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I know that in Java, if an exception is caught by a catch clause and its catch block throws an exception, control would pass throw the associated finally block (if any) before the thread is terminated. This does not appear to be the case in C#, however.

It is possible to almost mirror this behavior in C# is by putting a try-finally statement inside the try block of the try-catch statement with the catch block that throws the exception, but that would be a problem if, for example, the finally block is supposed to contain code that disposes a Stream Writer that is supposed to log the exception.

Is there a clean way to achieve java-like try-catch-finally exception handling behavior in C#?

Here's an update with the requested sample code:

StreamWriter writer = new StreamWriter("C:\\log.txt");
try
{
    throw new Exception();
}
catch (Exception e)
{
    writer.WriteLine(e.Message);
    throw e;
}
finally
{
    if (writer != null)
    {
       writer.Dispose();
    }
}

Add that code to a console application, run it, let the re-thrown exception go unhandled, attempt to delete C:\log.txt. You won't be able to, because control never passed through the finally block. Also, if you add a breakpoint to some line inside the finally block you will see that it doesn't get hit. (I'm using VS2005).

As far as I know, the only way to force control to pass through the finally block is if the re-thrown exception is handled by the catch block of an enclosing try block (if you took the code above and placed it inside the try block of another try-catch statement).

If the exception is not caught and is allowed to terminate the application, as in the sample code I provided, control won't pass through the finally block.

In Java it would. In C#, at least based on what I have seen, it would not.

J Smith
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    They both behave the same (at least if I understand your situation correctly). The associated finally block will be executed even if the catch block throws. Your question would be much clearer if you provided example code. – Mike Zboray Oct 08 '13 at 00:26

4 Answers4

2

No, this is incorrect. C# will always execute the finally block, even after an exception is thrown/re-thrown from the catch block. See When is finally run if you throw an exception from the catch block?.

Community
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musical_coder
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1

In the .NET Framework, when an exception occurs, the system will determine what if anything is going to catch that exception before any finally blocks execute. Depending upon various application settings, an attempt to throw an exception which will not be caught may kill the application instantly, without giving any finally blocks (or anything else) a chance to run.

If one wraps the Main method, as well as each thread, in

try
{
  ...
}
catch
{
  throw;
}

then any exception which is thrown within the try block will get caught. Even though it will be immediately re-thrown, any nested finally blocks will execute before the catch. There are some cases where this is desirable behavior; there are other cases where one may wish to e.g. perform some special logging if an exception isn't going to be caught (in some cases, the information one wishes to log may be destroyed if the finally blocks get a chance to run first). Within C#, there isn't any way to vary one's actions based upon whether an exception is going to be caught, but in VB.NET there are some ways via which that can be done; a VB.NET assembly which makes calls to C# code can give that code a way of knowing whether any exceptions thrown by an inner method will propagate out to the vb.net wrapper without being caught.

supercat
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  • Your explanation is consistent with the behavior I observed. Is this in the .NET Framework's specification? Also, what are the application settings that you referred to and how can they be configured? – J Smith Apr 04 '14 at 18:41
  • @JSmith: I've read about some of the settings, but don't remember the details, unfortunately. What happens up to the point the exception is found to be unhandled is clearly defined by the ECMA spec; after that point, an implementation may (as far as the spec is concerned) do what it likes, IIRC. – supercat Apr 04 '14 at 18:52
0

This does not appear to be the case in C#, however.

Is this what you are looking for.

Phillip Ngan
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As already stated, the finally block will always run once the catch block has been executed.

Edit: I just tried the sample code provided by the OP in a console application and lo and behold, it did not hit the finally block and had an error of "An unhandled exception of type 'System.Exception' occurred in ConsoleApplication1.exe". This was truly puzzling (besides the part of re-throwing the same exception in an endless loop) so I did a little investimagation and this is what I found:

If a exception occurs the CLR traverses up the call stack looking for a matching catch expression. If the CLR doen't finds a matching one, or the Exception gets re thrown each time, the Exception bubbles out of the Main() method. In that case Windows handles the Exception.

Event Handling of Console Applications is the easiest to understand, because there is no special Handling by the CLR. The Exception is leaving the Applications Thread if not caught. The CLR opens a window asking for debug or exit the application. If the user chooses to debug, the debugger starts. If the user chooses to close, the Application exits and the Exception is serialized and written to the console.

Moral of the story, do not re-throw the same exception from a catch block in a console application!.

JuStDaN
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  • "As already stated, the finally block will always run once the catch block has been executed." That might be true according to the specification, but it doesn't appear to always be true in practice. – J Smith Oct 08 '13 at 12:23
  • @J Smith Updated answer. – JuStDaN Oct 09 '13 at 00:29