Question

I have a question about ThreadStart delegates in vb.net. I am taking over a senior programmer’s project, and he is much more experienced then myself so I’m kind of lost on what he was trying to do. I am trying to do some research on Threading but there is a lot out there on this topic and don’t want to waste my time with stuff that doesn’t pertain to my error here. I will post the whole exception message and hopefully someone can point me in the direction of an article that I could read to learn more about it.

System.InvalidOperationException was unhandled
  Message=The thread was created with a ThreadStart delegate that does not accept a parameter.
  Source=mscorlib
  StackTrace:
       at System.Threading.Thread.Start(Object parameter)
       at LabelLibrary.LabelPrinter.Print(PrintQueue queue) in C:\Documents and Settings\bjorandb\Desktop\LabelPrintingService\LabelTemplates\clsLabelPrinter.vb:line 94
       at LabelLibrary.LabelPrinter.Print() in C:\Documents and Settings\bjorandb\Desktop\LabelPrintingService\LabelTemplates\clsLabelPrinter.vb:line 53
       at PrintApplyApplication.HomeController.PrintThread.Print() in C:\Documents and Settings\bjorandb\Desktop\LabelPrintingService\PrintApplyApplication\Controllers\HomeController.vb:line 85
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.runTryCode(Object userData)
       at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Here is the function that the error is getting thrown in, but I don’t know how much help it will be by itself.

Public Function Print(ByVal queue As PrintQueue) As PrintResult
    Dim result As PrintResult = Nothing
    Dim job As PrintSystemJobInfo = Nothing
    Dim thread As Threading.Thread = Nothing

    result = New PrintResult
    If queue Is Nothing Then
        result.Status = "Can not find printer " & queue.Name & "."
    ElseIf PrintFilePath Is Nothing Then
        result.Status = "Print File Path Cannot be nothing."
    Else
        Try
            thread = New Threading.Thread(AddressOf AddJob)
            thread.SetApartmentState(Threading.ApartmentState.STA)
            thread.Start(queue) <--- (Exception is being thrown here!!)
            result.Status &= "Print Sent Successfully"
        Catch ex As PrintJobException
            If ex.InnerException.Message = "File contains corrupted data." Then
                result.Status &= "Could not generate a label with given xps file. Check the xps file format and if it is corrupted."
            End If
            result.Status &= "There was an error printing."
            result.ErrorMessage = ex.Message
        End Try
        If queue.IsOutOfPaper Then
            result.Status &= "The printer is out of Paper."
        End If
        If queue.IsPaperJammed Then
            result.Status &= "The Printer is jammed."
        End If
        If (queue.IsOutOfMemory) Then
            result.Status &= "The Printer is out of memory."
        End If
    End If
    Return result
End Function

And here is the AddJob method

Private Sub AddJob()
    Dim job As PrintSystemJobInfo = Nothing

    Try
        job = LocalPrintServer.GetDefaultPrintQueue.AddJob("Text", PrintFilePath, False)
        job.Refresh()
        While Not (job.IsCompleted Or job.IsDeleted)
            job.Refresh()
        End While
    Catch ex As PrintJobException
    End Try
    If System.IO.File.Exists(PrintFilePath) Then
        System.IO.File.Delete(PrintFilePath)
    End If
End Sub

Answer 1

Without the code I can only speculate that you’re creating the thread using ThreadStart and then calling the overload of Start that takes a parameter.

You should either use ParameterizedThreadStart or simply call the Start method with no parameters.

Edit: Like I said, the thread start method call is not compatible with the definition of AddJob. Simply use Thread.Start().