanged = True
            txtUnit2.Text = GetTemp1(decUnit1)
            suppressTextBox2TextChanged = False

        End If
    End If

End Sub

Answer 1

使用System.Convert.ToDecimal()而不是仅仅将文本框值分配给Decimal变量，如下所示：

decResult1 = Convert.ToDecimal(txtUnit1.Text)

正如艾哈迈德在评论中所指出的那样，使用Decimal.TryParse()将允许你避免在演员表失败时System.Convert.ToDecimal()将抛出的异常，如下所示：

Dim number As Decimal

If Decimal.TryParse(txtUnit1.Text, number) Then
    ' Successfully cast text box text to a Decimal
Else
    ' Unable to cast text box text to a Decimal      
End If

更新：

以下是您的代码编辑：

Private Function GetLength1(ByVal decLengthUnit1 As Decimal) As Decimal
    Dim decResult1 As Decimal

    If cboUnitType.SelectedItem = "Length" Then
        ' converts kilometer to...
        If cbo1.SelectedItem = "Kilometer" Then
            If cbo2.SelectedItem = "Kilometer" Then
                Decimal.TryParse(txtUnit1.Text, decResult1)
            ElseIf cbo2.SelectedItem = "Meter" Then
                decResult1 = (decLengthUnit1 * 1000)
            ElseIf cbo2.SelectedItem = "Centimeter" Then
                decResult1 = (decLengthUnit1 * 100000)
            ElseIf cbo2.SelectedItem = "Millimeter" Then
                decResult1 = (decLengthUnit1 * 1000000)
            ElseIf cbo2.SelectedItem = "Mile" Then
                decResult1 = (decLengthUnit1 * 0.621371191)
            ElseIf cbo2.SelectedItem = "Yard" Then
                decResult1 = (decLengthUnit1 * 1093.613297)
            ElseIf cbo2.SelectedItem = "Foot" Then
                decResult1 = (decLengthUnit1 * 3280.83989)
            ElseIf cbo2.SelectedItem = "Inch" Then
                decResult1 = (decLengthUnit1 * 39370.07868)
            End If
        End If
    End If

    If decResult1 < 0 Then
        txtUnit2.ForeColor = Color.Red
    ElseIf decResult1 >= 0 Then
        txtUnit2.ForeColor = Color.Green
    End If

    Return decResult1.ToString().Trim("0")
End Function

用户输入为零或十进制？

1 个答案: