The syntax will change from language to language, but this is a general question.
What is the difference between this....
try
{
Console.WriteLine("Executing the try statement.");
throw new NullReferenceException();
}
catch (NullReferenceException e)
{
Console.WriteLine("{0} Caught exception #1.", e);
}
finally
{
Console.WriteLine("Executing finally block.");
}
and this....
try
{
Console.WriteLine("Executing the try statement.");
throw new NullReferenceException();
}
catch (NullReferenceException e)
{
Console.WriteLine("{0} Caught exception #1.", e);
}
Console.WriteLine("Executing finally block.");
I keep seeing it being used, so I assume there's a good reason to use finally, but I can't figure out how it's any different from just putting code after the statement since it will still run.
Is there ever a scenario where finally doesn't run?
In your example, it doesn't make a whole lot of difference.
Picture this, though:
try
{
Console.WriteLine("Executing the try statement.");
throw new NullReferenceException();
}
catch (SomeOtherException e)
{
Console.WriteLine("{0} Caught exception #1.", e);
}
finally
{
Console.WriteLine("Executing finally block.");
}
Console.WriteLine("Executing stuff after try/catch/finally.");
In this case, the catch won't catch the error, so anything after the whole try/catch/finally will never be reached. However, the finally block will still run.
try
{
throw new Exception("Error!");
}
catch (Exception ex)
{
throw new Exception(ex, "Rethrowing!");
}
finally
{
// Will still run even through the catch kicked us out of the procedure
}
Console.WriteLine("Doesn't execute anymore because catch threw exception");
It really depends - some other answers have very good reasons to use a Finally block. But I think the best reason is because you're doing exception handling. Things you do in a Finally block typically involve cleaning up resources to ensure proper continuation, regardless of whether or not an exception was thrown - to me that's still part of the exception handling, at least part of a "try something" operation.
IMHO the Finally scope highlights the fact that its code contains stuff that deserves special attention in case of an exception.
finally block is guaranted to be excuted.
So, in your example, results of both cases are looks same.
but if you use return or throw in your catch block, you can see what is difference.
Finally should be used to everything that needs to be done in order to keep a system consistent. This usually means release resources
Finally is always executed, no matter what exception was thrown. It should be used to release resources, in the following cases:
Finalize a connection
Close a file handler
Free memory
Close a database connection
Let me give a complete example. Imagine that that you are sending messages through the network. In pseudo-code:
// With finally | //Without finally
try{ | try{
send_message() | send_message()
} catch(NetworkError){ | } catch(NetworkError){
deal_with_exception() | deal_with_exception()
} finally { | }
finalizes_connection() | finalizes_connection()
} |
The only difference of both codes is when what is hold in the try block raises an exception that is not NetworkError, for example, MethodNotFound. In the first case, the method finalizes_connection() will be called, and in the second one, it will not.
A connection is naturally done through more than one program. So what happens in the case of a MethodNotFound exception to the other program? In the first case, your program will finish the connection and the other program and it will be happy. In the second case, the other program can be waiting for your response forever. What if the other program can only receive one connection per time? You just bugged the other program as well.
This would also apply for a file, for example, that you opened and other programs wouldn't be able to open for reading (in Windows). And for memory, it is never released and now you have a memory leak.
finally runs for both try and catch. It ensures that it will run, but it is not 100% guaranteed it will [some errors stop execution of code]
try block needs at least one catch or a finally.after executing all catch blocks the finally block will be executed.You can add any logic you need there which should be done ultimately.
Its a good practice to use finally to handle program crashes. finally will always run .If the function exits inside of the try catch block, or another error is thrown in either the try or the catch, the finally will still execute. You won't get that functionality not using the finally statement.
I don't know C#, but the purpose of a finally block in Java-ish languages is to ensure that system resources are relinquished, especially if garbage collection is irregular. It's the same principle for all. Consider the block
InputStream is = new FileInputStream("foo.txt");
OutputStream os = new FileOutputStream("D:/nonexistent/directory/bar.txt");
// Throws a FileNotFoundException.
