MANAGED EXCEPTION HANDLING in Visual Basic .NET

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CHAPTER 14 MANAGED EXCEPTION HANDLING
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... leave KeepGoing } catch [mscorlib]System.Exception handler HandlerBegins to AfterHandler ... KeepGoing: ... The IL disassembler by default outputs the EH clauses in the scope form at least those clauses that can be represented in this form. However, we have the option to suppress the scope form and output all EH clauses in their label form (command-line option /RAW). But let s suppose for the sake of convenience that we can shape the code in such a way that the contiguity condition is satisfied, allowing us to use the scope form. A single guarded block with multiple handlers in scope form will look like this: .try { // Guarded code ... leave KeepGoing } catch [mscorlib]System.StackOverflowException { // The exception handler #1 code ... leave KeepGoing } catch [mscorlib]System.Exception { // The exception handler #2 code ... leave KeepGoing } ... KeepGoing: ... Much more readable, isn t it The nested EH configuration shown earlier in Figure 14-1 is easily understandable when written in scope form: .try { .try { .try { // Guarded code ... leave L1 } catch A { // This code works when exception A is thrown ... leave L2
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CHAPTER 14 MANAGED EXCEPTION HANDLING
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} } // No need for leave here! finally { // This code works in any case ... endfinally } } // No need for leave here either! catch B { // This code works when exception B is thrown in guarded ... leave L3 }
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The filter EH clauses in scope form are subject to the same limitation: the handler block must immediately follow the guarded block. But in a filter clause the handler block includes first the filter block and then, immediately following it, the actual handler, so the scope form of a filter clause looks like this: .try { // Guarded code ... leave KeepGoing } filter { // Here we decide whether we should invoke the actual handler ... ldc.i4.1 // OK, let's invoke the handler endfilter } { // Actual handler code ... leave KeepGoing } And, of course, we can easily switch between scope form and label form within a single EH clause declaration. The general ILAsm syntax for an EH clause declaration is as follows: <EH_clause> ::= .try <guarded_block> <EH_type_specific> <handler_block> Where <guarded_block> ::= <label> to <label> | <scope> <EH_type_specific> ::= catch <class_ref> | filter <label> | filter <scope> | finally | fault <handler_block> ::= handler <label> to <label> | <scope > The nonterminals <label> and <class_ref> must be familiar by now, and the meaning of <scope> is obvious: code enclosed in curly braces.
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CHAPTER 14 MANAGED EXCEPTION HANDLING
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Processing the Exceptions
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The execution engine of the CLR processes an exception in two passes. The first pass determines which, if any, of the managed handlers will process the exception. Starting at the top of the EH table for the current method frame, the execution engine compares the address where the exception occurred to the TryOffset and TryLength entries of each EH clause. If it finds that the exception happened in a guarded block, the execution engine checks to see whether the handler specified in this clause will process the exception. (The rules of engagement for catch and filter handlers were discussed in previous sections.) If this particular handler can t be engaged for example, the wrong type of exception has been thrown the execution engine continues traversing the EH table in search of other clauses that have guarded blocks covering the exception locus. The finally and fault handlers are ignored during the first pass. If none of the clauses in the EH table for the current method is suited to handling the exception, the execution engine steps up the call stack and starts checking the exception against EH tables of the method that called the method where the exception occurred. In these checks, the call site address serves as the exception locus. This process continues from method frame to method frame up the call stack, until the execution engine finds a handler to be engaged or until it exhausts the call stack. The latter case is the end of the story: the execution engine cannot continue with an unhandled exception on its conscience, and the runtime executes all finally and fault handlers and then either aborts the application execution or offers the user a choice between aborting the execution and invoking the debugger, depending on the runtime configuration. If a suitable handler is found, the execution engine swings into the second pass. The execution engine again walks the EH tables it worked with during the first pass and invokes all relevant finally and fault handlers. Each of these handlers ends with the endfinally instruction (or endfault, its synonym), signaling the execution engine that the handler has finished and that it can proceed with browsing the EH tables. Once the execution engine reaches the catch or filter handler it found on its first pass, it engages the actual handler. What happens to the method s evaluation stack When a guarded block is exited in any way, the evaluation stack is discarded. If the guarded block is exited peacefully, without raising an exception, the leave instruction discards the stack; otherwise, the evaluation stack is discarded the moment the exception is thrown. During the first pass, the execution engine puts the exception object on the evaluation stack every time it invokes a filter block. The filter block pops the exception object from the stack and analyzes it, deciding whether this is a job for its actual handler block. The decision, in the form of int32 having the value 1 or 0 (engage the handler or don t, respectively), is the only thing that must be on the evaluation stack when the endfilter instruction is reached; otherwise, the IL verification will fail. The endfilter instruction takes this value from the stack and passes it to the execution engine. During the second pass, the finally and fault handlers are invoked with an empty evaluation stack. These handlers do nothing about the exception itself and work only with method arguments and local variables, so the execution engine doesn t bother providing the exception object. If anything is left on the evaluation stack by the time the endfinally (or endfault) instruction is reached, it is discarded by endfinally (or endfault). When the actual handler is invoked, the execution engine puts the exception object on the evaluation stack. The handler pops this object from the stack and handles it to the best of
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