Determine if executing in finally block due to exception being thrown

Question

Is it possible to determine if code is currently executing in the context of a finally handler as a result of an exception being thrown? I'm rather fond of using the IDisposable pattern to implement entry/exit scoping functionality, but one concern with this pattern is that you might not necessarily want the end-of-scope behavior to occur if an exception occurs in the body of the using. I'd be looking for something like this:

public static class MyClass
{
    public static void MyMethod()
    {
        using (var scope = MyScopedBehavior.Begin())
        {
            //Do stuff with scope here
        }
    }
}

public sealed class MyScopedBehavior : IDisposable
{
    private MyScopedBehavior()
    {
        //Start of scope behavior
    }

    public void Dispose()
    {
        //I only want to execute the following if we're not unwinding
        //through finally due to an exception:
        //...End of scope behavior    
    }

    public static MyScopedBehavior Begin()
    {
        return new MyScopedBehavior();
    }
}

There are other ways I can accomplish this (pass a delegate to a function that surrounds the call with particular behavior), but I'm curious if it's possible to do it using the IDisposable pattern.

---

Actually, this has apparently been asked and answered before here. It's possible to detect in a very hackish sort of way. I wouldn't actually use that technique, but it's interesting to know that it's possible.

Answer 1

The means of accomplishing this that I've seen require an extra method:

public static void MyMethod()
{
    using (var scope = MyScopedBehavior.Begin())
    {
        //Do stuff with scope here
        scope.Complete(); // Tells the scope that it's good
    }
}

By doing this, your scope object can track whether it's disposing because of an error, or a successful operation. This is the approach taken by TransactionScope, for example (see TransactionScope.Complete).

Answer 2

As a side point, IL allows you to specify SEH fault blocks that are similar to finally but are entered only when an exception is thrown - you can see an example here, about 2/3rds down the page. Unfortunately, C# doesn't expose this functionality.

Answer 3

I was looking for something similar for unit testing - I have a helper class I use to clean up objects after a test run and I want to keep the nice, clean 'using' syntax. I also wanted the option of not cleanup up if the test failed. What I came up with is to call Marshal.GetExceptionCode(). I don't know if this is appropriate for all cases, but for test code it seems to work fine.

Answer 4

The best I can come up with would be:

using (var scope = MyScopedBehavior.Begin())
{
  try
  {
    //Do stuff with scope here
  }
  catch(Exception)
  {
    scope.Cancel();
    throw;
  }
}

Of course, scope.Cancel() would make sure nothing happens in Dispose()

Answer 5

The following pattern avoids the problem with API misuse i.e. a scope completion method not being called i.e. omitted completely, or not being called because of a logical condition. I think this answers your question more closely and is even less code for the API user.

Edit

Even more straightforward after Dan's comment:

public class Bling
{
    public static void DoBling()
    {
        MyScopedBehavior.Begin(() =>
        {
            //Do something.
        }) ;
    }   
}

public static class MyScopedBehavior
{
    public static void Begin(Action action)
    {
        try
        {
            action();

            //Do additonal scoped stuff as there is no exception.
        }
        catch (Exception ex)
        {
            //Clean up...
            throw;
        }
    }
}   

Answer 6

I think the best way is to use write out try/catch/finally clause manually. Study an item from the first 'Effective c#" book. A good C# hacker should know exactly what using expands to. It has changed a bit since .Net 1.1 - you can now have several using one under another. So, use reflector, and study the un-sugared code.

Then, when you write your own code - either use the using or write your own stuff. It is not terribly hard, and a good thing to know.

You could get fancy with other tricks, but it feels too heavy, and even not efficient. Let me include a code sample.

LAZY WAY:

using (SqlConnection cn = new SqlConnection(connectionString))
using (SqlCommand cm = new SqlCommand(commandString, cn))
{
    cn.Open();
    cm.ExecuteNonQuery();
}

MANUAL WAY:

bool sawMyEx = false;
SqlConnection cn =  null;
SqlCommand cm = null;

try
{
    cn = new SqlConnection(connectionString);
    cm = new SqlCommand(commandString, cn);
    cn.Open();
    cm.ExecuteNonQuery();
}
catch (MyException myEx)
{
    sawMyEx = true; // I better not tell my wife.
    // Do some stuff here maybe?
}
finally
{
    if (sawMyEx)
    {
        // Piss my pants.
    }

    if (null != cm);
    {
        cm.Dispose();
    }
    if (null != cn)
    {
        cn.Dispose();
    }
}

Answer 7

It would be (IMHO very) helpful if there were a variant of IDisposable whose Dispose method accepted a parameter to indicate what exception, if any, was pending when it was run. Among other things, in the event that Dispose is unable to perform the expected cleanup, it would be able to throw an exception which includes information about the earlier exception. It would also allow a Dispose method to throw an exception if code "forgets" to do something that it was supposed to do within a using block, but not overwrite any other exception that might cause the using block to exit prematurely. Unfortunately, no such feature exists as of yet.

There are numerous articles which suggest means of using API functions to find out whether a pending exception exists. One major problem with such approaches is that it is possible that code may be running in a finally block for a try which completed successfully, but that may be nested in a finally block whose try exited prematurely. Even if a Dispose method could identify that such a situation existed, it would have no way of knowing which try block it "belonged" to. One could formulate examples where either situation applies.

As it is, the best approach is probably to have an explicit "success" method and assume failure if it's not called, and figure that the consequences of forgetting to call the "success" method should be obvious even if no exception is thrown. One thing that may be helpful as a simple utility method would be something like

T Success<T>(T returnValue)
{
  Success();
  return T;
}

thus allowing code like:

return scopeGuard.Success(thingThatMightThrow());

rather than

var result = thingThatMightThrow();
scopeGuard.Success();
return result;

Answer 8

Why not simply dispose from inside a try { } block at the very end, and not use a finally at all? This seems to be the behavior you're looking for.

This also seems more realistic in terms of how others might use your class. Are you sure that everybody who ever uses it will never want to dispose in the case of an exception? Or should this behavior be handled by the consumer of the class?