ifselectedindex changed , show data in a textbox - vb.net

Question

I've linked an access database to my form.
I have 1 table , 2 rows
1 = Researchtype short text
2 = Researchdetails (long text)
In my combobox1 i've binded my researchtype row so i can choose a type of research.
Question now: how can i bind the details data to the richtextbox below it in order to show the research data as soon as i choose a research type?
I've tried if else combos, try catch combos,
i'm thinking i'm actually overthinking the issue here.
What would be the easiest way to "select from dropdown" and show the result in textbox.
I'm a vb.net beginner
Public Class Onderzoeken
Private Sub Onderzoeken_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'TODO: This line of code loads data into the 'PatientenDatabaseDataSetX.tbl_OnderzoeksTypes' table. You can move, or remove it, as needed.
Me.Tbl_OnderzoeksTypesTableAdapter.Fill(Me.PatientenDatabaseDataSetX.tbl_OnderzoeksTypes)
End Sub
Private Sub cboxOnderzoek_SelectedIndexChanged(sender As Object, e As EventArgs) Handles cboxOnderzoek.SelectedIndexChanged
If cboxOnderzoek.SelectedItem = Nothing Then
cboxOnderzoek.Text = ""
Else
rtbBeschrijvingOnderzoek.Text = CStr(CType(cboxOnderzoek.SelectedItem, DataRowView)("OZ_Onderzoeksbeschrijving"))
End If
End Sub
End Class
I added the entire code of that page now , it's not much, but as stated: I added the binding source and displaymember "researchtype" to the combobox.
So when i start the form, i can choose a type of research.
Now i need to show the description of the research in the richtextbox

Answer 1

In the Form.Load...
I have a function that returns a DataTable that contains columns called Name and Type. I bind the ComboBox to the DataTable and set the DisplayMember to "Name". Each Item in the ComboBox contains the entire DataRowView. I set the TextBox to the first row (dt(0)("Type")) Type column value so the correct information will be displayed for the initial selection.
I put the code to change the textbox display in ComboBox1.SelectionChangeCommitted because the other change events will produce a NRE since .SelectedItem has not yet been set when the form loads. The commited event will only occur when the user makes a selection.
First, cast the SelectedItem to its underlying type, DataRowView. Then you want the value of the Type column. This value is assigned to the text property of the textbox.
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim dt = LoadCoffeeTable()
ComboBox1.DataSource = dt
ComboBox1.DisplayMember = "Name"
TextBox1.Text = dt(0)("Type").ToString
End Sub
Private Sub ComboBox1_SelectionChangeCommitted(sender As Object, e As EventArgs) Handles ComboBox1.SelectionChangeCommitted
TextBox1.Text = DirectCast(ComboBox1.SelectedItem, DataRowView)("Type").ToString
End Sub
Just substitute Researchtype for Name and Researchdetails for Type.

Answer 2

After using 'OleDbDataAdapter' to fill the dataset, you can set 'DisplayMember' and 'ValueMember' for your ComboBox. Every time the index of your ComboBox changes, it's 'ValueMember' will be displayed in richtextbox.
Here's the code you can refer to.
Private dataset As DataSet = New DataSet()
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim connString As String = "your connection String"
Using con As OleDbConnection = New OleDbConnection(connString)
con.Open()
Dim cmd As OleDbCommand = New OleDbCommand()
cmd.Connection = con
cmd.CommandText = "SELECT Researchtype, Researchdetails FROM yourtable"
Dim adpt As OleDbDataAdapter = New OleDbDataAdapter(cmd)
adpt.Fill(dataset)
End Using
ComboBox1.DisplayMember = "Researchtype"
ComboBox1.ValueMember = "Researchdetails"
ComboBox1.DataSource = dataset.Tables(0)
End Sub
Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
RichTextBox1.Text = ComboBox1.SelectedValue.ToString()
End Sub
Result of my test.