Needing Assistance with vb.NET Application - vb.net

Question

I don't normally post on forums because I try to find information for myself, and ask as an absolute last resort. I've tried scouring the net for answers, but I'm only receiving about half of the answer I'm looking for.
I'm currently building an application that deals with state law. There's one combo box and one text box. One for the offense title, and one for the numerical code for that particular code section. So say if I select "Kidnapping", it prepopulates the text box below it with "11-5-77", for example.
The method I've been using for, oh, about the last hour now, is:
If AWOffenseTitle.Text = "Kidnapping" Then
AWCN.Text = "11-5-77"
ElseIf AWOffenseTitle.Text = "False Imprisonment" Then
AWCN.Text = "11-5-78"
With AWOffenseTitle being the combo box name, and AWCN being the text box name. While this has proved to work perfectly well so far, I'm sure you can imagine with hundreds of offense titles, this is going to take a ridiculously long time. Well, I finally found a spreadsheet with offense titles and their respective title codes. What I'm looking to do is create two text files within a folder in the local directory "Offenses". One with a vertical list of offenses, and one with a vertical list of offense code numbers that populate the same lines in each. What I'm looking to do is populate the combo box with the contents of text file one (which I can do already), but then selecting an offense title will read the second text file and display it's proper title code. That's what has me at a loss. I'm relatively well-versed with vb.NET, but I'm not an expert by any means.
I'm hoping someone here will be able to provide a code example and explain it to me line-by-line so I can gain a better understanding. I want to get more proficient with VB although it's not so popular anymore. I've been using VB since 6.0, but not on a regular basis. More on a sporadic project kind of basis.
I really appreciate any assistance anyone might be able to provide, and if you need more information, I'd be glad to answer any questions. I tried to be as thorough as I could.
Thank you in advance!

Answer 1

First, you need to retrieve your data. I demonstrated using an Sql Server database containing a table named Offenses with columns named OffenseTitle and OffenseCode. You will have to change this code to match your situation.
Private Function GetOffenseData() As DataTable
Dim dt As New DataTable
Using cn As New SqlConnection("Your connection string"),
cmd As New SqlCommand("Select OffenseTitle, OffenseCode From Offenses;")
cn.Open()
dt.Load(cmd.ExecuteReader)
End Using
Return dt
End Function
As the Form loads, set the properties of the ComboBox. DisplayMember matches the name of the title column and ValueMember is the name of the code column.
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim dt = GetOffenseData()
ComboBox1.DisplayMember = "OffenseTitle"
ComboBox1.ValueMember = "OffenseCode"
ComboBox1.DataSource = dt
End Sub
Then when the selected item in the combo changes, just set the .Text property of TextBox to the SelectedValue in the combo and your code appears.
Private Sub ComboBox1_SelectionChangeCommitted(sender As Object, e As EventArgs) Handles ComboBox1.SelectionChangeCommitted
TextBox1.Text = ComboBox1.SelectedValue.ToString
End Sub
There are other ways to do this if your data source is other than a database. Please advise if you need additional help.

Answer 2

In addition to HardCode's comment and Mary's detailed answer, I can only add an answer that's somewhere in between them.
It might be the case, that the information is not taken from a database, but from another source, like a text/data file or a web service. So it might be useful to create an abstraction for the data source you actually use.
First, I create a class or struct that will hold the data for each combo box item.
Class Offense
Public ReadOnly Property Title As String
Public ReadOnly Property Code As String
Public Sub New(title As String, code As String)
Me.Title = title
Me.Code = code
End Sub
End Class
Next, you need a method that retrieves a list of offenses that you can bind to your combo box. It's entirely up to you how you fill/fetch the offenses list. I have simply hard coded your two values here.
Private Function GetOffenseData() As List(Of Offense)
Dim offenses As New List(Of Offense)
offenses.Add(New Offense("Kidnapping", "11-5-77"))
offenses.Add(New Offense("False Imprisonment", "11-5-78"))
Return offenses
End Function
At a certain moment (probably in your form's Load event handler), you need to initialize your combo box. Just like Mary did, I use data binding.
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
AWOffenseTitle.DropDownStyle = ComboBoxStyle.DropDownList
AWCN.ReadOnly = True
AWOffenseTitle.DisplayMember = NameOf(Offense.Title)
AWOffenseTitle.ValueMember = NameOf(Offense.Code)
AWOffenseTitle.DataSource = GetOffenseData()
End Sub
Note that I use the NameOf operator to get the desired property names of the Offense class. If you ever decide to rename the properties of your Offense class, you will be able to easily detect where they are used, since the compiler will complain if your code still uses the wrong property names somewhere.
Finally, the app needs to react to combo box value changes, so that the text box will show the corresponding offense code. Mary used an event handler for the SelectionChangeCommitted event, but I use a handler for the SelectedIndexChanged event instead:
Private Sub AWOffenseTitle_SelectedIndexChanged(sender As Object, e As EventArgs) Handles AWOffenseTitle.SelectedIndexChanged
AWCN.Text = AWOffenseTitle.SelectedValue
End Sub
(Up to now, I was not aware of the SelectionChangeCommitted event of the ComboBox control. I will need to look into this event to see if it is actually a better choice for this scenario, but I found that the SelectedIndexChanged event does the job just fine, so for now I sticked with that event, since I am more familiar with it.)