Rethrow Exception

CA2200 : Rethrow to preserve stack details. That’s one recommendation that as a developer you might have seen when running Code Analysis on your solution.

So what does it mean ? What does the guilty code looks like and what is the correct way to throw exceptions. Lets start with incorrect code first.

Considering following Code.

    class Program
    {
        static void Main( string[] args)
        {
            try
            {
                new IntermediateClass().IntermediateMethod();
            }
            catch ( Exception Ex)
            {
               
                throw;
            }
        }
    }

    class IntermediateClass
    {
        public void IntermediateMethod()
        {
            try
            {
                new ErrorClass().ErrorMethod();
            }
           catch ( Exception Ex)
           {
                throw  Ex;
            }
        }
    }
    
    class ErrorClass
    {
        public void ErrorMethod()
        {
            int k = 1;
            int j = 3 / --k;
        }
    }
 

We have 3 classes, the main class which invokes a method of IntermediateClass, which in turns invokes method from ErrorClass. The ErrorMethod has been designed purposely to throw an error. It doesn’t implement an exception handling mechanism of its own, and the exception is caught by the Intermediate Class. The Intermediate Class, in turns, throws the exception back to the main class. The Code Analysis Tool has problem with this very same line, where the Intermediate Class throws the exception.

Every Exception Object contains a property called Stack Trace, which carries a list of method call hierarchy starting from the method which threw the exception to the method which caught the exception. If you were to analysis the stack trace of the exception caught in the Main Class, you would find something in similar lines.

“ExceptionHandling.IntermediateClass.IntermediateMethod() in <Path>\IntermediateClass.cs:line 21\r\n at ExceptionHandling. Program. Main (String[] args) in <Path>\Program.cs”

As you can observe, the stack trace doesn’t contains the references to the ErrorMethod, but rather it is reset and traces method starting from the point the exception is rethrown, that is from the intermediate method. In order to preserve the stack information, when you need to rethrow the exception without specifying the exception explicitly. Following would be the modification that would be required in Intermediate Method.

    class IntermediateClass
    {
        public void IntermediateMethod()
        {
            try
            {
                new ErrorClass().ErrorMethod();
            }
           catch ( Exception Ex)
           {
                throw ;
            }
        }
    }

Now if you were to view the stack trace now, the method list would look slightly different.

“ExceptionHandling.ErrorClass.ErrorMethod() in <path>\ErrorClass.cs:line 14\r\n at ExceptionHandling. IntermediateClass. IntermediateMethod() in <path>\IntermediateClass.cs:line 21\r\n at ExceptionHandling.Program.Main(String[] args) in <path>\Program.cs:line “

So next time you rethrow an exception, don’t forget to ensure the exception is not explicitly specified.