MSDN Magazine - September 2008 - (Page 19)

GaURav Khanna CLR InsIde Out Unhandled Exception Processing In The CLR Unhandled exception processing shouldn’t it. If a managed frame calls into a native one, be a mystery. It’s actually quite useful to unthe native frame will be explicitly called out. derstand what happens during the process The topmost frame is the entry point frame since it gives a crashing application an opand the bottommost frame is the one most portunity to perform last-minute diagnostic recently executing code. logging about what went wrong. This diagFigure 1 is an example of one such call nostic information is valuable and may save stack. Here, Bar throws an exception, and thus you time in understanding the crash. the CLR will start walking the managed stack So what is unhandled exception processfrom Bar all the way up to Main, looking for Figure 1 Bar Throws an Exception ing? It’s a phase of the regular exception proan exception handler. In this example, Main cessing mechanism that is only triggered for serves as the entry point managed frame for an exception after all of the thread’s stack the thread in question. frames have been searched for an exception The diagram in Figure 2 shows another handler and none has been found. call stack where the managed code in Bar Here I will be discussing how unhandled calls, using P/Invoke, into native function exception processing for managed exceptions Nfunc, which throws an exception (for exis implemented by the CLR. But before I get ample, throws a C++ exception). Since the into its specifics, let’s see how managed exexception happened in native code, the CLR ception handling works in general. remains unaware of it unless the exception Note that I am assuming that you are fais not caught in the native function and miliar with the Windows® Structured Exreaches the managed Bar method. In such a ception Handling (SEH) mechanism and case, the CLR will create a managed exceprelated concepts. To learn more about them, Figure 2 Nfunc Throws an Exception tion object corresponding to the native exyou may want to refer to this excellent article ception and will start looking for exception by Matt Pietrek, “A Crash Course on the Depths of Win32 Struc- handlers starting from the frame containing Bar and continuing tured Exception Handling,” available at microsoft.com/msj/0197/Excep- all the way up to the Main method. tion/Exception.aspx. In the two examples, the exception handler could be either a catch block that matches the thrown exception’s managed type Managed Exception Handling per the type-matching rules, or a managed filter that agrees to The managed programming model uses the concept of excep- handle the exception after examining the exception object passed tions to notify callers up the stack about error conditions during to it by the CLR. run time. Typically, the caller of a method would know what kind For this example, let’s assume that Main has a catch block to of exception the method may throw (or raise), and, accordingly, the handle the exception. When it is found, but before execution is call is made from within the confines of a try block with an associ- resumed inside it, the CLR initiates the exception unwind operaated catch block or a managed filter with a handler block. tion (also known as the second pass in SEH terms) to invoke the When an exception is thrown by managed code (or one is raised finally clauses all the way from the point where the exception was asynchronously, like an access violation in unsafe managed code), raised to the point just before the exception handler that agreed to the CLR will begin walking the managed stack from the first man- handle the exception. For managed frames, this also results in the aged frame it finds closest to the point of the exception and will begin looking for a managed exception handler. Send your questions and comments to clrinout@microsoft.com. Unless otherwise noted, the stack grows downward and each Code download available at msdn.microsoft.com/magazine/cc135911. row represents a managed frame that calls into the frame below September 2008 19 http://microsoft.com/msj/0197/Exception/Exception.aspxc http://microsoft.com/msj/0197/Exception/Exception.aspxc http://msdn.microsoft.com/magazine/cc135911

Table of Contents Feed for the Digital Edition of MSDN Magazine - September 2008

MSDN Magazine - September 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Cutting Edge Hierarchy ID New Features for Microsoft SQL Server 2008 Prism Data Services Advanced WPF Test Run Security Briefs Foundations { End Bracket }

MSDN Magazine - September 2008

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