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Should I generally use "if" or "try" when working with directories/files

Usually, when I work with files and directories and I want to check that a path of a a directory or a file exists, I just use something like that:
if (Directory.Exists(path))
{
//Something...
}
However, if I understood this answer correctly, it is recommended to allow the exception still be thrown, meaning that rather than using if, use try.. catch.
Is that a general approach when working with files and directories or there are times that it is preferable using if(Directory.Exists... or something of that sort?
NOTE: After seeing the first responses, just wanted to clarify that the cases when certain directory/path might not exist is an expected and normal behavior.

You nearly always have to catch exceptions, especially for I/O errors, somewhere, lest the program simply be killed when one occurs.
In many scenarios, it makes sense to also first check for valid input (e.g. Directory.Exists()). This allows you to efficiently and in a user-friendly way report and respond to obvious user-error scenarios.
But you have no guarantee that the directory won't be deleted between the time you execute that call and the time you try to access it in some way. Or that if the directory is on a remote share, the network won't fail. Or that you won't have some other kind of I/O error.
There are a few exceptions that just aren't worth catching. Unexpected OutOfMemoryException for example (as opposed to some data structure simply getting too large), or other types of internal .NET failures. The likelihood of recovering from those types of errors is minimal. But for anything else, you at some point should be catching exceptions that could happen. Sometimes this simply means a top-level catch (Exception e), where you will log the exception in some way before exiting the program cleanly.
(I will note that exceptions that are uncaught and which cause the application to be terminated will generally get logged in the system event log. So as long as users are comfortable inspecting the log and retrieving exception information from there, then there's no need to catch all exceptions…just those you know what to do with).

100% depends on what you want to achieve.
If a path is invalid and you want to inform it to the user, you can choose between returning a success value or throwing an exception by what's more suitable for your code architecture.
In other cases, where something can go wrong without the user being responsible for it, or needing to be informed, you usually let exceptions get thrown and handle them in catch blocks in the appropriate places.
The thing to remember is that you can always perform an if check and throw an exception of your own in case of invalid state, rather than letting the invalid state cause an exception by itself. The main difference there is in the data thrown with the exception (stack trace, message, etc), but can also be expressed in performance (you wouldn't want to try a high-cost operation if you can first perform a low-cost check to make sure it will succeed). Try-catch blocks add a BIT of an overhead as well, but I wouldn't consider that to be significant.
The questions to ask are:
Does invalid state causes the whole operation from above to fail?
Where and how does the code handle invalid state?
Does the code "fix" the invalid state and tries again?
Is the invalid state originating from a user error or a code issue?
How costly the operation is, in contrast to how costly the validation check is?
Edit: Check out Yuval's link in the comment to get better understanding of the costs of try-catch blocks.
The validation check is almost always very low-cost, so in conclusion I would say: Perform the check, even if you intend to throw your own exception in case of invalid state.

Try Catch Should be used, except it never should?

So I have been doing some research into how I should be doing try-catch-finally blocks and there is some conflicting information in every post I read. Can someone clarify?
One common idea is to not catch exceptions that you do not know what to do with at that point in the code. The exception will bubble up until it presumably gets to a global exception handler if nothing else catches it. So at that point you display a message to the user that an unknown type of exception occurred, log it, etc.
Now after reading it sounds like this is the only exception handler that you will need? You should not be using it for flow control, so you should be checking if something is returned as null or is invalid causing the exception and correcting it in code. ie. testing for null and doing something about it before it can cause the exception.
But then there are other things such as running out of memory that you would not be able to test for, it would just occur during an operation and the exception would be thrown. But I would not be able to do anything about this either so this would be bubbled up to the global handler.
I think there are some that I am missing, like when dealing with files there can be some thrown from the external code. File not found exception seems like one that may come up often, so I would catch it and in the finally block gracefully close down anything I opened related to other code/processing and then notify the user and log it right there?
The only reason why you would want to catch an exception is for the finally part of the block to make sure that whatever you started before the exception is closed/finalized in a known state? But even then you would want to throw this exception after performing these tasks so the user is notified by the global exception handler, there is no point duplicating this code at this point?
So other than a global exception handler you would have try-catch-finally blocks for these scenarios.
So assuming that I am missing something here, there may be the possibility that you want to try and catch a specific type of exception and then do something with it. I cannot think of anything that you would want to do in the catch block though since the global one would log/notify the user and if you have an exception that usually means that there is no deal for the code to continue on.
Is there any easy way to know which exceptions will be thrown from which modules? I think the only way I have read is to read the MSDN or component suppliers documentation, other than that there is no way to know what exception you would be trying to catch if you were looking for a specific one (not sure why you would)
This question came up since in my application I had a section of code in a try-catch block, and it ended up that when an exception occurred it was either because an object was null, or a string was invalid. Once I wrote code to handle those scenarios the try-catch block is no longer needed, if an exception is encountered now there is nothing the code can do to recover so it should be logged and let the user know so it can be fixed.
But this goes against what I have been reading and what has been preached to me, bad code is code with no try-catch blocks. So how does this all tie together and what piece am I missing here?

The first part of your question is all correct: you should only catch exceptions that you know how to handle. Otherwise, just let them bubble up until they reach code that can handle them.
(Note that "handle" doesn't mean "log" or "display an error". It means to correct the problem that caused the exception, or work around it in some way.)
If they never encounter code that can handle them, or if they are unhandlable exceptions (like OutOfMemory), then they will eventually reach the global unhandled exception handler. This is where you will log the exception (if appropriate), display a generic error to the user (if appropriate), and more often than not, terminate the application. You cannot simply continue as if nothing happened—the exception indicates that the application is in an unexpected state. If you try and continue, you're just going to crash, or worse.
I think there are some that I am missing, like when dealing with files there can be some thrown from the external code. File not found exception seems like one that may come up often, so I would catch it and in the finally block gracefully close down anything I opened related to other code/processing and then notify the user and log it right there?
FileNotFound is a good example of an exception that you will want to handle locally. In the same method (or perhaps one level up, in your UI code) that attempts to load the file, you'll have a catch block for FileNotFound exceptions. If appropriate, display a friendly error message to the user and ask them to choose another file. If it's internal code, give up and try something else. Whatever you need to do. There are few good reasons for FileNotFound to bubble up outside of your code.
This is sort of like using exceptions for flow control, but unavoidable. There is no way to avoid using exceptions (or error codes) for I/O, so you just need to handle the failure case. You could try and verify that the file exists first, before trying to open it, but that would not solve the race issue wherein the file gets deleted or becomes otherwise inaccessible between the time your verification code runs and when you actually try and open it. So now all you've done is duplicated your error-handling code in two places, which serves little purpose.
You have to handle exceptions like FileNotFound locally. The further away from the code that throws, the less likely you can do anything reasonable about it.
Another good example of this, aside from I/O-related exceptions, is a NotSupportedException. For example, if you try to call a method that isn't supported, you might get this exception. You will likely want to handle it and have code in the catch block that falls back to a safe alternative.
The only reason why you would want to catch an exception is for the finally part of the block to make sure that whatever you started before the exception is closed/finalized in a known state? But even then you would want to throw this exception after performing these tasks so the user is notified by the global exception handler, there is no point duplicating this code at this point?
This does not require catching the exception. You can have a try block with only a finally block. A catch block is not required. In fact, this is precisely what using statement implements. If you have state that needs to be cleaned up in the event of an exception being thrown, you should implement the IDisposable pattern and wrap usage of that object in a using block.
Is there any easy way to know which exceptions will be thrown from which modules? I think the only way I have read is to read the MSDN or component suppliers documentation, other than that there is no way to know what exception you would be trying to catch if you were looking for a specific one (not sure why you would)
Precisely. This is not really a problem, though, since you are only catching the exceptions that you can do something about. If you don't know that a module can throw a particular exception, you obviously can't have written code that can handle that exception.
The documentation will tell you all of the important exceptions that you might need to handle, like FileNotFound, SecurityException, or what have you.
This question came up since in my application I had a section of code in a try-catch block, and it ended up that when an exception occurred it was either because an object was null, or a string was invalid. Once I wrote code to handle those scenarios the try-catch block is no longer needed, if an exception is encountered now there is nothing the code can do to recover so it should be logged and let the user know so it can be fixed.
Avoiding exceptions in the first place is always the best option. For example, if you can design your application so that a null object or invalid string is impossible, great. That is what we call robust code. In that case, you don't need to catch these exceptions because there's no way that you can handle it. You thought you already handled the problem, so if an exception is getting thrown anyway, it is a sign of a bug. Don't gloss over it with a catch block.
But sometimes, catch blocks are still necessary, and you write code inside of the catch block to handle the problem. In that case, there's probably no reason to re-throw the exception or log it, so you don't have any code duplication.
But this goes against what I have been reading and what has been preached to me, bad code is code with no try-catch blocks. So how does this all tie together and what piece am I missing here?
Completely wrong. I don't know where you've been reading that, but it is nonsense. Exceptions are exceptional conditions. If your code has catch blocks strewn all over it, that is a sign that you are doing it wrong. Either you're using exceptions for flow control, you're swallowing exceptions in a misguided attempt to "improve reliability", or you don't know about the global unhandled exception handler.
Doesn't sound like you're missing anything to me.
The only thing I feel compelled to mention that doesn't fit strictly into any of your questions is that sometimes you might want to catch an exception and rethrow it as a different exception. The most common situation where you would do this is if you were designing a library of re-usable code. Inside of the library, you might catch internal exceptions and, if you cannot handle them, rethrow them as general exceptions. The whole point of a library is encapsulation, so you shouldn't let exceptions bubble up that the caller cannot possibly do anything about.

There is no true guide for exceptions management (raising and handling). Every app has to decide what level of flow control should be used and how exception has to be raised/handled.
General rules are:
exceptions are raised in exceptional situations
handle exception you can handle and do meaningful things for your app so
exception raising can be used in flow control, actually it's the only way you can reliably handle flow control when you are dealing with devices, so hardware interrupts. (printers, bill validators, file transfer...)
The rest is up to you. The meaning of exception management is made by you.

Imagine you need to download some files from an FTP server, one you don't control. Of course you can't trust other people so you need to prepare for temporary outages and such. So you wrap your downloading code in a try-catch-block and look for WebException if you catch one you might check for FtpStatusCode.ActionNotTakenFileUnavailableOrBusy if that was the error you simply retry. Similarly if you call a web service a 404 might be trouble, but a 429 means that you wait a little and retry, because you had been rate-limited.
Usually you can know which exceptions can be thrown by experience or documentation but C# lacks checked exceptions. Things like NullPointerException or ArgumentNullException can be properly handled with guards in your code. But other things, like errors external dependencies can sometimes be caught and handled by you without crashing the application.

What's the best way to handle the exceptions and how to deal with them in asp.net

First, I'm already familiar with the simple exception handling syntax but I'm asking about the best place, the best time and the best way to deal with them.
I'm building an N-Layered application. so I think the DAL will sometime generate some errors to handle .. and I just learned about the SqlException class, what's the deal with that class ? I once saw a code that handles the SqlException then it handles Exception!
After knowing the practice and where I'm going to handle them, I'm planning to create a method to connect to the database and log the errors in a database so I could fix it but still I don't know what information should I collect to allow me identify the whole situation!
I thought exceptions handling was not a big deal. but every now and then I read some strange advices -that I never understood- on the questions comments but no one could answer me since it was some very old questions!
"Don't just explicitly catch
exceptions"
"the code that is used by
higher-layers in your application must
always only throw exceptions and never
worry about how to deal with them."
EDIT
What about Page_Error event and Application_Error .. I saw that they are a good practice for handling errors

