启用EnableRaisingEvents时打开图片触发NullReferenceException（Process为null）求助

Let's break down exactly what's going on here:

First, remember how System.Diagnostics.Process.Start() works when you open a file (not an executable directly): it uses Windows' file association system to launch the default app for that file type. The critical issue here is the return value of Process.Start() isn't always a valid Process object—sometimes it returns null.

Why does this happen with images but not .csv/.docx/.pdf? #

• For files like .docx, .pdf, or .csv, the default apps (Word, Adobe Acrobat, Excel) almost always spin up a new process for each document you open. That means Process.Start() can hand back a valid Process instance you can hook events to.
• For most image file types, the default viewer (like Windows Photos, or even some third-party tools) might reuse an already-running process instead of creating a new one. When this happens, Process.Start() returns null because no new process was created—your image just loads in the existing viewer window.

That’s exactly why the line process.EnableRaisingEvents = true throws a NullReferenceException: you’re trying to access a property on a null object.

Fixes to Try #

1. Add a null check first (immediate exception fix)
   This stops the crash right away, and lets you handle the case where no new process was created:

   var filePath = Path.Combine(DownloadDirectory, "GedDocuments", fullName);
   var process = System.Diagnostics.Process.Start(filePath);
   if (process != null)
   {
       process.EnableRaisingEvents = true;
       process.Exited += new EventHandler(closedProgram);
   }
   else
   {
       // Handle the scenario where the image opened in an existing process
       // Maybe log a message, or adjust your closedProgram logic if needed
   }
   

2. Launch the image viewer directly (for reliable process tracking)
   If you need to guarantee you can track the process, explicitly start the viewer executable and pass the image file as an argument. For example, using Windows Photos via Explorer:

   var imagePath = Path.Combine(DownloadDirectory, "GedDocuments", fullName);
   var process = Process.Start(new ProcessStartInfo("explorer.exe", $"\"{imagePath}\"") 
   {
       UseShellExecute = true
   });
   // Or target a specific viewer like Paint for bitmaps:
   // var process = Process.Start("mspaint.exe", imagePath);
   if (process != null)
   {
       process.EnableRaisingEvents = true;
       process.Exited += closedProgram;
   }
   

   Note: This ties you to a specific viewer, so you might want to let users configure their preferred app, or detect the default viewer first.

3. Alternative: Track file lock status (if you need to know when viewing ends)
   If your goal is to detect when the user is done with the image (not just when a process exits), tracking the process isn’t reliable (the viewer might stay open). Instead, you could monitor the file’s lock status—though this is more complex to implement.

Quick Side Note on Paths #

Replace your string concatenation with Path.Combine() to avoid hidden path issues (it handles directory separators correctly across systems):

var filePath = Path.Combine(DownloadDirectory, "GedDocuments", fullName);

内容的提问来源于stack exchange，提问作者Bekal