Small vb.net app error: InvalidOperationException was unhandled

I have a VB.NET Windows form that has two buttons and a textbox.
One button will just open and read a text file and perform an Asynchronous operation and display text. (So, I've just written the routine for the buttonReadAsync_Click event and need to get the error resolved before moving onto the second button)
Second button will open and readt a text file and perform a Synchronous operation.

I'm still working on how to understand threading and don't quite have the hang of it yet. I've described what the error is below and where it happens in my code.
I would appreciate an explanation on what I missed and why it threw this error.
Thanks,
Wallace

Option Explicit On
Option Strict On
 
Imports System.IO
Imports System.Text
Imports System.Threading
 
Public Class MainForm
 
    Dim fs As System.IO.FileStream
    Dim fileContents As Byte()
    Dim callBack As AsyncCallback
 
    Private Sub buttonReadAsync_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles buttonReadAsync.Click
 
        OpenFileDialog1.ShowDialog()
        callBack = New AsyncCallback(AddressOf fs_StateChanged)
        fs = New System.IO.FileStream(OpenFileDialog1.FileName, _
            IO.FileMode.Open, IO.FileAccess.Read, IO.FileShare.Read, 4096, IO.FileOptions.None)
        ReDim fileContents(CInt(fs.Length))
        fs.BeginRead(fileContents, 0, CInt(fs.Length), callBack, Nothing)
    End Sub <-- AFTER THIS LINE IS PROCESSED IT THROWS THE ERROR "Cross-thread operation not valid: Control 'textBoxResults' accessed from a thread other than the thread it was created on.
 
    Private Sub fs_StateChanged(ByVal AsyncResult As IAsyncResult)
        If AsyncResult.IsCompleted Then
            textBoxResults.Text = Encoding.UTF8.GetString(fileContents)
            fs.Close()
        End If
    End Sub
End Class

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