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Single method executer and error handler

I've written what I initially thought was a generic method executer and error handler for any method I add or might add in the future but after many hours of struggling and googling, I have resorted to going to forums.
Aim: To try and get away from individual error handling in a method and handle all errors in one single method. (hope this makes sense).
Code for Generic method executer and error handler:
internal static Tuple<SystemMessage, object> ExecuteAndHandleAnyErrors<T,TArg1>(this object callingMethod, params object[] args)
{
dynamic methodToExecute;
if (callingMethod.GetType() == typeof(Func<T, TArg1>))
{
methodToExecute = (callingMethod as Func<T,TArg1>);
}
else
{
methodToExecute = (callingMethod as Action<T, TArg1>);
}
try
{
var result = methodToExecute.DynamicInvoke(args);
return new Tuple<SystemMessage, object>(null,result);
}
catch (Exception ex)
{
return new Tuple<SystemMessage, object>(new SystemMessage
{
MessageText = ex.Message,
MessageType = SystemMessage.SystemMessageType.Error
}, null);
}
}
//This is the code for a sample method:
internal QuestionAnswerSet LoadQuestions(DataWrapper dataWrapper)
{
var taskExecuter = new Func<DataWrapper, QuestionAnswerSet> (InternalDeserializeObject<QuestionAnswerSet>);
var questionAnswerSet = taskExecuter.ExecuteAndHandleAnyErrors<DataWrapper, QuestionAnswerSet>(dataWrapper);
return questionAnswerSet.Item2 as QuestionAnswerSet;
}
my question is this: Is it possible that if the LoadQuestions method falls over, how do I catch the error and defer the error handling to the ExecuteAndHandleAnyErrors method without manually adding a try...catch statement to the LoadQuestions method?
Hope this makes sense.
thank u.
charles

You could wrap every call of LoadQuestions in its own call to ExecuteAndHandleAnyErrors.
However, this seems to be missing part of the point of exception handling. When using exceptions to communicate error, one usually doesn't "handle all errors in one single method". That one single method usually cannot deal with any possible exception sensibly. For example, could your method handle a ThreadAbortedException? What about an ArgumentException? Nor does one add a lot of try ... catch block all over the place.
In general, try to write try ... catch blocks that handle specific exceptions when your code can handle the failure sensibly (e.g., catching FileNotFoundException near where you open a file and triggering a dialog box or attempting to open a default file at a different path).
Often, an application will have one top-level try ... catch block in Main() to log any otherwise unhandled exceptions. Then it rethrows the exception/crashes the program/exits the program.

I solved it.
What I was doing was is seeing the first time the exception is thrown and not stepping further down by pressing F10.
THanks for all the help

I don't quite understand the workings of using/Disposable objects

I asked a question regarding returning a Disposable (IDisposable) object from a function, but I thought that I would muddle the discussion if I raised this question there.
I created some sample code:
class UsingTest
{
public class Disposable : IDisposable
{
public void Dispose()
{
var i = 0;
i++;
}
}
public static Disposable GetDisposable(bool error)
{
var obj = new Disposable();
if (error)
throw new Exception("Error!");
return obj;
}
}
I coded it this way deliberately, because then I call:
using (var tmp = UsingTest.GetDisposable(true)) { }
Using the debugger, I notice that the Dispose method never executes, even though we've already instantiated a Disposable object. If I correctly understand the purpose of Dispose, if this class actually had opened handles and the like, then we would not close them as soon as we had finished with them.
I ask this question because this behavior aligns with what I would expect, but in the answers to the related question, people seemed to indicate that using would take care of everything.
If using still somehow takes care of all of this, could someone explain what I'm missing? But, if this code could indeed cause resource leak, how would you suggest I code GetDisposable (with the condition that I must instantiate the IDisposable object and run code which could throw an exception prior to the return statement)?

