vb.net - 错误是从字符串“”到类型“双”的转换无效

Question

Private Sub btntambah_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btntambah.Click
    Dim n As Integer
    For n = 0 To lv1.Items.Count - 1
        If lv1.Items(n).Text = txtkdbrng.Text Then
            MsgBox("Data Buku Sudah Ada Dalam List")
            Exit Sub
        End If
    Next
    lv1.Items.Add(txtkdbrng.Text)
    lv1.Items(n).SubItems.Add(txtnmbrng.Text)
    lv1.Items(n).SubItems.Add(txtharga.Text)
    lv1.Items(n).SubItems.Add(txtjmlhhrg.Text)
    lv1.Items(n).SubItems.Add(txttotal.Text)
    lv1.Items(n).SubItems.Add(txtjmlpsn.Text)
    lv1.Items(n).SubItems.Add(txtspesifikasi.Text)
    txttotal.Text = Format(CDbl(Total()), "###, ###, ###")
    ClearBarang()
    txtkdbrng.Focus()
End Sub

Function Total() As Double
    Dim ttl As Double = 0
    If Not Double.TryParse(txttotal.Text, Total) Then
        Total = 0
    End If
End Function

Private Sub btncetak_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btncetak.Click
    If txtnosph.Text = "" Or lv1.Items.Count = 0 Then
        MsgBox("Data Belum Lengkap", MsgBoxStyle.Information, "INFORMASI")
    Else
        Dim nilaikembali1, nilaikembali2 As Integer
        objsph.PNoSph = txtnosph.Text
        objsph.PTglSph = Format(dtpsph.Value, "yyy-MM-dd")
        objsph.PKdCs = txtkdcstmr.Text
        nilaikembali1 = objsph.Simpan()
        'menyimpan ke tabel Ada menggunakan perulangan
        For x As Integer = 0 To lv1.Items.Count - 1
            objada.PNoSph = txtnosph.Text
            objada.PKdBrg = lv1.Items(x).SubItems(1).Text
            objada.PKdBrg = CDbl(lv1.Items(x).SubItems(2).Text)
            objada.PKdBrg = CDbl(lv1.Items(x).SubItems(3).Text)
            nilaikembali2 = objada.Simpan()
            Dim objbarang As New ClsBarang
            objbarang.PKdBrg = lv1.Items(x).SubItems(0).Text
        Next
        If nilaikembali1 = 1 And nilaikembali2 = 1 Then
            MsgBox("Data Berhasil Disimpan", MsgBoxStyle.Information, "INFORMASI")
        End If
        ClearForm()
        ClearBarang()
        ClearCustomer()
        txtnosph.Text = objsph.AutoNumber
    End If
End Sub

Answer 1

问题在于这段代码：

Function Total() As Double
    Dim ttl As Double = 0
    If Not Double.TryParse(txttotal.Text, Total) Then
        Total = 0
    End If
End Function

首先，您使用的是与函数名称相匹配的不太知名的 VB6 样式隐式返回变量，而是使用Double在您的代码中声明的命名ttl并返回值，如下所示：

Function Total() As Double
    Dim ttl As Double = 0
    If Double.TryParse(txttotal.Text, ttl) Then
        Return ttl
    Else
        ' Attempted conversion of text to Double type failed
        ' Do something here, raise error, alert user, etc.
        ' Returning zero may or may not be acceptable, if it is return zero
        Return 0
    End If
End Function

使用该Return语法会产生更清晰的代码，因为它不会假设您的代码读者知道与函数概念名称匹配的隐式变量。始终Return在Function.

注意：如果总值为零并不表示存在问题，那么您将需要更改函数以返回Nullable(Of Double)而不是仅仅返回Double，然后您可以Else将结果的一部分更改为TryParse()：

Function Total() As Nullable(Of Double)
    Dim ttl As Double = 0
    If Double.TryParse(txttotal.Text, ttl) Then
        Return ttl
    Else
        ' Attempted conversion of text to Double type failed
        ' So return null
        Return Nothing
    End If
End Function

现在你的Total函数的调用者需要考虑Nothing被返回并做出相应的反应。

Answer 2

这意味着显式或隐式转换/转换为 double 的变量实际上是一个空字符串，无法继续。