The variable is is created successfully, and takes up system resources. Processes can only use a fixed number of file handles at a time. The variable os is never created, and an exception is thrown, and bubbles up to the caller. In the process, is goes out of scope, and becomes eligible for garbage collection.
However garbage collections are never guaranteed to happen, or they may happen at some indefinite time in the future. So, the system resources taken by is may never be freed. This can be costly, or can crash the program if it happens enough times. So, finally blocks were put into Java. Other languages have similar constructs. In C++, destructors are invoked deterministically. LISPy languages have unwind-protect, but those are usually packaged in with-foo macros.
In Java 6 and lower, one would do this:
try {
is = new FileInputStream("foo.txt");
os = new FileOutputStream("D:/nonexistent/directory/bar.txt");
// work...
} finally {
if (is != null) {
try {
is.close();
} catch (IOException ignored) {}
}
if (os != null) {
try {
os.close();
} catch (IOException ignored) {}
}
}
You can't just call is.close() because that might throw, and then os will never be closed. You have to check for null too. Sane people used Jakarta Commons-IO's IOUtils.closeQuietly() methods to replace the block:
} finally {
IOUtils.closeQuietly(is);
IOUtils.closeQuietly(os);
}
But Java 7 introduced a better solution: try-with-resources. C# 4 probably came first with something similar, Microsoft being quicker on the uptake than Snoracle.
try (
is = new FileInputStream("foo.txt"),
os = new FileOutputStream("D:/nonexistent/directory/bar.txt")
) {
// work...
}
finally always always runs. finally is like the catcher that never misses anything. In the example you mentioned, yes finally doesnt add any value. But finally is usually used to dispose/ release resources.
Related
In other words, how are these two different?
try
{
// Something that can throw an error
}
catch
{
// Handle the error
}
finally
{
// Something that runs after the try statement
}
vs.
try
{
// Something that can throw an error
}
catch
{
// Handle the error
}
// Something that runs after the try statement
finally block always executes.
You can be sure, that this block will be executed no matter what.
Literally it is something like:
Try something, catch some exceptions(if told to and if they are there) and execute the
finally block finally.
If there is a break in the try block or an exception, it may cause the program to halt. In cases like these code that is mandarory to be executed, like closing open connections and returning items to the connection pools are writen in the finally block. The finally block ensures that the code written inside it will be executed.
If you only ever use general catches (i.e. catch without args or catch(Exception ex)), whether to put code in finally or after try/catch is basically a stylistic choice, as both a finally block and code after a try/catch will execute in any situation (barring deliberate escape mechanisms such as return).
However, if you use a narrow catch, and the exception isn't caught, what will happen is the finally block will still execute, however the code after the try/catch won't.
What is the difference between 2 conditions? Every time when method1 or method2 runs, there should be a code block that is required to run. It seems to me that 2 method are the same.
// example method1
void Method1(void)
{
try
{
// do something
}
catch (Exception ex)
{
// do something
}
finally
{
// do something whenever method1 runs
}
}
// example method2
void Method2(void)
{
try
{
// do something
}
catch (Exception ex)
{
// do something
}
// do something whenever method2 runs
}
Finally block seems to be unnecessary for me.
In your first example, you could re-throw the exception and the code inside the finally would still run. This would not be possible in the second example.
If you choose not to re-throw the exception, then yes there is little difference. However, this is considered bad form - very rarely should you need to consume an exception that you cannot explicitly handle.
It is a keyword to help you with code execution flow. When you throw an exception the execution flow of the code is affected (like using return), the finally keyword allows you to express that when an exception occurs (or you return from a try) you still want execution to do something as it's leaving.
To answer the question facetiously, it is a must when you need it and not when you don't.