The reason it's never called is because of the way you allocate it. The "tmp" variable is never allocated at all, because the GetDisposable(bool) function never returns due to the fact that you threw an exception.
If you were to say instead the following,
using (var tmp = new Disposable())
{
throw new ArgumentException("Blah");
}
then you would see that IDisposable::Dispose() does indeed get called.
The fundamental thing to understand is that the using block has to get a valid reference to the IDisposable object. If some exception occurs so that the variable declared in the using block does not get assigned then you are out of luck, because the using block will have no knowledge of the IDisposable object.
As for returning an IDisposable object from a function, you should use a standard catch block inside of the function to call Dispose() in the event of a failure, but obviously you should not use a using block because this will dispose the object before you are ready to do so yourself.

Depending upon what semantics you want for GetDisposable, this is probably how I would implement it:
public static Disposable GetDisposable(bool error)
{
var obj = new Disposable();
try
{
if (error)
throw new Exception("Error!");
return obj;
}
catch (Exception)
{
obj.Dispose();
throw;
}
}

This is because the tmp variable is never assigned. It's something you need to be careful of with disposable objects. A better definition for GewtDisposable would be:
public static Disposable GetDisposable(bool error)
{
var obj = new Disposable();
try
{
if (error)
throw new Exception("Error!");
return obj;
}
catch
{
obj.Dispose();
throw;
}
}
Because it ensures that obj is disposed.

The IDisposable interface simply guarantees that the class that implements it has a Dispose method. It does nothing in regard to calling this method. A using block will call Dispose on the object when the block is exited.

You create an IDisposable in GetDisposable but since you exit the function by throwing an exception, it never gets returned and hence tmp never gets assigned. The using statement is shorthand for
var tmp = UsingTest.GetDisposable(true);
try { }
finally
{
if(tmp != null) tmp.Dispose();
}
and you never reach the try block. The solution in your example is to check the error flag before creating the disposable obj:
public static Disposable GetDisposable(bool error)
{
if (error)
throw new Exception("Error!");
return new Disposable();
}

A related question is Handling iDisposable in failed initializer or constructor and I think the answer is that if you want to avoid leaking disposable objects from a failed constructor, you'll have to smuggle out a copy of the object from the constructor (e.g. stash it in a passed-in container, or assign it to a passed-by-reference variable) and wrap the constructor call in a catch block. Icky, but I don't know how to do it better. VB.net can actually manage a little better than C# because of how its initializers work.

Catching exceptions thrown in the constructor of the target object of a Using block

using(SomeClass x = new SomeClass("c:/temp/test.txt"))
{
...
}
Inside the using block, all is fine with treating exceptions as normal. But what if the constructor of SomeClass can throw an exception?

Put your using into the try catch f.e.
try
{
using(SomeClass x = new SomeClass("c:/temp/test.txt"))
{
...
}
}
catch(Exception ex)
{
...
}

Yes, this will be a problem when the constructor throws an exception. All you can do is wrap the using block within a try/catch block. Here's why you must do it that way.
using blocks are just syntactic sugar and compiler replaces each using block with equivalent try/finall block. The only issue is that the compiler does not wrap the constructor within the try block. Your code after compilation would have following conversion in the IL.
//Declare object x of type SomeClass.
SomeClass x;
//Instantiate the object by calling the constructor.
x = new SomeClass("c:/temp/test.txt");
try
{
//Do some work on x.
}
finally
{
if(x != null)
x.Dispose();
}
As you can see from the code, the object x will not be instantiated in case when the constructor throws an exception and the control will not move further from the point of exception raise if not handled.
I have just posted a blog-post on my blog on this subject last night.
I'm just now wondering why C#
designers did not wrap object
construction within the try block
which according to me should have been
done.
EDIT
I think I found the answer why C# does not wrap object construction into try block generated in place of the using block.
The reason is simple. If you wrap both declaration and instantiation within the try block then the object would be out of scope for the proceeding finally block and the code will not compile at because, for finally block the object hardly exists. If you only wrap the construction in the try block and keep declaration before the try block, even in that case the it will not compile since it finds you're trying to use an assigned variable.

I threw a quick test program together to check this, and it seems that the Dispose method does not get called when an exception is thrown in the constructor;
class Program
{
static void Main(string[] args)
{
try
{
using (OtherClass inner = new OtherClass())
{
Console.WriteLine("Everything is fine");
}
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
Console.Read();
}
}
class OtherClass : IDisposable
{
public OtherClass()
{
throw new Exception("Some Error!");
}
void IDisposable.Dispose()
{
Console.WriteLine("I've disposed my resources");
}
}
Output :
Some Error!
If you don't throw the exception..
Output :
Everything is fine
I've disposed my resources
Presumably this is because the object was never created, so there's nothing to call Dispose on.
I'm not sure what would happen if the constructor had already allocated some resources which would normally require a proper clean up through Dispose and the exception occurred afterwards though.

