Question

Is it possible to print in Silverlight without blocking the UI thread?

I have to print a lot of pages, and consequently my UI freezes for a while. I would like to print on a background thread instead (and update a progress bar if possible), but can’t figure out how.

I tried calling the Print() method of my PrintDocument inside the DoWork() method of a BackgroundWorker, but that gives me an UnauthorizedAccessException "Invalid cross-thread access".

It seems that even initiating a PrintDocument is not possible outside the UI thread:
PrintDocument pd = new PrintDocument(); in BackgroundWorker.DoWork() throws the same exception.

I found a lot of posts where people say that printing has to happen on the UI thread, but no documentation of this. I’m new to multithreading, so any pointers in the right direction would be appreciated.

Answer 1

I had this problem and came across this question which unfortunately didn’t have the answer i was hoping for. But I thought that for anyone else who comes across this problem, this may at least shed some light.

I was following this article on printing in Silverlight, It works like a charm for regular printing on the UI Thread, but for actually trying to print on a separate thread I don’t think it’s possible. I switched out the last line of code in the example

printDocument.Print("SLPrintDemo document");

with an alternate one to see if it would work

new Thread(() => printDocument.Print("SLPrintDemo document")).Start();

To see if the print job itself would spawn in a separate thread. However though the code compiles and runs fine, the document does not print. The reason seems to be that once the Print command is fired, it then fires up the printing options dialog for choosing printer and other options etc. At this point it is no longer on the UI thread so nothing happens ( No exceptions, so i’m assuming they’re swallowed somewhere)

So as far as I can tell at the moment, there is no way to print in Silverlight that is not in the UI thread.