Thursday, March 4, 2010

Rethrowing exceptions to preserve stack details

In my last post, I described our plan at work to introduce FxCop into our development process. The first rule we will be enforcing is RethrowToPreserveStackDetails.

The following example violates the rule:

   12   public void MyTest()
   13   {
   14       Process();
   15   }
   17   public void Process()
   18   {
   19       try
   20       {
   21           int result = Calculator.Divide(12, 0);
   22       }
   23       catch (DivideByZeroException ex)
   24       {
   25           // React, likely by logging
   26           throw ex; // <- This is wrong
   27       }
   28   }

Executing this code results in the following logged callstack
  ...FxCopTests.Process() in C:\test\FxCopTests.cs:line 26
  ...FxCopTests.MyTest() in C:\test\FxCopTests.cs:line 14
Note that the callstack ends with line 26, which is in the catch block. In this example the real exception location isn't hard to find. Unfortunately, production code is rarely this simple.

The problem here is that calling "throw ex" causes the callstack info to be created at that point. If you were creating a new exception and throwing it this would be desired behavior. With an existing exception you don't want the original callstack to be overwritten.

The fix for this code is easy enough - replace:

  throw ex;



After this change, the callstack correctly points to the line throwing the exception
  ...Calculator.Divide(Int32 x, Int32 y) in C:\test\Calculator.cs:line 11
  ...FxCopTests.Process() in C:\test\FxCopTests.cs:line 26
  ...FxCopTests.MyTest() in C:\test\FxCopTests.cs:line 14

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