Further Reading
To be on the safe side, before you attempt to start making use of this keyword, please read the documentation for it:
http://msdn.microsoft.com/en-us/library/zwc8s4fz.aspx
And the exception handling keywords in general:
http://msdn.microsoft.com/en-us/library/s7fekhdy.aspx
Examples
Catch an exception to do something with it, then re-throw it. Use finally to call any tidy-up code:
try
{
OpenConnectionToDatabase();
// something likely to fail
}
catch (Exception ex)
{
Log(ex);
throw;
// throw ex; // also works but behaves differently
}
// Not specifying an exception parameter also works, but you don't get exception details.
//catch (Exception)
//{
// Log("Something went wrong);
// throw;
//}
finally
{
CloseConnectionToDatabase();
}
Don't register any interest in catching exceptions, but use finally to tidy-up code:
try
{
OpenConnectionToDatabase();
// something likely to fail
}
finally
{
CloseConnectionToDatabase();
}
Return from your try because it looks nicely formatted, but still use finally to tidy-up code:
try
{
OpenConnectionToDatabase();
return 42;
}
finally
{
CloseConnectionToDatabase();
}
As you know the code written inside the finally block always runs.
Please have a look onto the following point written below, it will clear your all confusion.
Finally is used for Resource management. Mainly for releasing an resources. It always runs independent upon the Exception.
As we know catch is used for handle an exception, but sometimes it fails to handle to an External exception. Then the finally block is used to handle that exception to perform the operation.
The code in the finally block will run anyway after the try-catch, it is very usefull for clean up.
try
{
// open resources
}
catch (Exception ex)
{
// something bad happened
}
finally
{
// close resources that are still opened
}
This will behave very differently depending on whether you return from the try, for example. Also - the finally will run even if the catch throws an exception (or re-throws the original exception), which will not happen without the finally.
So: it isn't required, but it will behave differently. So if you want the code to happen, put it in the finally.
In many ways, try/finally is much more common than try/catch or try/catch/finally.
You do not absolutely have to have the finally block, however, having it guarantees that the code within it will always run (unless there is an exception in the finally!).
Consider the following:
void Method2(void)
{
try
{
// do something
}
catch (Exception ex)
{
// do something
throw;
}
// do something whenever method2 runs
}
the code after the try/catch will not execute if an exception is thrown. Additionally, if the code within the catch block has an error that causes an exception (such as your logging throwing an unexpected exception) the code that should have been in the finally will not run, leaving and cleanup undone.
Also a return statement will cause that code to not be run, while the finally will still be executed (also, here you can see that the catch can be skipped too, allowing any exceptions to propogate upwards - AFTER executing the finally):
void Method2(void)
{
try
{
// do something
return
}
finally
{
// do something whenever method2 runs
}
}
Whenever you have cleanup code that must be run at the end of a method, use finally (or if your objects implement IDisposable use the using statement).
The finally block ensures that any code within it ALWAYS gets executed so if you have a return statement inside your try block or rethrow an exception within your catch block, the code inside the finally block will always execute.
It is a must if you need to ensure that something happens regardless (e.g. disposing of a resource etc)
The big difference is that try...catch will swallow the exception, hiding the fact that an error occurred. try..finally will run your cleanup code and then the exception will keep going, to be handled by something that knows what to do with it.
Are there situations where it is appropriate to use a try-finally block without a catch block?
You would use it to ensure some actions occur after the try content or on an exception, but when you don't wish to consume that exception.
Just to be clear, this doesn't hide exceptions. The finally block is run before the exception is propagated up the call stack.
You would also inadvertently use it when you use the using keyword, because this compiles into a try-finally (not an exact conversion, but for argument's sake it is close enough).
try
{
TrySomeCodeThatMightException();
}
finally
{
CleanupEvenOnFailure();
}
Code running in finally is not guaranteed to run, however the case where it isn't guaranteed is fairly edge - I can't even remember it. All I remember is, if you are in that case, chances are very good that not running the finally isn't your biggest problem :-) so basically don't sweat it.
Update from Tobias: finally will not run if the process is killed.
Update from Paddy: Conditions when finally does not execute in a .net try..finally block
The most prevalent example you may see is disposing of a database connection or external resource even if the code fails:
using (var conn = new SqlConnection("")) // Ignore the fact we likely use ORM ;-)
{
// Do stuff.
