How to only accept certain values inside a textbox - vb.net

Question

I made a flashcard application where the user can change the difficulty by entering a number inside a textbox.
Sub UpdateDifficultyLevel(front As String, difficulty As Integer)
'The parameters are represented by question marks in the query
Dim sql = "UPDATE flashcards SET difficulty = ?
WHERE Front = ?"
'This using statement The Using statement makes sure that any "unmanaged resources" are released after they've been used.
Using conn As New OleDbConnection("provider=microsoft.ACE.OLEDB.12.0;Data Source=flashcard login.accdb"), 'Establish connection
cmd As New OleDbCommand(sql, conn)
cmd.Parameters.Add("#difficulty", OleDbType.Integer).Value = difficulty 'Updates database with parameters
cmd.Parameters.Add("#front", OleDbType.VarWChar).Value = front 'Updates database with parameters
conn.Open() 'Opens connection
cmd.ExecuteNonQuery() 'Executes
End Using
End Sub
Private Sub btnSubmit_Click(sender As Object, e As EventArgs) Handles btnSubmit.Click
Dim difficulty As Integer 'Sets difficulty as integer
If Integer.TryParse(TxtDifficulty.Text, difficulty) Then
Dim front = txtFront.Text 'Defines front as variable which is equal to txtfront.text
UpdateDifficultyLevel(front, difficulty) 'Calls subroutine
Else
MsgBox("Please enter a number between 1 and 3") ' tells user that the difficulty must be a number
End If
End Sub
This works where the user can only enter an integer but how would I make it so they can only enter an integer between 1 and 3

Answer 1

You can utilize ErrorProvider to feedback to the user that the input is wrong. And reuse the validate function to get the parsed integer when used (so the logic is only in one place)
Private errorMessage As String = "Please enter a number between 1 and 3"
Private Function validateInput(ByRef difficulty As Integer) As Boolean
Return Integer.TryParse(TxtDifficulty.Text, difficulty) AndAlso difficulty >= 1 AndAlso difficulty <= 3
End Function
Private Sub TxtDifficulty_TextChanged(sender As Object, e As EventArgs) Handles TxtDifficulty.TextChanged
If Not validateInput(Nothing) Then
ErrorProvider1.SetError(TxtDifficulty, errorMessage)
TxtDifficulty.SelectAll()
Else
ErrorProvider1.SetError(TxtDifficulty, "")
End If
End Sub
Private Sub btnSubmit_Click(sender As Object, e As EventArgs) Handles btnSubmit.Click
Dim difficulty As Integer 'Sets difficulty as integer
If validateInput(difficulty) Then
Dim front = txtFront.Text 'Defines front as variable which is equal to txtfront.text
UpdateDifficultyLevel(front, difficulty) 'Calls subroutine
Else
MsgBox(errorMessage) ' tells user that the difficulty must be a number
End If
End Sub
There are many other, possibly better, ways to do this however, but this sticks with your current design.

Answer 2

Use a NumericUpDown control instead, with its Increment set to 1, MinValue set to 1 and MaxValue set to 3. It looks just like a textbox and, bonus, you can change the number within using the arrow keys
You can also consider a MaskedTextBox, RadioButtons, ComboBox, ListBox, even 3 different buttons to start an Easy, Medium or Hard game..
A big part of effective UI design is in using tools designed for purpose and not bothering the user with an endless succession of error messages that they have to read, understand and apply corrections for. A great example of that gone wrong is a cellphone with a Symbian OS; they were renown for bothering the user with incessant messages.
If you have an opportunity to design a UI so the user simply can't get it wrong, rather than shouting at them when they do, take it; your iPhone doesn't present a Qwerty keyboard when you're dialling a number..