Exception handling is a big deal, and it's not simple to design a good strategy for that.
First of all, some general rules:
Exceptions occur when the running code is completely unable to go ahead, so maybe it tried to handle some internal exceptions but ultimately failed. Think about TCP connection: if a damaged packet arrives, it's an exception, but TCP protocol can handle it. If too many are damaged, an I/O or socket exception is thrown
Exceptions can not always be handled. In almost all cases, when you get an exception from underlying layers you are unable to run corrective code. If your application depends on a DB and that is offline, when you get the exception about it you can only display an error message
Exceptions can be unexpected, and can reveal design or implementation flaws. For example, an implementation flaw can be the situation in which you have a redundant DB but when you fail to connect to frist mirror you don't try with the second
For the third point, it's important to log exceptions and periodically analyse logs to find any weird situation. So, let's begin with the concrete answer.
First of all
think about "handling" the exception. When you write every single code line, think about the possible problems that may prevent it from completing, and think about the possible corrective actions. if any are possible. An error message is not a good handling way, it's the latest strategy.
Don't start to write try-catch(Exception), but prefer specific exceptions. If you need to parse strings to numbers etc, then expect FormatException, if you need to cast from Object to your type expect InvalidCastException
When you write lower-level layers
don't hesitate to throw exceptions!! Don't do like many folks do, ie. return null or use (like ANSI C) a boolean return value and reference parameters. Exceptions are there for that. If you can handle an exception (ie. you don't find a local file but you know you have a remote backup, so handle FileNotFoundException by calling the remote mirror, but if you can't still connect then ultimately throw) then do it and try to resume computation, but if you cannot then throw. And don't forget to throw the inner exception, if present, because it is helpful for logging in the highest layer.
Basically, you can still decide to throw an exception on your own even if you don't catch any! And this is highly recommended especially when function parameters are invalid!
Another good option is to still log in the underlying layers. You actually want to log no matter an exception occurs.
When you log
remember to give an adequate severity to the messages. If you find via code that your DB is offline, that's not an unexpected exception. Still log it as an error, but don't worry about code bugs when you investigate the logs. Instead, if you catch an exception that your code is unable to recognize (a NullReferenceException is a classic example) then log with highest severity, ie. fatal, to give it maximum priority!
A good strategy for ASP.NET
can surely be based upon Page.OnError method. If you have a base page class for all of the pages of your site, you should definitely override that method. In that method, you should first log your exception.
You also shouldn't abuse of try-catch(Exception) blocks, because if you don't catch an exception you can't handle with catch, you will have to handle it via OnError.
When you run such a method, don't immediately think about Server.RemoveError(). You can prefer to have a static HTML page for HTTP 500 error (that is triggered when an unhandled exception bubbles to ASP.NET runtime) that displays a courtesy message to the user.
Briefly
Don't hesitate to throw in underlying layers if anything strange occurs
As said by your advice, don't handle exceptions you are unable to handle (if you catch an exception you can't handle, rethrow it)
LOG!!!!!!!!!!!!!!!!!
Don't disclose exception details to final users on a public website, never!! By default, ASP.NET prevents that from occurring, but you could still use OnError to print stack trace
Use OnError, or Application_Error as single central point to handle all unexpected exceptions
Periodically examine logs against error/fatal messages to find issues with your code, then think about maintaining/debugging/fixing it

Take a look at elmah. It's a logger for asp.net. Renders all errors on a nice summary page.
http://code.google.com/p/elmah/
The best way to handle exceptions is in the specific layer they apply to. If it is a constraint volation, for example, 2 users with the same name, you should let that bubble up to the UI and alert the user.
Same goes with any business rule violations. Those should bubble up to the UI so the end user knows what went wrong.
A SQL Connectivity error is best handled in the DAL...etc..