}
Compiles into something like:
SqlConnection conn;
try
{
conn = new SqlConnection("");
// Do stuff.
}
finally
{
if (conn != null)
conn.Dispose();
}
Good Explaination using code:
void MyMethod1()
{
try
{
MyMethod2();
MyMethod3();
}
catch(Exception e)
{
//do something with the exception
}
}
void MyMethod2()
{
try
{
//perform actions that need cleaning up
}
finally
{
//clean up
}
}
void MyMethod3()
{
//do something
}
If either MyMethod2 or MyMethod3 throws an exception, it will be caught by MyMethod1. However, the code in MyMethod2 needs to run clean up code, e.g. closing a database connection, before the exception is passed to MyMethod1.
http://forums.asp.net/t/1092267.aspx?Try+without+Catch+but+with+finally+doesn+t+throw+error+Why+no+syntax+error+
using is equivalent to try-finally. You will only use try-finally when you want to do some clean up inside finally and don't care about the exception.
The best approach will be
try
{
using(resource)
{
//Do something here
}
}catch(Exception)
{
//Handle Error
}
Doing so even clean up called by using fails, your code will not fail.
There are some condition when finally will not get executed.
If there is any StackOverflowException or ExecutingEngineException.
Process is killed from external source.
If you have, for example an unmanaged resource you create and use in the try block, you can use the finally block to ensure you release that resource. The finally block will always be executed despite what happens (e.g. exceptions) in the try block.
E.g. the lock(x) statement is really:
System.Threading.Monitor.Enter(x);
try { ... }
finally
{
System.Threading.Monitor.Exit(x);
}
The finally block will always get called to ensure the exclusive lock is released.
You need a finally block, when no matter which (if any) exceptions are caught or even if none are caught you still want to execute some code before the block exits. For instance, you might want to close an open file.
See Also try-finally
try/finally: when you do not want to handle any exceptions but want to ensure some action(s) occur whether or not an exception is thrown by called code.
I don't know anything about C#, but it seems that anything you could do with a try-finally, you could more elegantly do with a using statement. C++ doesn't even have a finally as a result of its RAII.
Here is a situation where you might want to use try finally: when you would normally use a using statement, but can't because you are calling a method by reflection.
This won't work
using (objMsg = Activator.CreateInstance(TypeAssist.GetTypeFromTypeName("omApp.MessagingBO")))
{
}
instead use
object objMsg = null;
try
{
objMsg
= Activator.CreateInstance(TypeAssist.GetTypeFromTypeName("myAssembly.objBO"));
strResponse = (string)objMsg.GetType().InvokeMember("MyMethod", BindingFlags.Public
| BindingFlags.Instance | BindingFlags.InvokeMethod, null, objMsg,
new object[] { vxmlRequest.OuterXml });
}
finally
{
if (objMsg!=null)
((IDisposable)objMsg).Dispose();
}
Have a look at the following link:
https://softwareengineering.stackexchange.com/questions/131397/why-use-try-finally-without-a-catch-clause
It depends on the architecture of your application and the operation you are performing in the block.
1.we can use the try block without catch but we should use the catch/finally,
any one of them.
2.We can't use only try block.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Why use finally in C#?
In C#, what is the point of having a finally clause?
eg.
try {
// do something
}
catch (Exception exc)
{
// do something
}
// do something
Won't the code at the end execute anyway? What is the point of a finally block?
Finally is for the event that even the catch throws an exception, plus it allows you to exewcute code on success and failure, the finally will ALWAYS be executed. ALWAYS.
Ok, except when the application is killed at the system level or the computer explodes.
The finally ensures that anything in the block will be executed, regardless if the previous two statements are activated.
A good example would be to release database resources.
Try-Catch-Finally
Example:
try
{
//Open a database connection
}
catch
{
//Catch exception, database connection failed
}
finally
{
//Release the opened database connection resources
}
There are pieces of your code that you want to execute regardless of the success of previous code. By using Try/Catch/Finally you are able to benefit from the error handling.
