Develop Reference

Explain "finally"'s use in try-catch-finally blocks

I read that finally key make a try-catch block final work, even function throw exception or not. But I wonder what is different if I don't put a code inside a finally block (like Function_2 below), which is the way I'm using to coding. Thank You!
void Function_1()
{
try
{
throw new Exception();
}
catch
{
}
finally //Have finally block
{
Other_Function();
}
}
void Function_2()
{
try
{
throw new Exception();
}
catch
{
}
Other_Function(); //Don't have finally block
}

if I dont put a code inside a finally block (like Function_2 below)
if you don't put code inside finally , until unless you won't get an exception that code block will be executed.
but if you get an exception before that code block (which was not kept inside finally) will not be executed as the control returns from there itself.
but if you keep your code in finally , it will be executed irrespective of the situation.
Example:1 without finally block
try
{
//throw exption
}
catch
{
//return from here
}
//below statement won't get executed
Other_Function();
Example:2 with finally block
try
{
//throw exption
}
catch
{
//return from here if finally block is not available
//if available execute finally block and return
}
finally
{
//below statement surly gets executed
Other_Function();
}

A finally block, refers to a block of statements that is always executed, regardless of unexpected events or exceptions that may occur during an application's execution. The execution of a finally block is intended to release resources, such as database connections, which are usually available in limited quantities.
From MSDN:
Usually, when an unhandled exception ends an application, whether or
not the finally block is run is not important. However, if you have
statements in a finally block that must be run even in that situation,
one solution is to add a catch block to the try-finally statement.
Alternatively, you can catch the exception that might be thrown in the
try block of a try-finally statement higher up the call stack. That
is, you can catch the exception in the method that calls the method
that contains the try-finally statement, or in the method that calls
that method, or in any method in the call stack. If the exception is
not caught, execution of the finally block depends on whether the
operating system chooses to trigger an exception unwind operation.

Code in finally blocks is always executed. Finally provides a construct for ensuring the correct execution of programs. It ensures a block of statements are always reached before the enclosing method is exited.
his program shows how the finally clause is part of the control flow in programs. In this program, a random number is generated. This value is used to determine whether to throw an exception, immediately return, or do nothing.
using System;
class Program
{
static void Main()
{
try
{
// Acquire random integer for use in control flow.
// ... If the number is 0, an error occurs.
// ... If 1, the method returns.
// ... Otherwise, fall through to end.
int random = new Random().Next(0, 3); // 0, 1, 2
if (random == 0)
{
throw new Exception("Random = 0");
}
if (random == 1)
{
Console.WriteLine("Random = 1");
return;
}
Console.WriteLine("Random = 2");
}
finally
{
// This statement is executed before the Main method is exited.
// ... It is reached when an exception is thrown.
// ... It is reached after the return.
// ... It is reached in other cases.
Console.WriteLine("Control flow reaches finally");
}
}
}
Source

Why must a return statement precede a throw statement in a catch block?

The code below will complain
try
{
session.Save(obj);
return true;
}
catch (Exception e)
{
throw e;
return false; // this will be flagged as unreachable code
}
whereas this will not:
try
{
session.Save(obj);
return true;
}
catch (Exception e)
{
return false;
throw e;
}
I dont get it...I thought my csc101 told me that return statements should always be the last statement in a function and that it exits the function and return control to the calling code. Why does this defy my professor's logic, and why does only one of these generate a warning?

return will exit the method; throw will also exit the method, assuming it is not inside the try. It can only exit once!
So regardless of the order - the first of the throw / return effectively end the method.
As more general feedback, though: if the intent is to return false upon failure, all you need is:
try
{
session.Save(obj);
return true;
}
catch
{
return false;
}
Personally, I would say that this is bad code - it hides the actual problem from the caller, making it very hard to debug. It tells us nothing of why it failed. I would say that the better approach is simply to let the exception bubble. In that case, there is no point returning true, because we would never return false - and there is no point catching an exception just to re-throw it. So the entire method becomes:
session.Save(obj);
(nothing else required whatsoever)
If your question is "why does only one of these generate a warning": a fair question, but the compiler isn't required to spot either of them for you. Perhaps it should spot it. I suspect that gmcs would spot this and warn about it - the compiler in mono is far more willing to point out stupidity.
Edit: as expected, [g]mcs outputs:
Program.cs(15,13): warning CS0162: Unreachable code detected
Program.cs(28,13): warning CS0162: Unreachable code detected
for the code below - so it does indeed report both uses as warnings:
class Program
{
static void Main() { }
static void DoSomething() { }
bool ReturnFirst()
{
try
{
DoSomething();
return true;
}
catch
{
return false;
throw; // line 15
}
}
bool ThrowFirst()
{
try
{
DoSomething();
return true;
}
catch
{
throw;
return false; // line 28
}
}
}

You are wrong: both your examples raise the Dead code compiler error because both throw and return mark the exit point of a method and no further code is allowed beyond that point.
However, whether the compiler allows it or not, the code below either the throw or the return is still dead and will never get a chance to execute.
(NOTE: this question was initially tagged as Java and my first sentence pertains to Java compiler semantics)

Because any code after the return statement within a code block will be unreachable.

