Hi, Consider following line will appending text in textbox while BackgroundWorker is running asynchronously. (Based on previous treads and regarding to jmcilhinney pointed BackgroundWorkers are for huge process-occupying things not UI, you have to make it a sort of pause or invoke to access UI update thing)
Code:
    Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
        While (CLIENT.Connected)
            Try
                RECEIVE = STREAM.ReadLine
                Me.Convtxt.Invoke(New MethodInvoker(Function()
                                                        Convtxt.AppendText("YOU: " + RECEIVE + vbNewLine)
                                                    End Function))
                RECEIVE = ""
            Catch ex As Exception
                MessageBox.Show(ex.Message.ToString())
            End Try
        End While
    End Sub
This code works with warnings on underlined line. But since I'm using Option Strict = On, Mentioned line throws an error:
Cannot infer a return type. Consider adding an 'As' clause to specify the return type.

How can I rectify it?