This should not be a problem with a well-designed class. Remember the overall question:
public class HoldsResources : IDisposable
{
public HoldsResources()
{
// Maybe grab some resources here
throw new Exception("whoops");
}
}
using (HoldsResources hr = new HoldsResources())
{
}
So, the question is, what should you do about the resources allocated by the HoldsResources constructor before it threw an exception?
The answer is, you shouldn't do anything about those resources. That's not your job. When it was decided that HoldsResources would hold resources, that brought the obligation to properly dispose of them. That means a try/catch/finally block in the constructor, and it means proper implementation of IDisposable to dispose of those resources in the Dispose method.
Your responsibility is to use the using block to call his Dispose method when you're through using the instance. Nothing else.

When you get a hold of resources in the ctor that are not subject to garbage collection, you have to make sure to dispose of them when things go south.
This sample shows a ctor which will prevent a leak when something goes wrong, the same rules apply when you allocate disposables inside a factory method.
class Sample
{
IDisposable DisposableField;
...
public Sample()
{
var disposable = new SomeDisposableClass();
try
{
DoSomething(disposable);
DisposableField = disposable;
}
catch
{
// you have to dispose of it yourself, because
// the exception will prevent your method/ctor from returning to the caller.
disposable.Dispose();
throw;
}
}
}
Edit: I had to change my sample from a factory to a ctor, because apparantly it wasn't as easy to understand as I hoped it would be. (Judging from the comments.)
And of course the reason for this is: When you call a factory or a ctor, you can only dispose of its result. When the call goes through, you have to assume that everything's okay so far.
When calling a ctor or factory you don't have to do any reverse-psychoanalysis to dispose of anything you can't get hold of anyways. If it does throw an exception, it is in the factories/ctor's responsibility to clear anything half-allocated before re-throwing the exception.
(Hope, this time, it was elaborate enough...)

Can a scope block with the "using" keyword react to exceptions?

I have the need to do some logging within my code. I'm required to use an internal company-developed library to record some information. Here's how it works.
Recorder recorder = Recorder.StartTiming();
DoSomeWork();
recorder.Stop(); // Writes some diagnostic information.
To ensure that Stop() is always called, I created a wrapper class that allows a clean "using" block.
using (RecorderWrapper recorderWrapper = new RecorderWrapper) // Automatically calls Recorder.StartTiming() under the covers
{
DoSomeWork();
} // When the recorderWrapper goes out of scope, the 'using' statement calls recorderWrapper.Dispose() automatically - which calls recorder.Stop() under the covers
it's worked well so far. However, there's a change my company is requiring, that would look something like this on the original code:
Recorder recorder = Recorder.StartTiming();
try
{
DoSomeWork();
}
catch (Exception ex)
{
recorder.ReportFailure(ex); // Write out some exception details associated with this "transaction"
}
recorder.Stop(); // Writes some diagnostic information.
I'd like to avoid try/catches in all my 'using' scope blocks with RecorderWrapper. Is there a way I can accomodate the "ReportFailure()" call and still leverage the 'using' scope block?
Specifically, I want everyone on my team to "fall into a pit of success", i.e. make it easy to do the right thing. To me, this means making it really hard to forget to call recorder.Stop() or forget the try/catch.
Thanks!