This answer is based on C# and may or may not be applicable to Java.
In this case, you do not actually need the return statement. throw will be the last step of the function.
In this example, both return and throw will end the current function. Regardless of which way around you put them, then first will always prevent the second from being reachable.
NOTE: The exception to when a throw statement would end the function is if it was to be wrapped in a try block. In this case, the throw function would end execution of the remaining try block code, and move to the most relevant catch block - or finally block if a catch is not applicable.
Your code should look like this:
try
{
session.Save(obj);
return true;
}
catch(Exception e)
{
throw e;
}
However, there is not much point in having the try/catch anyway if all you are doing is re-throwing the exception.
To specifically answer your only question:
Why does this defiles my professor's logic?
Well either your professor is wrong, or you have misunderstood them

The "return false;" in the catch block is unreachable because of the "throw e;" just before it. When the code executes in the catch block the first line is a throw which means you immediately throw the exception to the calling method and therefore any following code does not get executed.
try
{
session.Save(obj);
return true;
}
catch(Exception e)
{
throw e; //Throws exception to calling method
return false; //this will be flagged as unreachable code
}
I hope this helps.

C# try catch continue execution

I have a question that might seem fairly simple (of course if you know the answer).
A certain function I have calls another function but I want to continue execution from the caller even though the callee has thrown an exception. Let me give you an example:
something function1()
{
try
{
//some code
int idNumber = function2();
//other code that need to execute even if function2 fails
return something;
}
catch(Exception e)
{//... perhaps something here}
}
EDIT: function1 also has a return statement so nothing can in fact crash on the way
In function2 I need to do stuff but I only need to log if anything fails, example:
int function2()
{
try
{
//dostuff
}
catch(Exception e)
{
//Log stuff to db
}
}
ok, now my question is, what should I do if I wanted to continue execution in function1 even if function 2 throws an error?
Sometimes I mix up if I should do throw; or throw e; or throw nothing at all (leave catch block empty)

Leaving the catch block empty should do the trick. This is almost always a bad idea, though. On one hand, there's a performance penalty, and on the other (and this is more important), you always want to know when there's an error.
I would guess that the "callee" function failing, in your case, is actually not necessarily an "error," so to speak. That is, it is expected for it to fail sometimes. If this is the case, there is almost always a better way to handle it than using exceptions.
There are, if you'll pardon the pun, exceptions to the "rule", though. For example, if function2 were to call a web service whose results aren't really necessary for your page, this kind of pattern might be ok. Although, in almost 100% of cases, you should at least be logging it somewhere. In this scenario I'd log it in a finally block and report whether or not the service returned. Remember that data like that which may not be valuable to you now can become valuable later!
Last edit (probably):
In a comment I suggested you put the try/catch inside function2. Just thought I would elaborate. Function2 would look like this:
public Something? function2()
{
try
{
//all of your function goes here
return anActualObjectOfTypeSomething;
}
catch(Exception ex)
{
//logging goes here
return null;
}
}
That way, since you use a nullable return type, returning null doesn't hurt you.

Why cant you use the finally block?
Like
try {
} catch (Exception e) {
// THIS WILL EXECUTE IF THERE IS AN EXCEPTION IS THROWN IN THE TRY BLOCK
} finally {
// THIS WILL EXECUTE IRRESPECTIVE OF WHETHER AN EXCEPTION IS THROWN WITHIN THE TRY CATCH OR NOT
}
EDIT after question amended:
You can do:
int? returnFromFunction2 = null;
try {
returnFromFunction2 = function2();
return returnFromFunction2.value;
} catch (Exception e) {
// THIS WILL EXECUTE IF THERE IS AN EXCEPTION IS THROWN IN THE TRY BLOCK
} finally {
if (returnFromFunction2.HasValue) { // do something with value }
// THIS WILL EXECUTE IRRESPECTIVE OF WHETHER AN EXCEPTION IS THROWN WITHIN THE TRY CATCH OR NOT
}

Or you can encapsulate the looping logic itself in a try catch e.g.
for(int i = function2(); i < 100 /*where 100 is the end or another function call to get the end*/; i = function2()){
try{
//ToDo
}
catch { continue; }
}
Or...
try{
for(int i = function2(); ; ;) {
try { i = function2(); return; }
finally { /*decide to break or not :P*/continue; } }
} catch { /*failed on first try*/ } finally{ /*afterwardz*/ }

just do this
try
{
//some code
try
{
int idNumber = function2();
}
finally
{
do stuff here....
}
}
catch(Exception e)
{//... perhaps something here}
For all intents and purposes the finally block will always execute. Now there are a couple of exceptions where it won't actually execute: task killing the program, and there is a fast fail security exception which kills the application instantly. Other than that, an exception will be thrown in function 2, the finally block will execute the needed code and then catch the exception in the outer catch block.

