Why .net program on linux under mono fails to run?

Question

I ran my Delphi-prism (.NET) program on Linux under Mono. It ran for awhile and crashed with the following error message on the terminal. But the same program runs perfectly fine on Windows 7

Can anyone tell me why?

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object
  at System.Windows.Forms.XEventQueue+PaintQueue.Dequeue () [0x00000] in <filename unknown>:0
  at System.Windows.Forms.XplatUIX11.GetMessage (System.Object queue_id, System.Windows.Forms.MSG& msg, IntPtr handle, Int32 wFilterMin, Int32 wFilterMax) [0x00000] in <filename unknown>:0
  at System.Windows.Forms.XplatUI.GetMessage (System.Object queue_id, System.Windows.Forms.MSG& msg, IntPtr hWnd, Int32 wFilterMin, Int32 wFilterMax) [0x00000] in <filename unknown>:0
  at System.Windows.Forms.Application.RunLoop (Boolean Modal, System.Windows.Forms.ApplicationContext context) [0x00000] in <filename unknown>:0
  at System.Windows.Forms.Application.Run (System.Windows.Forms.ApplicationContext context) [0x00000] in <filename unknown>:0
  at System.Windows.Forms.Application.Run (System.Windows.Forms.Form mainForm) [0x00000] in <filename unknown>:0
  at Millennia.Program.Main (System.String[] args) [0x00000] in <filename unknown>:0

Answer 1

It looks like your program is using Windows Forms. Support for Windows Forms is not the best in mono. Mono provides the runtime and libraries, but if you want to make a cross-platform program you need to consider the UI libraries you make use of as well.

Update (2016):

My original answer is probably wrong. According to this page, support for Windows Forms 2.0 is complete, but there may still be a number of bugs, I haven't tried it. However if you use a third-party UI library, for example Infragistics, they probably won't work due to native calls.

Have a look at this page for more info on porting Winforms apps, including the tool MoMA that can help.

Answer 2

I had the same problem (.net, Linux, Mono). My program has a provider-subsrciber pattern. The provider is on another thread. The main form is subscribed to the provider. When it is fired:

    private void RefreshLabels(ParameterMap pm)
    {
        string StateValueKey = string.Empty;
        string svkValue = string.Empty;

        if (InvokeRequired)
        {
            BeginInvoke(new RefreshItemsDelegate(RefreshLabels), new object[] { pm });
        }
        else
        {

..... it comes across from the other thread and does its job.

Another subscriber fired an event on the main form also. I ignored this event and the problem DISAPPEARED.