Because there are certain objects that you should perform cleanup on, otherwise it will cause issues for your application.
The constantly used example is using a SqlConnection:
SqlConnection conn = new SqlConnection(connString);
try
{
conn.Open();
throw new ArgumentException();
}
catch(SqlException ex)
{
}
In this case, the SqlConnection is left open with no way of closing it because you handle a SqlException, but ArgumentException is thrown. If you used a finally block, this wouldn't happen because the finally block code would execute:
try
{
conn.Open();
throw new ArgumentException();
}
catch(SqlException ex)
{
}
finally
{
conn.Dispose();
}
The finally block is garaunteed to run when an exception is caught, even if the exception handling block throws more exceptions. Often used for cleaning up certain kinds of resources such as open file handles, network connections, etc...
Good examples here:
http://dotnetperls.com/finally
http://www.csharp-station.com/Tutorials/lesson15.aspx
The Finally ensures that whatever is in it will fire if the try works or fails. In a scenario where a catch occurs you are not guaranteed that everything at the last //do something will occur.
The code in the Finally block always executes, regardless of if an exception was thrown or not. The code following the exception block may or may not execute if the path returns in the catch.
I understand how try-catch works and how try-finally works, but I find myself using those (usually) in two completely different scenarios:
try-finally (or using in C# and VB) is mostly used around some medium-sized code block that uses some resource that needs to be disposed properly.
try-catch is mostly used either
around a single statement that can fail in a very specific way or
(as a catch-all) at a very high level of the application, usually directly below some user interface action.
In my experience, cases where a try-catch-finally would be appropriate, i.e., where the block in which I want to catch some particular exception is exactly the same block in which I use some disposable resource, are extremely rare. Yet, the language designers of C#, VB and Java seem to consider this to be a highly common scenario; the VB designers even think about adding catch to using.
Am I missing something? Or am I just overly pedantic with my restrictive use of try-catch?
EDIT: To clarify: My code usually looks like this (functions unrolled for clarity):
Try
do something
Aquire Resource (e.g. get DB connection)
Try
do something
Try
do something that can fail
Catch SomeException
handle expected error
do something else...
Finally
Close Resource (e.g. close DB connection)
do something
Catch all
handle unexpected errors
which just seems to make much more sense than putting any of the two catches on the same level as finally just to avoid indentation.
A quote from MSDN
A common usage of catch and finally
together is to obtain and use
resources in a try block, deal with
exceptional circumstances in a catch
block, and release the resources in
the finally block.
So to make it even more clear, think of the code that you want to run, in 99% of the cases it runs perfectly well but somewhere in the chunk there might occure an error, you don't know where and the resources created are expensive.
In order to be 100% sure that the resources are disposed of, you use the finally block, however, you want to pin-point that 1% of cases where the error occures, therefore you might want to set up logging in the catch-ing-section.
That's a very common scenario.
Edit - A Practical Example
There is some good examples here: SQL Transactions with SqlTransaction Class. This is just one of the many ways to use Try, Catch & Finally and it demonstrates it very well, even though a using(var x = new SqlTranscation) might be efficient some times.
So here goes.
var connection = new SqlConnection();
var command = new SqlCommand();
var transaction = connection.BeginTransaction();
command.Connection = connection;
command.Transaction = transaction;
Here comes the more interesting parts
///
/// Try to drop a database
///
try
{
connection.Open();
command.CommandText = "drop database Nothwind";
command.ExecuteNonQuery();
}
So let's imagine that the above fails for some reason and an exception is thrown
///
/// Due to insufficient priviligies we couldn't do it, an exception was thrown
///
catch(Exception ex)
{
transaction.Rollback();
}
The transaction will be rolled back! Remember, changes you made to objects inside the try/catch will not be rolled back, not even if you nest it inside a using!
///
/// Clean up the resources
///
finally
{
connection.Close();
transaction = null;
command = null;
connection = null;
}
Now the resources are cleaned up!
Not an answer to your question, but a fun fact.