Do you mean you want to execute code in function1 regardless of whether function2 threw an exception or not? Have you looked at the finally-block? http://msdn.microsoft.com/en-us/library/zwc8s4fz.aspx

In your second function remove the e variable in the catch block then add throw.
This will carry over the generated exception the the final function and output it.
Its very common when you dont want your business logic code to throw exception but your UI.

C# - Throwing exception in class

I`m writing class. Here is one of functions:
public string GetAttribute(string attrName)
{
try
{
return _config.AppSettings.Settings[attrName].Value;
} catch(Exception e)
{
throw new ArgumentException("Element not exists", attrName);
return null;
}
}
Then, I am using it in the main form MessageBox.Show(manager.GetAttribute("not_existing_element"));
Visual Studio throws an Exception at line:throw new ArgumentException("Element not exists", attrName);
but, I am want to get an Exception at line MessageBox.Show(manager.GetAttribute("not_existing_element"));
How can I do that?
P.S: Sorry for bad English.

You are misusing exception handling. In your code, if you get (for example) a NullReferenceException, you will catch it and then throw an ArgumentException.
Rewrite your method to not have any exception handling:
public string GetAttribute(string attrName)
{
return _config.AppSettings.Settings[attrName].Value;
}
This way, you are not resetting the stack trace and swallowing the original exception.
In terms of getting an exception on the calling line - you will never be able to get an exception at a line that isn't throwing an exception.

A couple of things:
First, you'll get an unreachable code warning for the return null statement in your catch, because the throw will execute before the return. You can simply delete the return null statement.
Secondly, I'm not sure what you mean by getting the exception at the MessageBox line, but I think you mean you want to catch it there. Wrap the call to MessageBox in a try-catch.
try
{
MessageBox.Show(manager.GetAttribute("not_existing_element"));
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}

Order of execution of try catch and finally block

I am confused about the order of try, catch and finally block execution.
I also want to know when should I use try-catch block and what should I put in the try-catch block?
I also want to know if some exception comes in try block then if an action is taken corresponding to try block then which one is executed first catch or finally (which is always to be executed)?
After the execution of these two does control return to try block or it leave it?

If you have (note: this is not valid C#, see below for a valid example):
try {
// ... some code: A
} catch(...) {
// ... exception code: B
} finally {
// finally code: C
}
Code A is going to be executed. If all goes well (i.e. no exceptions get thrown while A is executing), it is going to go to finally, so code C is going to be executed. If an exception is thrown while A is executed, then it will go to B and then finally to C.
As an example, here's a valid C# code block from http://msdn.microsoft.com/en-us/library/dszsf989.aspx:
public class EHClass
{
void ReadFile(int index)
{
// To run this code, substitute a valid path from your local machine
string path = #"c:\users\public\test.txt";
System.IO.StreamReader file = new System.IO.StreamReader(path);
char[] buffer = new char[10];
try
{
file.ReadBlock(buffer, index, buffer.Length);
}
catch (System.IO.IOException e)
{
Console.WriteLine("Error reading from {0}. Message = {1}", path, e.Message);
}
finally
{
if (file != null)
{
file.Close();
}
}
// Do something with buffer...
}
}
The reason to use try/catch/finally is to prevent your program to fail if there is an error in some code (A in the above example). If there is a problem, you can use catch part to catch the problem and do something useful, such as inform the user, log the exception to a log file, try again or try something different that you suppose might work instead of what you tried originally.
finally is used to ensure that some cleanup is performed. E.g. in A you might try to open a file and read it. If opening succeeds, but read fails, you will have an open file dangling. What you would like in that case is to have it closed, which you would do in finally block - this block always gets executed, guaranteeing the closing of the file.
Take a look here for more info:
http://msdn.microsoft.com/en-us/library/0yd65esw.aspx
http://www.c-sharpcorner.com/UploadFile/puranindia/75/Default.aspx

A try ... catch block is used to catch exceptions. In the try block you put the code that you expect may raise an exception.
If no exception occurs then the code in the try block completes as expected. If there's a finally block then that will execute next.
If an exception does occur then execution jumps to the start of the first matching catch block. Once that code is complete the finally block (if it exists) is executed. Execution does not return to the try block.

