.Net - Explain how exceptions are handled by the common language
runtime in .NET. - June 03, 2009 at 11:00 AM by Shuchi Gauri
Explain how exceptions are handled by the common language runtime in .NET.
The CLR uses a technique generally referred to as a two-pass exception review
process. What this means is that the CLR will process an exception in two
passes. In the first pass, the CLR will determine if there is a handler for the
exception. This is done by reviewing the entries in the SEH table; specifically
it looks at the Try Offset and Try Length flags to see if the exception
occurred within a guarded block, and if so, whether the Flags entry dictates
that a handler exists for this type of occurrence. Let's assume that the CLR
did find a handler during the first pass. At that point the CLR begins a second
pass of the SEH table during which it will work through the execution phase of
the exception management process. So we can divide the two passes into a
discovery pass, in which we determine whether there is a handler in this method
context to handle the exception; and an execution pass, in which we actually
execute the handler and any special rules.
When code throws an exception, the CLR looks up the call stack looking for a
catch filter to handle the exception. When it finds the relevant catch block,
before executing the code, it will execute all code in all finally blocks -
starting from the try block that threw the exception and stopping with the
catch filter that matches the exception. when the CLR encounters an exception
for a method it will use the descriptors in the SEH table to determine how to
handle the exception, which code block is affected, and what handler should be
invoked.
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