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I'll typically only write custom exceptions in the Model (Business) layer. The reason for doing this is so the presentation layer can consume the exception and display a message which is non-technical and meaningful to the user. Probably breaches best practices but it works well example:
Model receives an OutOfRangeException, Model creates a MyModelException with the message "The number of passengers must be greater than 0" and throws the exception to the presentation layer.
Architecture is extensible, code is minimal.
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Wow! What a response. Thanks to everyone. Some very good advice in there, and lots to think about. I'm glad I asked; that was really helpful.
Best wishes, Patrick
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Patrick. There are some questions which it is a joy to answer, and this is definitely one of them. Good question.
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Patrick Skelton wrote: According to some books I have read, the exceptions built into .NET should generally be reserved for .NET's own use
If that was the case, why the hell are those classes public? I have never read anything like that.
You should be creating custom exceptions only when your code interacts with something external. For instance, a service should return custom exceptions back to client in order to hide the complete information of the actual exception. Not because you are ashamed of it but it doesn't makes sense client knowing details of services. Same logic I use in case anything wrong happens in database. Apart from my data layer code, none of the layers get the complete trace of the error.
Apart from all the custom exceptions, I also keep one catch block taking in Exception as a type (in the outermost code layer) just in case we get an exception which I have not handled properly.
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In my opinion there are two situations which REQUIRE the creation of your own exception class.
1. You have a business or data layer checking errors from data entry and throwing an exception. Example: Customer name is missing. By having an exception class specific to your application, the user interface can then create custom error messages attach them to the appropriate user control and display them in a user friendly way that is specific to your application. This also allows for easy implementation of internationalization (localization) by allowing you to create a module that will pick the language specific error message for each individual situation. You don't really want to do that kind of localization at the lower levels.
2. You are implementing a service that crosses machine boundaries. In that case, you want an exception class that serializes well and provides the necessary detail to both debug any situation and to provide sufficiently detailed error messages that will be useful to the end user.
Personally, I am still trying to get used to the idea that throwing exceptions is an appropriate coding practice at an application level. I come from an assembly language and UNIX background where "exceptions" were machine-level faults that totally destroy the application code context, so it was something you would never do in application code. But in the .Net world, throwing exceptions is just as appropriate for handling application errors as returning an error code would be in the UNIX world.
Robert Tanenbaum
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Hi, Robert,
Thanks for that.
I know what you mean about exceptions. I too cut my teeth on assembly language, and even when I started venturing into C/C++ (in video games), exceptions were to be avoided wherever possible. In fact, one game I worked on shipped with a release exception handler that simply did while( 1 );.
Best wishes, Patrick
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Note : This one is Extreme Hypothetical dream
Is it possible to build a CPU that understands Itermediate Language straight away. ( No JIT, No unmanaged)
What could be the blocking factor's if we can't take it to the next Consumer Electronic Show (CES) 
Regards,
Vythees
Miles to go before sleep...
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Theoretically, it would be possible. However, if you removed JIT from the equation, for instance, how would you expect the application to be optimised? If you remove unmanaged, then how do you expect to accomplish tasks such as file writing?
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I think(dream) current branch prediction technique in CPU can be scaled to optimize the code.
I think we won't design a unmanaged process. We can design a process which understands AppDomains. Which will track all the resource instance. ( Hope GC will be embedded in the BIOS itself too).
Not sure If I am making clear.
Regards,
Vythees
Miles to go before sleep...
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It's an interesting idea, but how do you go about ensuring that you can enhance the underlying platform? How do you separate out the runtime from the processor itself?
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Hmm.., I need to backtrack from the idea of placing GC in Bios. Better It should be a managed OS which should be built purely by IL.
Its could be like scaling a CLR itself to a complete OS.
Hope when we build we don't have bussiness compulsion to support backward compatibility (16bit DOS PAC-Man game's) 
Regards,
Vythees
Miles to go before sleep...
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Have you checked out Singularity at MS Research. It doesn't run MSIL natively, but is a completely CLR(ish) O/S, with many improvements. Very Radical.
Singularity[^]
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Or even better is the COSMOS project. They include a compiler to turn IL into ASM, but everything else is written in C#
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Goooood questions.
Modern processors decode opcodes into microcode when loading code into the CPU cache (at least, last time I checked. Those buggers change so fast!)
That would be a decent place for optimizaitons, though I agree it's not fun to throw that away.
But then, there are two things:
- the code in the cache can be optimized to current calling conditions
- optimizations independent of calling conditions can be written back to the in-memory code
For the second, you need some "SystemLevel" code that allows unsafe operations, e.g. ILProcessor.Primitives.Out(portNumber, Value). It needs to be different from .NET's "unsafe" in that you can declare infection stoppage - i.e. "this class is a safe wrapper around its unsafe operations".
Implementing the optimizations in hardware would probably be painfully stupid. However, the write-back-optimizations need to be done only once, and can be done in "native" code. State dependent on-the-fly optimizations could be done in a small "helper CPU" running some hardwired FORTRAN code.
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I haven't heard of any serious attempt to create a CIL processor yet, and I don't expect any now. It would be very similar to a Java Bytecode Processor, see here[^]; I remember aJile was one of the first of those. Nothing much came from any of those AFAICR, as they did not find a sweat spot in the market while RISC processors and RISC technologies in general were making huge inroads in the processor market.
Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.
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Thanks for the answer.
I think a sweet spot may be certain middleware and asp.net appserver where pure performance in demand.
Hope one day I could see one. ( Again. Sure I will not use  )
Regards,
Vythees
Miles to go before sleep...
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You're welcome.
Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.
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Should be possible. At least, there are CPU which can execute Java Bytecode.
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Thanks. Almost broken my Immagination
Regards,
Vythees
Miles to go before sleep...
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I couldn't care less about a processor that runs MSIL.
The one I really want to see is the proc that can run encrypted code natively, without first decrypting it.
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How would that work? Everyone would have to use the same encryption key and then everyone could decrypt the code leaving you with no protection.
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I didn't say I knew how it would work.
But I did say it would be nice if you didn't have to decrypt it to run it.
Just a wild thought with no concern of how it would work or its implementation.
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There is a Paper on this at IEEExplore[^].
You have to be a subscriber to read it though. Unless you can find it elsewhere.
Henry Minute
Do not read medical books! You could die of a misprint. - Mark Twain
Girl: (staring) "Why do you need an icy cucumber?"
“I want to report a fraud. The government is lying to us all.”
I wouldn't let CG touch my Abacus!
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Henry Minute wrote: You have to be a subscriber to read it though
and are you?
Henry Minute wrote: Unless you can find it elsewhere.
Wouldn't that breach the copyright?
Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum
Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.
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Luc Pattyn wrote: and are you?
Nope. 
Apart from CP I don't subscribe to anything that would allow someone like me to subscribe.
Luc Pattyn wrote: Wouldn't that breach the copyright?
Possibly, although not certainly. Posilutely, maybe.
Henry Minute
Do not read medical books! You could die of a misprint. - Mark Twain
Girl: (staring) "Why do you need an icy cucumber?"
“I want to report a fraud. The government is lying to us all.”
I wouldn't let CG touch my Abacus!
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