Re-throwing exceptions.

Today we will talk about re-throwing exceptions. In many cases, after examining a caught exception, it doesn't fulfill some requirements and we must re-throw it. This is a very basic and simple action in C# but for the Java developers there is a trap. In the aforementioned scenario, in Java we would just write:

throw e;

were e is the exception we caught.
Why don't we do the same in C# and see what will happen. Below is a sample code that illustrates such situation:

   class Program
   {
       void m1() { m2(); }
       void m2() { m3(); }
       void m3() { m4(); }
       void m4()
       {
           throw new Exception("A sample exception.");
       }
       static void Main(string[] args)
       {
           try
           {
               new Program().m1();
           }
           catch(Exception e)
           {
               throw e;
           }
       }
   }

Now lets execute it and examine the stack trace:

System.Exception was unhandled
Message=A sample exception.
Source=LearningCsharp
StackTrace:
at ConsoleApplication1.Program.Main(String[] args) in C:\Path\Program.cs:line 220
…

What did happen here? We somehow lost the whole stack trace. Now it looks like the exception was created within the Main method – in a real life scenario that may cause many problems when troubleshooting.
How to do it correctly in C# and preserve the whole stack trace? Here is the answer:

   class Program
   {
       void m1() { m2(); }
       void m2() { m3(); }
       void m3() { m4(); }
       void m4()
       {
           throw new Exception("A sample exception.");
       }
       static void Main(string[] args)
       {
           try
           {
               new Program().m1();
           }
           catch(Exception e)
           {
               throw;
           }
       }
   }

Yes! Simple

throw;

solves the issue. Now our stack trace will look as follows:

System.Exception was unhandled
Message=A sample exception.
Source=LearningCsharp
StackTrace:
at ConsoleApplication1.Program.m4() in C:\Path\Program.cs:line 210
at ConsoleApplication1.Program.m3() in C:\Path\Program.cs:line 207
at ConsoleApplication1.Program.m2() in C:\Path\Program.cs:line 206
at ConsoleApplication1.Program.m1() in C:\Path\Program.cs:line 205
at ConsoleApplication1.Program.Main(String[] args) in C:\Path\Program.cs:line 220
...

Looks much better – we can see which method thrown the exception and thus find the source of the issue.
So Java developers – bear that difference in mind and avoid confusion when analyzing a half empty stack trace! :)