The Microsoft implementation of the C# compiler actually cannot handle try-catch-finally. When we parse the code
try { Foo() }
catch(Exception e) { Bar(e); }
finally { Blah(); }
we actually pretend that the code was written
try
{
try { Foo(); }
catch(Exception e) { Bar(e); }
}
finally { Blah(); }
That way the rest of the compiler -- the semantic analyzer, the reachability checker, the code generator, and so on -- only ever has to deal with try-catch and try-finally, never try-catch-finally. A bit of a silly early transformation in my opinion, but it works.
Example:
Try
Get DB connection from pool
Insert value in table
do something else...
Catch
I have an error... e.g.: table already contained the row I was adding
or maybe I couldn't get the Connection at all???
Finally
if DB connection is not null, close it.
You can't really get a more "common" example. Pity that some people still forget to put the close connection in the Finally, where it belongs, instead of in the Try block... :(
I often write code that looks like this:
Handle h = ...
try {
// lots of statements that use handle
} catch (SomeException ex) {
// report exception, wrap it in another one, etc
} catch (SomeOtherException ex) {
// ...
} finally {
h.close();
}
So maybe you are just being overly pedantic ... e.g. by putting a try / catch around individual statements. (Sometimes it is necessary, but in my experience you don't usually need to be so fine-grained.)
There is nothing wrong with nesting try/catch/finally blocks. I actually use this quite often. I.e. when I use some resource that needs to be disposed or closed but I want only a single catch block around a larger code unit that needs to be aborted if some error occurs inside it.
try {
// some code
SomeResource res = new SomeResource();
try {
res.use();
} finally {
res.close();
}
// some more code
} catch( Exception ex ) {
handleError( ex );
}
This closes the resource as early as possible in either case (error or not) but still handles all possible errors from creating or using the resource in a single catch block.
I think you are quite right. From the .Net Framework Design Guidelines, written by some of the top architects at Microsoft:
DO NOT overcatch. Exceptions should
often be allowed to propagate up the
call stack.
In well-written code, try-finally [or
using] is far more common than
try-catch. It might seem
counterintuitive at first, but catch
blocks are not needed in a surprising
number of cases. On the other hand,
you should always consider whether
try-finally [or using] could be of use
for cleanup.
page 230 section 7.2
I would nearly always use try-catch-finaly in cases where you need to dispose something in all cases and you use the case to log the error and/or inform the user.
How about something like:
Dim mainException as Exception = Nothing
Try
... Start a transaction
... confirm the transaction
Catch Ex as Exception
mainException = Ex
Throw
Finally
Try
... Cleanup, rolling back the transaction if not confirmed
Catch Ex as Exception
Throw New RollbackFailureException(mainException, Ex)
End Try
End Try
Assuming here that the RollbackFailureException includes an "OriginalException" field as well as "InnerException", and accepts parameters for both. One doesn't want to hide the fact that an exception occurred during the rollback, but nor does one want to lose the original exception which caused the rollback (and might give some clue as to why the rollback happened).
Another use would be to dispose a file handle to an e-mail attachment when using the System.Web.Mail mail objects to send e-mail. I found this out when I had to programatically open a Crystal Report, save it to disk, attach it to an e-mail, and then delete it from disk. An explicit .Dispose() was required in the Finally to make sure I could delete it, especially in the event of a thrown exception.
In my experience, cases where a try-catch-finally would be appropriate, i.e., where the block in which I want to catch some particular exception is exactly the same block in which I use some disposable resource, are extremely rare. Yet, the language designers of C#, VB and Java seem to consider this to be a highly common scenario; the VB designers even think about adding catch to using.
you:
try {
//use resource
} catch (FirstException e) {
// dispose resource
// log first exception
} catch (SecondException e) {
// dispose resource
// log first exception
} catch (ThirdException e) {
// dispose resource
// log first exception
}
me:
try {
//use resource
} catch (FirstException e) {
// log first exception
} catch (SecondException e) {
// log first exception
} catch (ThirdException e) {
// log first exception
} finally {
// dispose resource
}
feel defference?)