The how/when/where to catch exceptions may depend on what your trying to do exactly, its difficult to give an exact catch all always correct answer.
As to your specific questions,
I just learned about the SqlException
class, what's the deal with that class
? I once saw a code that handles the
SqlException then it handles
Exception!
Its good practice to handle the specific exception you believe may occur, if your not sure what type this exception is you can just 'Exception', if you want something specific to occur on a 'SQLException' and something else to happen with an 'Exception' then there is certainly nothing wrong with writing code that handles both.
"Don't just explicitly catch
exceptions"
I believe this is refering to code like this
try
{
int i = 1/0;
}
catch(Exception e)
{
//do nothing
}
This exception will be caught but you'll never know it happened, hence this is not a good idea, and the person using the code will be scratching their head as to whats going on.

I think what you are asking here is a Error/Exception Handling Strategy for any application.
I think it includes:
Where - All places where you think an exception can occur or which need more monitoring like DB calls, External Service Calls, Use of Arrays, User Input Parsing, Type Casting and so on...
How - All you high level layers should throw the exception and it should be captured at the entry point and processed to understand the root cause. Usually you do this in Application_Error() where you catch the exception and log it for troubleshooting. How you log an exception is upto you. A Log File or DB driven log is an option based on your requirements and available resources.

IMO apart from extremely rare circumstances I only ever use exception handling for I/O related code where there are interactions with services and file systems whose functionality and maintenance is beyond the control of my applications.
I have always considered the use try/catch statements to manipulate the logic (flow-of-control) in a program in the same way if/else statement work to be extremely bad practice. Most common exceptions can be avoided if you use the tools at hand correctly.

Where should I handle the exceptions, in the BLL, DAL or PL?

Which is the best place to handle the exceptions ? BLL, DAL or PL ?
Should I allow the methods in the DAL and BLL to throw the exceptions up the chain and let the PL handle them? or should I handle them at the BLL ?
e.g
If I have a method in my DAL that issues "ExecuteNonQuery" and updates some records, and due to one or more reason, 0 rows are affected. Now, how should I let my PL know that whether an exception happened or there really was no rows matched to the condition. Should I use "try catch" in my PL code and let it know through an exception, or should I handle the exception at DAL and return some special code like (-1) to let the PL differentiate between the (exception) and (no rows matched condition i.e. zero rows affected) ?

It makes no sense to let an exception that is thrown in the DAL bubble up to the PL - how is the user supposed to react if the database connection could not be established?
Catch and handle exceptions early, if you can handle them. Do not just swallow them without outputting a hint or a log message - this will lead to severe difficulties and bugs that are hard to track.

The short answer is it depends!
You should only ever handle an exception if you can do something useful with it. The 'something useful' again depends on what you are doing. You may want to log the details of the exception although this isn't really handling it and you should really re-throw the exception after logging in most circumstances. You may want to wrap the exception in some other (possibly custom) exception in order to add more information to the exception. As #mbeckish touches on, you may want to try to recover from the exception by retrying the operation for example - you should be careful not to retry forever however. Finally (excuse the pun) you may want to use a finally block to clean up any resources such as an open DB connection. What you choose to do with the exception will influence where you handle it. It is likely that there isn't a great deal of useful things that can be done with many exceptions other than to report to the user that an error has occurred in which case it would be more than acceptable to handle the exception in the UI layer and report the problem to the user (you should probably log the exception as well, further down your layers).
When throwing exceptions yourself, you should only ever throw exceptions in 'exceptional'circumstances as there is a big overhead in throwing exceptions. In your example you suggest you may be thinking of throwing an exception if no records are updated by your operation. Is this really exceptional? A better thing to do in this situation would be to return the number of records updated - this may still be an error condition that needs to be reported to the user, but isn't exceptional like the command failing because the connecction to the DB has gone down.
This is a reasonable article on exception handling best practices.

