[VB.NET 2008] How to write in a textbox from another thread

steve_9496613

Hi everybody, I have a simple project with a form and a class in another file to manage the COM port (from the article "Smart Device Print Engine for Compact Framework" by Orkun GEDiK). I open the COM port and I start its thread for receiving data. I want to write the received data in a textbox in the form. To do this, every byte I receive, I assign the value of this byte to a byte variable of the form called byt and I call from inside the loop of the thread a function in the form that stores all the bytes in a string dati (in hexadecimal format):

Public Sub ScriviTesto2()
Dim str As String

If InvokeRequired Then
  Invoke(New Action(AddressOf ScriviTesto2))
  Return
End If

Try
  str = Hex(byt)
  dati &= str & " "
Catch ex As Exception
  MessageBox.Show(ex.ToString)
End Try

End Sub

This seems to work because if I run the code step by step I see that the string dati grows byte by byte and contains the correct values. When there are no more bytes to receive, always from inside the loop of the thread, I call another function of the form that should write my string in a text box:

Public Sub ScriviTesto3()

If InvokeRequired Then
  Invoke(New Action(AddressOf ScriviTesto3))
  Return
End If

TB1.Text &= dati & vbCrLf

End Sub

...this function write nothing in the text box... why? The string data contains the right values, is not empty. I tried another way: I don't call the last funcion (ScriviTesto3()) but, when there are no more bytes to receive, I click a button of the form that do the same:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
TB1.Text &= dati & vbCrLf
End Sub

In this case, when I click the button the string dati is empty... I don't know why. I add the declaration of the involved variables:

Public Class Form1
Public byt As Byte
Protected dati As String
...

Can anyone help me? Thanks in advance.

Lost User

steve_9496613 wrote:

I call another function of the form that should write my string in a text box:

Pass the data to display there as a parameter.

steve_9496613 wrote:

In this case, when I click the button the string dati is empty... I don't know why.

Access to the form's field isn't thread-safe. You could lock it in a property, but that slow down things nicely. Hence, I'd suggest to create a list of bytes on your receiver-thread, and to pass this as a paramater to the form, say, once a second.

Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]

steve_9496613

Thanks Eddy. I have modified the first function as you suggested to do for the second one:

Public Sub ScriviTesto2(ByVal dato As Byte)
Dim str As String

If InvokeRequired Then
  Invoke(New Action(Of Byte)(AddressOf ScriviTesto2), dato)
  'Invoke(New Action(AddressOf ScriviTesto2))
  Return
End If

Try
  'str = Hex(byt)
  'dati &= str & " "
  str = Hex(dato)
  dati &= str & " "
  TB1.Text &= Hex(dato) & " "
  'TB1.Text = dati
Catch ex As Exception
  MessageBox.Show(ex.ToString)
End Try

End Sub

In the receiver thread I call this function every byte I receive, passing to it the byte just received. As before the string dati grows byte by byte so I can say that I correctly pass the parameter dato to this function but nothing is written in the text box TB1. Writing byte by byte in a textbox is not the fastest way but I just wanted to see if passing data as parameter to the function could be the solution. Perhaps the text box is not the right objet to use or, more likely, I'm missing something important. I did another try: as you suggested I created a byte array in the receiver thread, I fill it with the bytes received and at the end of data reception I call another function in form1 passing to it the byte array:

Public Sub ScriviTesto4(ByVal datiIn As Byte())
Dim i As Int32

If InvokeRequired Then
  Invoke(New Action(Of Byte())(AddressOf ScriviTesto4))
  Return
End If

For i = 0 To (datiIn.Count - 1)
  TB1.Text &= Hex(datiIn(i)) & " "
Next
TB1.Text &= vbCrLf

End Sub

Running step by step in debug I see that the array datiIn contains the right bytes and also the TB1.Text contains the right values but I see nothing in the text box in the form! What I'm doing wrong?

Lost User

steve_9496613 wrote:

Perhaps the text box is not the right objet to use or, more likely, I'm missing something important.

It's not optimal, but it should display some text. That's what it's there for.

steve_9496613 wrote:

Running step by step in debug I see that the array datiIn contains the right bytes and also the TB1.Text contains the right values but I see nothing in the text box in the form!

Did you create the textbox on the mainthread?

For i = 0 To (datiIn.Count - 1)
string x = Hex(datiIn(i)) & " "
TB1.Text = TB1.Text + x
Next

Can you write the string x to the debugger?

Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]

steve_9496613

Eddy Vluggen wrote:

Did you create the textbox on the mainthread?

I added the textbox to the form1 in the designer, it should be in the main thread. In form1 I declared an instance of the class to manage the COM:

Dim NewCom As CommDev

In the Load event of the form I create the new object:

NewCom = New CommDev()

In the New function of the class some parameters are set, like baud rate, parity, etc. When I open the COM port, I call an Init function that opens the COM port, sets buffer sizes, sets the device control block, sets timeouts, creates and starts the receiver thread.

Eddy Vluggen wrote:

Can you write the string x to the debugger?

I used your code instead of mine and I can see the value of the string x, I mean that I can "add an expression control" (I don't know if this is the right translation of the italian "Aggiungi espressione di controllo" that appear in the right click menu over a variable during debug) to the string x and see its right value in the bottom windows of Visual Studio. Nothing appears in the textbox... I made also another try:

Public Sub ScriviTesto4(ByVal datiIn As Byte())
Dim i As Int32

If InvokeRequired Then
  Invoke(New Action(Of Byte())(AddressOf ScriviTesto4))
  Return
End If

For i = 0 To (datiIn.Count - 1)
  TB1.Text &= Hex(datiIn(i)) & " "
Next
ScriviLabel(TB1.Text)  'write the textbox text to a label in the form
TB1.Text &= vbCrLf

End Sub

Private Sub ScriviLabel(ByVal stringa As String)
Label1.Text = stringa
End Sub

What I can see in the debug is that TB1.Text, stringa and Label1.Text all contains the right values but nothing appear in the window of my application. It seems to be a "visualization" problem, perhaps related to threads, but I'm not able to see the error.

Lost User

Can you add another label on the form?

Private Sub ScriviLabel(ByVal stringa As String)
Label1.Text = "Hello world"
Label2.Text = stringa
End Sub

This is the point where I'd paranoidly check the colour of my font.

Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]

steve_9496613

Hi Eddy, thanks for your efforts. I add the second label but no label takes the right content in the form; in the debug the Text properties contain my string and the "Hello world" string. I also call the Refresh method of labels and form but nothing changes. Before I did another project where I used the SerialPort object of .NET, everything was in the form1 and I used the DataReceived event of the COM to write in the textbox the received data, always using the Invoke method, and everything was OK. In this project I don't use the SerialPort object but the API in coredll.dll (CreateFileW, ReadFile, SetCommMask, etc.) because I need to catch different information from the COM port and all this is in a separate class/file. This is the main difference between the two projecs, in addition to the fact that one works and the other not... Sorry but I really don't know what is THE fundamental information I'm missing to solve this problem, if you have any questions, please feel free to ask.