You should almost never use try/catch.
You should only catch exceptions that you can actually correct, and only when you're expecting them. Otherwise, let the caller handle the exception - or not.
If used, any catch clauses are executed first - only one of them.
Then, finally is "finally" executed.
This has been stated better in many places, but I'll try. The following code:
try
{
// Do something here
}
catch (Exception ex)
{
MessageBox.Show("Friendly error message");
}
does not fix the exception. It hides the exception so that the problem will never be fixed. That code has no idea which exception was thrown, because it will catch all of them, and it does nothing to correct the problem - it just tells the user a polite fiction.
The fact of the matter is that the code above should be replaced with the following:
// Do something here
This way, if the caller of this method knows how to fix particular problems, then the caller can fix them. You will not have removed that option from the caller.
If the caller does not know how to fix the problem, then the caller should also not catch the exception.
Here is an example (from MSDN) of using exceptions in a reasonable manner. It's a modified form of the example in the documentation of the SmtpFailedRecipientsException Class.
public static void RetryIfBusy(string server)
{
MailAddress from = new MailAddress("ben#contoso.com");
MailAddress to = new MailAddress("jane#contoso.com");
using (
MailMessage message = new MailMessage(from, to)
{
Subject = "Using the SmtpClient class.",
Body =
#"Using this feature, you can send an e-mail message from an application very easily."
})
{
message.CC.Add(new MailAddress("Notifications#contoso.com"));
using (SmtpClient client = new SmtpClient(server) {Credentials = CredentialCache.DefaultNetworkCredentials})
{
Console.WriteLine("Sending an e-mail message to {0} using the SMTP host {1}.", to.Address, client.Host);
try
{
client.Send(message);
}
catch (SmtpFailedRecipientsException ex)
{
foreach (var t in ex.InnerExceptions)
{
var status = t.StatusCode;
if (status == SmtpStatusCode.MailboxBusy || status == SmtpStatusCode.MailboxUnavailable)
{
Console.WriteLine("Delivery failed - retrying in 5 seconds.");
System.Threading.Thread.Sleep(5000); // Use better retry logic than this!
client.Send(message);
}
else
{
Console.WriteLine("Failed to deliver message to {0}", t.FailedRecipient);
// Do something better to log the exception
}
}
}
catch (SmtpException ex)
{
// Here, if you know what to do about particular SMTP status codes,
// you can look in ex.StatusCode to decide how to handle this exception
// Otherwise, in here, you at least know there was an email problem
}
// Note that no other, less specific exceptions are caught here, since we don't know
// what do do about them
}
}
}
Note that this code uses try/catch to surround a small piece of code. Within that try/catch block, if an SmtpException or SmtpFailedRecipientsException is thrown, we know what to do about it. If, for instance, we were to catch IOException, we would not know what it meant, or what to do about it. Any exception you don't actually know how to correct should not be caught, except maybe to add information to the exception, log it, and rethrow.

Here is an example:
try
{
someFunctionThatWorks();
functionThatThrowsAnException(); // As soon as this function throws an exception we are taken to the catch block
anotherFunction(); // <-- This line will never get executed
}
catch(Exception e)
{
// Here you can handle the exception, if you don't know how to handle it you should not be catching it
// After this you will not be taken back to the try block, you will go right to the finally block
}
finally
{
// Code here is always executed at the very end, regardless of whether an exception was thrown or not
}

I'd like to elaborate a bit on this and extend #icyrock.com answer with scenario when you rethrow the exception in the catch block so it is handled lower on the execution stack...
I gave it a try with the following code:
static void Main(string[] args)
{
try
{
// pick one:
// NormalExcecution();
// TroubleExcecution();
}
catch
{
Console.WriteLine("block D");
}
Console.ReadKey();
}
private static void NormalExcecution()
{
try
{
Console.WriteLine("block A");
}
catch (Exception)
{
Console.WriteLine("block B");
throw;
}
finally
{
Console.WriteLine("block C");
}
}
private static void TroubleExcecution()
{
try
{
Console.WriteLine("block A");
throw new Exception();
}
catch (Exception)
{
Console.WriteLine("block B");
throw;
}
finally
{
Console.WriteLine("block C");
}
}
So when there is no exception in block A, then the sequence is as follows (exception handling blocks are never hit):
Block A
Block C
When there's some problem with block A, the sequence is as follows:
block A
block B
block C
block D
Another words, the occurring exception is first handled by block B, then the finally clause is executed, only after that the exception is rethrown and handled lower on the execution stack (block D).
Please mind I may be wrong with what is actually going on under the hood of the .NET framework - I just present the results I observed :)