in vb.net can you make the combo box only composed of some items like you can't type in it?

I am working on a project of a ticket reservation system. I have a submission button that also works as data validation for some inputs like number of tickets chosen and email ,...
I want to make the user is only able to choose one of the items from a combo box, or make one item default because when I open the form while inputing data you can still write anything in the combo box

If airportlist.Text = "" Then
    problems &= "**Airport not selected**" & vbCrLf
Else
    var_of_aeroport = airportlist.Text
End If

Private Sub airportlist_SelectedIndexChanged(sender As Object, e As EventArgs) Handles airportlist.SelectedIndexChanged
    airportlist.DropDownStyle = ComboBoxStyle.DropDownList
End Sub

I tried this but the problem is when you haven't selected an item yet.

You can set the DropDownStyle property of your ComboBox control in Design mode (when laying out the controls on your form/user control).

Also, in Design mode, you can populate the items in the list by editing the Items property of the ComboBox.

Finally, you can use the ComboBox's SelectedIndex property to preset the item you want it to show by default. Here, in my Form's Load event, I set ComboBox1's SelectedIndex property to 0 (first item):

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
    ComboBox1.SelectedIndex = 0
End Sub

When the ComboBox is shown, the first item is selected.

This answer is a little more than you may be looking for but I hope it provides some learning Here is what the form looks like just consider the Save New Link
And understand I like my own Error Messages I call them Alerts and they are made with Show Dialog forms OK here is the form and the code If you need more help with the code flow feel free to expand your question

Here is the code for adding the data to the database when you click
Save New Link

 Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click

    If cbType.Items.Contains(cbType.Text) Then
        'Combobox Contains a Match
    Else
        gvalertType = "15"
        frmAlert.ShowDialog()
        cbType.Text = ""
        cbType.DroppedDown = True
        cbType.Focus()
        Return
    End If

    If tbSiteName.Text = "" Then
        gvalertType = "2"
        frmAlert.ShowDialog()
        tbSiteName.Focus()
        Return
    End If

    If strType = "" Then
        gvalertType = "1"
        frmAlert.ShowDialog()
        cbType.Text = ""
        cbType.DroppedDown = True
        cbType.Focus()
        Return
    End If

    If tbUrl.Text = "" Then
        gvalertType = "3"
        frmAlert.ShowDialog()
        tbUrl.Focus()
        Return
    End If

Here is all the code in the frmAlert

Public Class frmAlert

Private Sub frmAlert_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    If gvalertType = "1" Then
        tbAlert.Text = vbCrLf & "Select A  Site Type"
    ElseIf gvalertType = "2" Then
        tbAlert.Text = vbCrLf & "Enter the Site Name"
    ElseIf gvalertType = "3" Then
        tbAlert.Text = vbCrLf & "Enter the Site URL"
    ElseIf gvalertType = "4" Then
        tbAlert.Text = vbCrLf & "No Clicking Here" & vbCrLf & "Click on Site Url "
    ElseIf gvalertType = "5" Then
        tbAlert.Text = vbCrLf & "Cell Is Empty" & vbCrLf & "Click on Site Url "
    ElseIf gvalertType = "6" Then
        tbAlert.Text = vbCrLf & "Site Type Can't be EMPTY" & vbCrLf & "Enter Site Type to Save New Type"
    ElseIf gvalertType = "7" Then
        tbAlert.Text = vbCrLf & "Site Type Can't be EMPTY" & vbCrLf & "Select a Current Type to Delete"
    ElseIf gvalertType = "8" Then
        tbAlert.Text = vbCrLf & "You are Creating DUPLICATES" & vbCrLf & "NO DUPLICATES"
    ElseIf gvalertType = "9" Then
        tbAlert.Text = vbCrLf & "Check the URL it is" & vbCrLf & "NOT Well Formed"
    ElseIf gvalertType = "15" Then
        tbAlert.Text = vbCrLf & "That Site Type NOT Available" & vbCrLf & "Select One of These Site Types"

    End If
End Sub

Private Sub btnOK_Click(sender As Object, e As EventArgs) Handles btnOK.Click
    Close()
End Sub

End Class

Here is what the Alert Form looks like