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:
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 :)
