Why is this variable constantly resetting? - vb.net

Question

I'm working on a program in VB.net that uses the form load event to prompt for password. I have this working, but I am supposed to be showing the attempt number you are at if you fail. However, my code is always returning "Attempt #1" and never increasing to #2, #3, etc, and I am unsure why it is constantly being reset.
Private Function checkPassword(passwordGuess As String)
Dim isValid As Boolean = False
Dim password As String = "941206"
Dim attemptCounter As Integer
If isValid = False Then
If password <> txtPassword.Text Then
attemptCounter += 1
MessageBox.Show("Login Unsuccesful.",
"Attempt #" & attemptCounter)
Else
isValid = True
MessageBox.Show("Login Successful.",
"Attempt #" & attemptCounter)
Me.Text = "Attempt #" & attemptCounter
End If
End If
End Function
Private Sub btnConfirm_Click(sender As Object, e As EventArgs) Handles btnConfirm.Click
Dim password As String
password = txtPassword.Text
checkPassword(password)
End Sub

Answer 1

You could create a class to store information about attempts that persists across function calls:
Public Class attempt
Public Shared counter As Integer = 0
End Class
Private Function checkPassword(passwordGuess As String)
Dim isValid As Boolean = False
Dim password As String = "941206"
If isValid = False Then
If password <> txtPassword.Text Then
attempt.counter += 1
MessageBox.Show("Login Unsuccesful.",
"Attempt #" & attempt.counter)
Else
isValid = True
MessageBox.Show("Login Successful.",
"Attempt #" & attempt.counter)
Me.Text = "Attempt #" & attempt.counter
End If
End If
End Function
Private Sub btnConfirm_Click(sender As Object, e As EventArgs) Handles btnConfirm.Click
Dim password As String
password = txtPassword.Text
checkPassword(password)
End Sub