The error is Conversion from string "" to type 'Double' is not valid - vb.net

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

The problem is with this code:
Function Total() As Double
Dim ttl As Double = 0
If Not Double.TryParse(txttotal.Text, Total) Then
Total = 0
End If
End Function
First off you are using the less well known VB6 style implicit return variable that matches the name of the function, instead use the declared Double in your code named ttl and return the value, like this:
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
Using the Return syntax produces much clearer code, in that it does not assume the reader of your code knows the implicit variable that matches the name of the function concept. Always use Return in a Function.
Note: If a total value of zero is not indicative of a problem, then you will need to alter your function to return Nullable(Of Double) instead of just Double and then you can alter the Else portion of the TryParse() result to this:
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
Now the caller of your Total function will need to account for Nothing being returned and react accordingly.

Answer 2

This means the variable being explicitly or implicitly casted/converted to double is actually an empty string and cannot be proceed.