This is a huge topic with lots of unneeded controversy (people with loud voices giving bad info!) If you're willing to deal with that, follow s1mm0t's advice, it is mostly agreeable.
However if you want a one-word answer, put them in the PL. Serious. If you can get away with it put your error handling in a global exception handler (all errors should log and give a code to look up the log in production for security reasons (esp if web), but give the full details back during development for speed reasons).
Edit: (clarification) you have to deal with some errors everywhere - but this is not an 'every function' norm. Most of the time let them bubble up to the PL and handle .NET's global error with your own code: log the full call stack from there via a common routine that is accessible from all 3 layers via event handlers (see EDIT at bottom of message). This means you will not have try/catch sprinkled thru all your code; just sections you expect and error and can handle it right there, or, non-critical sections, with which you log the error and inform the user of unavailable functionality (this is even more rare and for super-reliable/critical programs)
Aside from that, when working with limited-resource items, I often use the 'using' keyword or try/finally/end try without the catch. for multithreading lock/mutex/re-entry prevention flags/etc. you also need try/finally in ALL cases so your program still works (especially stateful apps).
If you're using exceptions improperly (e.g., to deal with non-bugs when you should be using the IF statement or checking it an iffy operation will work before you try it), this philosophy will fall apart more.
A side note, in thick client apps especially when there is the possibility of losing significant amounts or the users' input, you may be better with more try/catches where you attempt to save the data (marked as not-yet-valid of course).
EDIT: another need for at least having the logging routine in the PL - this will work differently depending on the platform. An app we're working on shares the BLL/DAL with 3 PL versions: an ASP.Net version, a winforms version, and a console app batch mode regression testing version. The logging routine that is called is actually in the BLL (the DAL only throws errors or totally handles any it gets or re-throws them). However this raises an event that is handled by the PL; on the web it puts it in the server's log and does web-style error message display (friendly message for production); in WinForms a special message window appears with tech support info, etc. and logs the error behind the scenes (developers can do something 'secret' to see the full info). And of course in the testing version it is a much simpler process but different as well.
Not sure how I'd have done that in the BLL except for passing a parameter 'what platform,' but since it doesn't include winforms or asp libraries that the logging depends on, that still would be a trick.

The layer that knows what to do to set things right should be the layer that handles the exception.
For example, if you decide to handle deadlock errors by retrying the query a certain number of times, then you could build that into your DAL. If it continues to fail, then you might want to let the exception bubble up to the next layer, which can then decide if it knows how to properly handle this exception.

All layers in your application should manage exceptions gracefullly. This is know as a cross cutting corncern, because it appears in all your layers.
I belive that using a framework like Enterprise Exception Block with unity, you will end up with a better code overall.
Take a look at this post
http://msdn.microsoft.com/en-us/library/ff664698(v=PandP.50).aspx
It will take sometime to master it, but there are lots of examples and screencast around there.

How to handle exceptions depends on technical and business needs. For complex or highly important database updates I include out params that pass a small list of known errors backup to the DL. This way, known error scenarios can be programmatically solved in some cases. In other cases the error needs to be logged and the user should be notified of an error.
I make a practice of notifying a human being of errors. Sure, logging will give us detailed information, but it's no replacement for the response time of a human being. Not only that, but why force developers to watch system logs just to see if things are going south? Talk about unnecessary cost.
If you have time to define potential errors/exceptions and programmatically solve them, then by all means do it. Many times errors/exceptions are unexpected. That's why it is important to be prepared for that unexpected and what better way to do that than involving a human being.
Overall, one should be on the defensive when planning exception handling. Programs grow or they die. A part of growing is introducing bugs. So don't spin your wheels trying to kill them all.

The question to you is where is the exception relevant? If it is a data access exception it should be caught in the DAL. If it is a logic exception it should be caught in the BLL. If it is a presentation exception then in the PL.
For instance if your DAL throws an exception it should return a null or a false or whatever the case may be to your BLL. Your BLL should know what to do if the DAL returns a null, maybe it passes it right through, maybe it tries calling another function, etc. The same goes with your PL if the BLL passes through a null from the DAL or returns something specific of its own then the presentation layer should be able to notify the end user that there was an issue.
Of course you won't get the verbose exception messages, but that is a good thing as far as your users are concerned. You should have a flexible logging system to catch these exceptions and report them to a database or an ip:port or whatever you decide.
Essentially you need to think in terms of separation of concerns if the concern is a data issue or a logic issue it should be handled accordingly.

