In the InvalidOperationException catch block, a throw statement appears without any identification of the exception to throw (throw is on its own), even though an exception instance (exception) appears in the catch block scope that could be rethrown. Throwing a specific exception would update all the stack information to match the new throw location. As a result, all the stack information indicating the call site where the exception originally occurred would be lost, making it significantly more difficult to diagnose the problem. For this reason, C# supports a throw statement or expression without the explicit exception reference as long as it occurs within a catch block.5 This way, code can examine the exception to determine if it is possible to fully handle it and, if not, rethrow the exception (even though not specified explicitly) as though it was never caught and without replacing any stack information.
Throwing Existing Exceptions without Replacing Stack Information
There is a mechanism that enables the throwing of a previously thrown exception without losing the stack trace information in the original exception.6 This allows you to rethrow exceptions, for example, even from outside a catch block and, therefore, without using throw;. Although it is fairly rare to need to do this, occasionally exceptions are wrapped or saved until the program execution moves outside the catch block. For example, multithreaded code might wrap an exception with an AggregateException. The System.Runtime.ExceptionServices.ExceptionDispatchInfo class is specifically designed to handle this scenario through the use of its static Capture() and instance Throw() methods. Listing 11.3 demonstrates rethrowing the exception without resetting the stack trace information or using an empty throw statement.
Listing 11.3: Using ExceptionDispatchInfo to Rethrow an Exception
Task task = WriteWebRequestSizeAsync(url);
catch (AggregateException exception)
exception = exception.Flatten();
With the ExceptionDispatchInfo.Throw() method, the compiler doesn’t treat the code as a return statement in the same way that it might a normal throw statement. For example, if the method signature returned a value but no value was returned from the code path with ExceptionDispatchInfo.Throw(), the compiler would issue an error indicating no value was returned. On occasion, therefore, developers may be forced to follow ExceptionDispatchInfo.Throw() with a return statement even though such a statement would never execute at runtime—the exception would be thrown instead.
Language Contrast: Java—Exception Specifiers
C# has no equivalent to Java’s exception specifiers. With exception specifiers, the Java compiler is able to verify that all possible exceptions thrown within a function (or a function’s call hierarchy) are either caught or declared as possibly rethrown. The C# team considered this option and concluded that the maintenance burden that it imposed was not worth the perceived benefit. Therefore, it is not necessary to maintain a list of all possible exceptions throughout a particular call stack, but neither is it feasible to easily determine the possible exceptions. (As it turns out, this wasn’t possible for Java, either. Calling virtual methods or using late binding, such as reflection, made it impossible to fully resolve at compile time which exceptions a method could possibly throw.)