C# 5.0 Programmer's Reference

216 ❘ CHAPTER 9 Error Handling
Global Exception Handling
Normally, you should try to catch an error as close as possible to the place where it occurs. An error
is easiest to fix if you catch it right away in the method where it happens.
However, bugs often arise in unexpected places. Unless you protect every method with error-handling
code (a fairly common strategy), a bug may arise in code that you have not protected.
Errors, Errors, Everywhere
Some sources of errors are completely beyond your control. For example, power
surges, static electricity, intermittent short circuits, or even stray radiation striking
exactly the right part of a chip can make the computer’s hardware misbehave, so
code that should work correctly fails. There’s little you can do to anticipate these
kinds of errors but you can use global error handling to try to recover from them.
Of course, that doesn’t excuse you from rigorously checking your code for errors.
The vast majority of bugs are due to real mistakes in the code or data rather than to
magical cosmic rays flipping a single bit on a memory chip.
When you write a global exception handler, you probably won’t know where the exception will
occur, so it’s hard to figure out how to keep the program running effectively. You can log the error,
save data if necessary, and possibly display a message for the user before closing the program. You
can make the program attempt to ignore the error and continue running, but it may be difficult to
actually fix the problem.
Let the User Escape
If you decide to make the program ignore the error and keep running, display a
message first and give the user a chance to close the program. Otherwise, the program
might get stuck in an infinite loop where it ignores the error and then retries
whatever operation caused the error in the first place.
How you install a global error handler depends on the type of program you run. The following
sections explain how to install global exception handlers for Windows Forms, WPF, and console
applications.
Note that global exception handlers don’t work inside Visual Studio. In all three kinds of programs,
if the program throws an unhandled exception, Visual Studio springs into action and catches the
error. To test the programs, you need to run a compiled executable program outside of Visual Studio.
Windows Forms Applications
To catch unhandled exceptions in a Windows Forms application, add an event handler to the
Application object’s ThreadException event. The WindowsFormsGlobalException example
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