System.Diagnostics.Process.Start() strange behaviour

Question

I have an application that uses Process.Start(string Filename) to open up a file. This method overload will cause the system to invoke the appropriate application based on the filename extension. In my case, it will typically be a WORD, PPT, PDF, JPG etc... Some type of viewable document. Ultimately, I need to launch the process and then later in the application I need to do something with the MainWindowHandle value.

I've found at least three different cases that cause Process.Start(string filename) to return different ways...

For the first and second cases below, assume I'm opening up two .pdf files (but the same thing appears to happen if I was opening two .ppt or two .doc files as well)...

Case 1: If AcroRd32 isn't running and I do something like..

Process p = Process.Start("yada.pdf");
p.WaitForInputIdle();
p.Refresh();

things work as expected. The value p.MainWindowHandle is populated correctly. No problem with this case.

Case 2: Now assume at the time of the call to Process.Start() AcroRd32 is already running on a previously opened pdf file. Now things get weird. Referring to the code below (some error-checking logic removed for clarity), after the call to Process.Start() the value of p.MainWindowHandle is zero (even though the window is created), and p.MainWindowTitle is empty. Then I sleep for 1 second and the handle is still zero, but the mainwindowtitle is now populated (even though I did NOT call p.Referesh() after the sleep. Each of the trace statements that have "<<<<" in the comments are printed at runtime.

Process p = Process.Start("SomeFileName.pdf");
p.WaitForInputIdle();
p.Refresh();
if (p.MainWindowHandle == 0)
    DebugTrace("MainWindowHandle is zero, why??");   //<<<<
if (p.MainWindowTitle.Length == 0)
    DebugTrace("MainWindowTitle is null");           //<<<<
Thread.Sleep(1000);
if (p.MainWindowHandle == 0)
    DebugTrace("MainWindowHandle is still zero.");   //<<<<
if (p.MainWindowTitle.Length == 0)
    DebugTrace("MainWindowTitle is null");
else
    DebugTrace("MainWindowTitle: " + p.MainWindowTitle);  //<<<<

I'm sure this has something to do with the fact that AcroRd32 is already running, but I have no control over that and I do need to get the value of p.MainWindowHandle. Any ideas how to handle this?

Case 3: Then onto the third case: in some cases, Process.Start() will return null even when it successfully opens the file. I found this to be the case with .jpg files, but I'm sure that just depends on the application that has been assigned to the .jpg extension. The Process.Start("file.jpg") returns null if the application is "Windows Photo Viewer" but if I change that to be "Paint" it doesn't return null. What's up with that? And how can I get the handle then?

Ok, all done, sorry for the detail, but hopefully I'm explaining the situation(s) I'm trying to work through!

Answer 1

I suspect that windows start a new process with acroRd32, which then passes along the given filepath to the running instance and then shuts down.

This is how it is usually done for avoiding multiple instance of a program...