You might be able to create a method on the recorder to hide this:
public void Record(Action act)
{
try
{
this.StartTiming();
act();
}
catch(Exception ex)
{
this.ReportFailure(ex);
}
finally
{
this.Stop();
}
}
So your example would then just be:
recorder.Record(DoSomeWork);

You could always try something like:
Edit by 280Z28: I'm using a static StartNew() method here similar to Stopwatch.StartNew(). Make your Recorder class IDisposable, and call Stop() from Dispose(). I don't think it gets any more clear than this.
using (Recorder recorder = Recorder.StartNew())
{
try
{
DoSomeWork();
}
catch (Exception ex)
{
recorder.ReportFailure(ex);
}
}

You could continue to use the RecorderWrapper you have, but add a TryExecuting method that accepts a lambda of what you want to happen add runs it in a try/catch block. eg:
using (RecorderWrapper recorderWrapper = new RecorderWrapper) // Automatically calls Recorder.StartTiming() under the covers
{
recorderWrapper.TryExecuting(() => DoSomeWork());
}
Inside RecorderWrapper:
public void TryExecuting(Action work)
{
try { work(); }
catch(Exception ex) { this.ReportFailure(ex); }
}

You could copy the pattern used by TransactionScope, and write a wrapper that must be actively completed - if you don't call Complete(), then the Dispose() method (which gets called either way) assumes an exception and does your handling code:
using(Recorder recorder = Recorder.StartTiming()) {
DoSomeWork();
recorder.Complete();
}
Personally, though, I'd stick with try/catch - it is clearer for maintainers in the future - and it provides access to the Exception.

No, a using block is only syntactic sugar for a try/finally block. It doesn't deal with try/catch. At that point you're going to be left with handling it yourself since it looks like you need the exception for logging purposes.

A using block is effectively a try/finally block that calls dispose on the object in question.
So, this:
using(a = new A())
{
a.Act();
}
is (i think, exactly) equivalent to this:
a = new A();
try
{
a.Act();
}
finally
{
a.Dispose();
}
And you can tack your catches onto the end of the try block.
Edit:
As an alternative to Rob's solution:
Recorder recorder = Recorder.StartNew()
try
{
DoSomeWork();
}
catch (Exception ex)
{
recorder.ReportFailure(ex);
}
finally
{
recorder.Dispose();
}

Oops, I hadn't noticed that a new instance of Recorder was being created by StartTiming. I've updated the code to account for this. The Wrap function now no longer takes a Recorder parameter but instead passes the recorder it creates as an argument to the action delegate passed in by the caller so that the caller can make use of it if needed.
Hmmm, I've needed to do something very similar to this pattern, lambdas, the Action delegate and closures make it easy:
First define a class to do the wrapping:
public static class RecorderScope
{
public static void Wrap(Action<Recorder> action)
{
Recorder recorder = Recorder.StartTiming();
try
{
action(recorder);
}
catch(Exception exception)
{
recorder.ReportFailure(exception);
}
finally
{
recorder.Stop();
}
}
}
Now, use like so:
RecorderScope.Wrap(
(recorder) =>
{
// note, the recorder is passed in here so you can use it if needed -
// if you never need it you can remove it from the Wrap function.
DoSomeWork();
});
One question though - is it really desired that the catch handler swallows the exception without rethrowing it? This would usually be a bad practice.
BTW, I'll throw in an addition to this pattern which can be useful. Although, it doesn't sound like it applies to what you're doing in this instance: Ever wanted to do something like the above where you want to wrap some code with a set of startup actions and completion actions but you also need to be able to code some specific exception handling code. Well, if you change the Wrap function to also take an Action delegate and constrain T to Exception, then you've got a wrapper which allows user to specify the exception type to catch, and the code to execute to handle it, e.g.:
public static class RecorderScope
{
public static void Wrap(Action<Recorder> action,
Action<Recorder, T1> exHandler1)
where T1: Exception
{
Recorder recorder = Recorder.StartTiming();
try
{
action(recorder);
}
catch(T1 ex1)
{
exHandler1(recorder, ex1);
}
finally
{
recorder.Stop();
}
}
}
To use.. (Note you have to specify the type of exception, as it obviously cannot be inferred. Which is what you want):
RecorderScope.Wrap(
(recorder) =>
{
DoSomeWork();
},
(recorder, MyException ex) =>
{
recorder.ReportFailure(exception);
});
You can then extend this pattern by providing multiple overloads of the Wrap function which take more than one exception handler delegate. Usually five overloads will be sufficient - it's pretty unusual for you to need to catch more than five different types of exceptions at once.

Don't add another level of indirection. If you need to catch the Exception, use try..catch..finally and call Dispose() in the finally block.

What is the best way to execute sequential methods?