All About Exceptions: What to do and Where to log?

My question actually comes in two parts hence the ambiguous title.
Part One
As far as I'm aware, you should never swallow an exception. Not even logging it and forgetting about. In general cases, I try to solve an exception and retry the code - for example, let's say I get a FileNotFound exception.
I prompt the user to check the file exists and try again, offering another file chooser dialog and hoping for the best. Failing attempting to solve the problem I end up notifying the user and logging the exception. I've been told that this isn't the correct thing to do in a catch block, so am I doing it right by attempting to solve the issue?
I can't think what else I should do. I suspect I'm being fed misinformation - I'm a gullable soul.
Part Two
Creating a log in my program directory to log an exception is fine I think, but again I'm being told that exceptions should be written to the windows eventlog. Is this correct? Under what circumstances should you write to the event log?
Silly questions need silly answers.
Edit:
There is no context to this question other than a general vague domain. My friend and I were blabbering about the right things to do in particular circumstances.

First off if you ever hear the word Never your ears should perk up... That is why they are called "Best Practices" and not "Rules written in Stone that you must follow..."
here is Microsoft's Exception Handling Best Practices Guide
And there are going to be plenty others...
It really boils down to you as a developer, your teams standards, your customer, etc. What do you want the application to do?
Question 1: Do you want the application to be able to continue on if an exception it thrown? Then I would "swallow" the exception.
Question 2: Is there a benefit to logging a particular exception to the event log or is it just going to bloat it with useless information, You may want to write every exception to the log during development and testing and have verbose information and then in production streamline it... I hope I have answered your question even though there really isn't an generic one...
I would say you should have some general guidelines and then if you have more specific situations then it would be a good time to re-post to this site and get some feedback from people that have tried different routes and can speak to the pros and cons.

The Code Analysis Team Blog is a great place to start on this topic. Also look at
Martin Fowler - Fail Fast
MSDN on Exception Handling
Checked vs Unchecked Exceptions
The second part of your question really depends. In many applications where you need central exception reporting, writing to the event log is a good idea. There are plenty of other cases where it would be a waste of time to do that, you'll have to use your own judgment on that.

Part One
Generally, you don't want to have exception generating behaviour in a catch block.
try
{
ExceptionThrowingMethod();
}
Catch(Exception ex)
{
//Log It
//Try Again
ExceptionThrowingMethod();
}
Clearly, the second exception will be uncaught, and you generally don't want to have try-catches nested within a catch-block.
Generally your catch block should
Log the error. Always. Even if you set it to your lowest logging level, and never read those logs.
Determine whether your current state is recoverable. (Are the right variables set or null? Did it break during a critical function, or between them?)
If you can recover, set some variables that indicate 'try-again', and allow execution to flow OUT of the catch-block. If you cannot recover, try to add some context, and then re-throw the error.
Catch blocks are for error recovery, not for regular execution. So, even through FileNotFound is an exceptional occurrence, prompting the user to try and locate their file is not, and so it should happen in its own try-catch (or loop back to the initial one).
Part Two
Generally, I would prefer writing logs to their own directory, because that way I know exactly where they are, and I also know that everything in the log is relevant. If your application is a critical application, (I.E. a service that needs to be running for a framework to work) then you might consider logging to the eventviewer. There's also the everybody wins method of logging to both. You could have thorough logs in your program directory, and log any critical errors to your event viewer.
Without knowing what reason you were given to log to the event viewer, I can't tell whether or not it's good advice.

Here are some best-practices for exception handling.
Best practices for exception management in Java or C#

I found this to answer part two of my question and it seems from a bit of further research that logging exceptions to the event log isn't a mysterious and dark practice. Thanks for your help everyone.