Cast non valido dalla stringa "" al tipo 'Double'.

Imports System.Data.OleDb
Imports System.IO
Public Class Form1
    Public Function SommaPrezzi() As Double
        Dim i As Integer
        Dim totale As Double
        Dim totale2 As Double
        Dim null1 As String = ""

        For i = 0 To ListBox1.Items.Count - 1 'Se usi il Vb.net metti listaprezzi.items.count
            totale = totale + ListBox1.Items(i)
        Next
        SommaPrezzi = totale

        ListBox1.Items.Add(null1)
        ListBox1.Items.Add("Ore complessive")
        ListBox1.Items.Add(SommaPrezzi)
    End Function

    Public Function SommaPrezzi2() As Double
        Dim i As Integer
        Dim totale2 As Double
        Dim prodotto2 As Integer
        Dim null1 As String = ""

        For i = 0 To ListBox2.Items.Count - 1 'Se usi il Vb.net metti listaprezzi.items.count
            totale2 = totale2 + ListBox2.Items(i)
        Next
        SommaPrezzi2 = totale2


        ListBox2.Items.Add(null1)
        ListBox2.Items.Add("Ore complessive")
        ListBox2.Items.Add(SommaPrezzi2)
    End Function

    Public Function DataGridView() As Integer
        Dim somma As Integer
        Dim i As Integer

        For i = 0 To ListBox2.Items.Count - 1 'Se usi il Vb.net metti listaprezzi.items.count
           'Si frema qua----->'  somma = somma + ListBox2.Items(i)
        Next
        DataGridView = somma
    End Function
    Dim Conn As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0; Data Source=C:\Users\ingfr\Documents\Database1.accdb")
    Dim Conn2 As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0; Data Source=C:\Users\ingfr\Documents\Database1.accdb")
    Dim Conn3 As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0; Data Source=C:\Users\ingfr\Documents\Database1.accdb")
    Dim reader2 As OleDbDataReader
    Dim reader3 As OleDbDataReader
    Dim da As OleDbDataAdapter
    Dim cmb As OleDbCommandBuilder
    Dim reader As OleDbDataReader
    Dim ds As DataSet = New DataSet()
    Dim tables As DataTableCollection
    Dim dt As New DataTable
    Dim source1 As New BindingSource
    Dim bs As New BindingSource() ' BindingSource
    Dim SQLString As OleDbCommand
    Dim SQLString2 As OleDbCommand
    Dim SQLString3 As OleDbCommand

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        ListBox1.Items.Clear()
        SQLString = New OleDbCommand("SELECT * FROM bianca WHERE commessa='" + TextBox1.Text + "'", Conn)
        Conn.Open()
        reader = SQLString.ExecuteReader()
        While (reader.Read())
            ListBox1.Items.Add(reader.GetString(1))
        End While
        reader.Close()
        Conn.Close()

        ListBox2.Items.Clear()
        SQLString2 = New OleDbCommand("SELECT * FROM gaetano WHERE commessa='" + TextBox1.Text + "'", Conn2)
        Conn2.Open()
        reader2 = SQLString2.ExecuteReader()
        While (reader2.Read())
            ListBox2.Items.Add(reader2.GetString(1))
        End While
        reader2.Close()
        Conn2.Close()
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs)
        Call SommaPrezzi()
    End Sub

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    End Sub

    Private Sub TextBox2_TextChanged(sender As Object, e As EventArgs)

    End Sub

    Private Sub Button4_Click(sender As Object, e As EventArgs)

    End Sub

    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Gaetano.Click
        Dim cnn As New OleDbConnection("Provider= Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\ingfr\Documents\Database1.accdb")

        With cnn
            .ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\ingfr\Documents\Database1.accdb"
            .Open()
        End With


        Dim dt As New DataTable
        Dim da As New OleDb.OleDbDataAdapter("SELECT Costo_orario from Costi WHERE Nome='Gaetano'", cnn)

        da.Fill(dt)
        If dt.Rows.Count > 0 Then
            With ComboBox2
                .Items.Clear()
                For i As Integer = 0 To dt.Rows.Count - 1
                    .Items.Add(dt.Rows(i).Item("Costo_orario"))
                Next
                .SelectedIndex = -1
            End With
        End If

        With cnn
            .Close()
            .Dispose()
        End With
    End Sub

    Private Sub Button4_Click_1(sender As Object, e As EventArgs) Handles Bianca.Click
        Dim cnn As New OleDbConnection("Provider= Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\ingfr\Documents\Database1.accdb")

        With cnn
            .ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\ingfr\Documents\Database1.accdb"
            .Open()
        End With


        Dim dt As New DataTable
        Dim da As New OleDb.OleDbDataAdapter("SELECT Costo_orario from Costi WHERE Nome='Bianca'", cnn)

        da.Fill(dt)
        If dt.Rows.Count > 0 Then
            With ComboBox1
                .Items.Clear()
                For i As Integer = 0 To dt.Rows.Count - 1
                    .Items.Add(dt.Rows(i).Item("Costo_orario"))
                Next
                .SelectedIndex = -1
            End With
        End If

        With cnn
            .Close()
            .Dispose()
        End With
    End Sub

    Private Sub Button3_Click_1(sender As Object, e As EventArgs) Handles Button3.Click
        Dim prodotto As Integer
        Dim null1 As String = ""

        prodotto = SommaPrezzi() * ComboBox1.Text
        ListBox1.Items.Add(null1)
        ListBox1.Items.Add("Costo Complessivo")
        ListBox1.Items.Add(prodotto)

        Dim prodotto2 As Integer

        prodotto2 = SommaPrezzi2() * ComboBox2.Text
        ListBox2.Items.Add(null1)
        ListBox2.Items.Add("Costo Complessivo")
        ListBox2.Items.Add(prodotto2)

        Dim totale As Integer

        totale = prodotto + prodotto2
        ListBox3.Items.Add("Costo Totale Commessa")
        ListBox3.Items.Add(null1)
        ListBox3.Items.Add(totale)

        DataGridView1.Rows.Insert(0, "Gaetano", DataGridView)
    End Sub
End Class