Working on a project where a sequential set of methods must be run every x seconds. Right now I have the methods contained within another "parent method", and just sequentially call them right after another.
class DoTheseThings()
{
DoThis();
NowDoThat();
NowDoThis();
MoreWork();
AndImSpent();
}
Each method must run successfully without throwing an exception before the next step can be done. So now I wrapped each of those methods with a while and try..catch, then in the catch execute that method again.
while( !hadError )
{
try
{
DoThis();
}
catch(Exception doThisException )
{
hadError = true;
}
}
This seems smelly and not very dry. Is there a better way to do this so I'm not wrapping any new functionality in the same methods. Isn't some kind of Delegate collection the proper way to implement this?
Is there a more "proper" solution?

Action[] work=new Action[]{new Action(DoThis), new Action(NowDoThat),
new Action(NowDoThis), new Action(MoreWork), new Action(AndImSpent)};
int current =0;
while(current!=work.Length)
{
try
{
work[current]();
current++;
}
catch(Exception ex)
{
// log the error or whatever
// maybe sleep a while to not kill the processors if a successful execution depends on time elapsed
}
}

Isn't some kind of Delegate collection the proper way to implement this?
Delegate is a possible way to solve this problem.
Just create a delegate something like:
public delegate void WorkDelegate();
and put them in arraylist which you can iterate over.

I have a personal religious belief that you shouldn't catch System.Exception, or more accurately, you should only catch the exceptions you know how to handle.
That being said, I am going to assume that each one of the methods that you are calling are doing something different, and could result in different exceptions being thrown. Which means you would likely need to have different handlers for each method.
If you follow my religion as well, and the second statement is true, then you are not repeating code unnecessarily. Unless you have other requirements, my recommendations to improve your code would be:
1) Put the try-catch in each method, not around each method call.
2) Have the catches within each method catch ONLY the exceptions you know about.
http://blogs.msdn.com/fxcop/archive/2006/06/14/631923.aspx
http://blogs.msdn.com/oldnewthing/archive/2005/01/14/352949.aspx
http://www.joelonsoftware.com/articles/Wrong.html
HTH ...

your example seems ok.. its a dry one but will do the job well!! actually if this methods execute db access.. you can use transaction to ensure integrity...
if your dealing with shared variables for multi threader programs.. it is cleaner to use synchronization.. the most important thing in coding is that you write the proper code... that has less bugs.. and will do the task correctly..

public void DoTheseThings()
{
SafelyDoEach( new Action[]{
DoThis,
NowDoThat,
NowDoThis,
MoreWork,
AndImSpent
})
}
public void SafelyDoEach( params Action[] actions )
{
try
{
foreach( var a in actions )
a();
}
catch( Exception doThisException )
{
// blindly swallowing every exception like this is a terrible idea
// you should really only be swallowing a specific MyAbortedException type
return;
}
}

What would be the reason that an error was occuring?
If this were a resource issue, such as access to something like a connection or object, then you might want to look at using monitors, semaphores, or just locking.
lock (resource)
{
Dosomething(resource);
}
This way if a previous method is accessing the resource, then you can wait until it releases the resource to continue.
Ideally, you shouldn't have to run a loop to execute something each time it fails. It is failing at all, you would want to know about the issue and fix it. Having a loop to always just keep trying is not the right way to go here.

I'd do what Ovidiu Pacurar suggests, only I'd use a foreach loop and leave dealing with array indexes up to the compiler.

Simple delegate approach:
Action<Action> tryForever = (action) => {
bool success;
do {
try {
action();
success = true;
} catch (Exception) {
// should probably log or something here...
}
} while (!success);
};
void DoEverything() {
tryForever(DoThis);
tryForever(NowDoThat);
tryForever(NowDoThis);
tryForever(MoreWork);
tryForever(AndImSpent);
}
Stack approach:
void DoEverything() {
Stack<Action> thingsToDo = new Stack<Action>(
new Action[] {
DoThis, NowDoThat, NowDoThis, MoreWork, AndImSpent
}
);
Action action;
while ((action = thingsToDo.Pop()) != null) {
bool success;
do {
try {
action();
success = true;
} catch (Exception) {
}
} while (!success);
}