Hi there my program is a simple adding calculation.

I have some products that when ticked ask for a input box that ask for the quanity of the type needed.

The user then selects the calculate button to calculate the price of the selections

My program works fine when every item is selected (via tick boxes)

but crashes with the error "Conversion from string "" to type 'Double' is not valid. " when a selection is not ticked here is my code

i have been told that my conversion procedure is outdated but this is what my lecturer has taught us and his marking requirments need to keep using this old method

Code:
Public Class Form1 
 
    
    Private Sub CheckBox1_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox1.CheckedChanged 
        'declare variables 
 
        Dim network As Double 
 
        Dim network_num As Double 
 
        Dim network_total As Double 
 
        'Make Text boxes Visable when ticked 
 
        textbox_network_no.Visible = True 
 
        TextBox_netcost.Visible = True 
 
        TextBox_networksub.Visible = True 
 
        'Error trapping 
 
        Try 
 
            'Inout box displayed to ask how much network cards are needed 
 
            network = InputBox("How Many 100Mbit Network Cards Would You Like?") 
 
        Catch ex As Exception 
 
            'simple message 
 
            MsgBox("sorry could not accept the input") 
 
            'repeat error message 
 
            network = InputBox("How Many 100Mbit Network Cards Would You Like?") 
 
        End Try 
 
        'set network_num to 35 
 
        network_num = CDbl(35) 
 
        'display network num in textbox 
 
        TextBox_netcost.Text = CDbl(network_num) 
 
        'display ammount ordered in textbox 
 
        textbox_network_no.Text = CDbl(network) 
 
        'calculation for network_total 
 
        network_total = network * network_num 
 
        TextBox_networksub.Text = CDbl(network_total) 
 
    End Sub 
 
    Private Sub CheckBox2_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox2.CheckedChanged 
 
        Dim hdd As Double 
 
        Dim hdd_num As Double 
 
        Dim hdd_total As Double 
 
        'Make Text boxes Visable when ticked 
 
        textbox_hdd_no.Visible = True 
 
        TextBox_hddcost.Visible = True 
 
        TextBox_hddsub.Visible = True 
 
        'Error trapping 
 
        Try 
 
            'Inout box displayed to ask how much network cards are needed 
 
            hdd = InputBox("How Many Hard Drives Would You Like?") 
 
        Catch ex As Exception 
 
            'simple message 
 
            MsgBox("sorry could not accept the input") 
 
            'repeat error message 
 
            hdd = InputBox("How Many Hard Drives Would You Like?") 
 
        End Try 
 
        'set hdd_num to 120 
 
        hdd_num = CDbl(120) 
 
        'display hdd num in textbox 
 
        TextBox_hddcost.Text = CDbl(hdd_num) 
 
        'display ammount ordered in textbox 
 
        textbox_hdd_no.Text = CDbl(hdd) 
 
        'calculation for hdd_total 
 
        hdd_total = hdd * hdd_num 
 
        TextBox_hddsub.Text = CDbl(hdd_total) 
    End Sub 
 
    Private Sub CheckBox3_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox3.CheckedChanged 
 
        Dim MOT As Double 
 
        Dim MOT_num As Double 
 
        Dim MOT_total As Double 
 
        'Make Text boxes Visable when ticked 
 
        textbox_MOT_no.Visible = True 
 
        TextBox_MOTcost.Visible = True 
 
        TextBox_MOTsub.Visible = True 
 
        'Error trapping 
 
        Try 
 
            'Inout box displayed to ask how much MOT are needed 
 
            MOT = InputBox("How MOT Health check Would You Like?") 
 
        Catch ex As Exception 
 
            'simple message 
 
            MsgBox("sorry could not accept the input") 
 
            'repeat error message 
 
            MOT = InputBox("How MOT Health check Would You Like?") 
 
        End Try 
 
        'set MOT_num to 39 
 
        MOT_num = CDbl(39) 
 
        'display MOT num in textbox 
 
        TextBox_MOTcost.Text = CDbl(MOT_num) 
 
        'display ammount ordered in textbox 
 
        textbox_MOT_no.Text = CDbl(MOT) 
 
        'calculation for MOT_total 
 
        MOT_total = MOT * MOT_num 
 
        TextBox_MOTsub.Text = CDbl(MOT_total) 
    End Sub 
 
    Private Sub Btn_calculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Btn_calculate.Click 
        Dim MOT_total As Double 
        Dim hdd_total As Double 
        Dim network_total As Double 
        Dim total As Double 
 
        MOT_total = CDbl(TextBox_MOTsub.Text) 
        hdd_total = CDbl(TextBox_hddsub.Text) 
        network_total = CDbl(TextBox_networksub.Text) 
 
        total = MOT_total + hdd_total + network_total 
 
        TextBox_total.Text = CDbl(total) 
 
    End Sub 
End Class
thank you.

Im new to VB so id imagine ther are 1000000 mistakes