Internals of Exception Handling

Exceptions are runtime bugs that occur due to illegal action taken by the user. The Exception occurs whenever the object state is corrupted and which causes the program to terminate. Based on the severity of the exception we choose either to continue with the program by proper handling the code or rethrow the exception again to the parent block and eventually terminate the whole program.

In this post, I am not going to talk about how to throw/re-throw exceptions or even write the best practices on Exception handling, but I will go on with some of the hidden facts of exception handling which you might want to know and later on will involve IL to check how exceptions are generated.

The Basics

Exception handling is one of the weakest section of CLR. Even though I like most of the things that CLR brings to us, but I definitely disagree if the exception handling is one amongst it. Even the exception handling as been evolved with the system is enhanced very well recently with the introduction of Uncatchable Exceptions, RuntimeWrappedException, code contacts etc. Lets discuss some of the major enhancements to the exception system.

The .NET exception handling is made up of three sections :

1. try : In this segment you need to write only the portion which can potentially throw errors. 
2. catch : This section can overload based on the type of Exception and will be used to handle the exception which occurred in the Try block.
3. finally : This block executes irrespective of try and catch. We write the clean up tasks here as it ensures to run even anything occurs. 

Internals of loops (While, For and ForEach)

Practically speaking, a loop is the primary building block of a program. We use loop to repeat a set of actions for a certain interval. Now if you think of these intervals, it could be a traversal from one number (called as start index) to another number (called as end index). Very often or probably out of 10 such loop eight times you loop a collection such that you start from 0 and loop until you point to the end of the sequence.

In C# (and VB.NET) we use while, do-while, for and foreach loop to loop through a set of instructions. In this post I will try to demonstrate the basic loops for a while and later on take on a bit about foreach loop and its requirement and finally go deep into its internals.

If you want to see all my Internal Series, you are welcome to follow the link :
Internals to .NET

Implementation of an Observer

Hi Folks,

As few people told me to implement an observer in my last post where I just showed how to use it, here is the post where I am going to clear out confusions for you. If you have read my other post, you might already know what is an Observer and why it is required. Lets recap this a bit more.

An observer is a container which observes each element individually and notifies you when the object state is modified. The observer should contain methods that enables you to subscribe or unsubscribe individually so that when you subscribe for a  notification, it will keep on creating notification until you explicitly unsubscribe the Observer.

In .NET base class library, there are two interfaces introduced viz, IObservable and IObserver. These interfaces gives you a standard to develop Observable pattern and also recommends you to use it rather than doing it of your own. Microsoft also builds forth its Reactive Framework (I will discuss about it later) based on Observer pattern and lets us use it when Observer is required. 

In this post, I will discuss how you could use IObserver and IObservable to implement you own notifiers.

Download Sample - 33KB

IObserver and IObservable - A New addition to BCL

With the introduction of new interfaces, it is time to get it on with discussion. With the current release of VS2010, there were two interfaces that were introduced viz, IObservable and IObserver. Here in the post, I am going to discuss about these interfaces and its connection to Push based approach on Reactive Framework.

IObserver and IObservable as a Dual to Enumerables

First, it should be noted, IObserver and IObservable is actually the mathematical dual of IEnumerable and IEnumerator. Based on iterator pattern, IEnumerable is actually a repository of elements that made up the objects. The IEnumerable holds all the objects and it uses IEnumerator to get each individual objects from the repository. The few methods in IEnumerator which the IEnumerable uses is MoveNext and Current. So for each iteration, the Enumerator calls MoveNext and assigns it to Current which is later on sent back to the external environment.

So if you consider the interface IEnumerable and IEnumerator it looks like :

public interface IEnumerator<out T> : IDisposable
{
          T Current { get; }
          bool MoveNext();
          void Reset();
 }

public interface IEnumerable<out T> : IEnumerable
{
     IEnumerator<T> GetEnumerator();
}

So the IEnumerator has MoveNext which is called every when we need to yield next element from the store. The MoveNext sets the Current item and sends it back to the Environment. So IEnumerable might be considered as Pull based approach and it is used for sequential retrieval of objects.

IObservable and IObserver introduced to BCL recently as stated is mathematical dual of IEnumerable and IEnumerator. Lets see the interfaces a bit :

public interface IObserver<in T>
{
    void OnCompleted();
    void OnError(Exception error);
    void OnNext(T value);
}

and for IObservable it is :

public interface IObservable<out T>
{
      IDisposable Subscribe(IObserver<T> observer);
}

Hence, if you see the difference between the two Interfaces, IEnuerator has Current and MoveNext. These methods are used to Pull objects from the repository. IObserver has OnNext which is used to Push objects to the repository. Again, if you look into IEnumerable, it uses GetEnumerator to pull back the object of IEnumerable, while IObservable has a Subscribe method which is used to push an Observer to the Observable. Hence you can easily say, Observable interfaces in BCL is a dual to Enumerables where the Former uses Push based approach and the later uses pull based approach.

Garbage Collection Algorithm with the use of WeakReference

We all know .NET objects deallocates memory using Garbage Collection. Garbage collection is a special process that hooks in to the object hierarchy randomly and collects all the objects that are not reachable to the application running. Let us make Garbage collection a bit clear before moving to the alternatives.

Garbage Collection Algorithm

In .NET, every object is allocated using Managed Heap. We call it managed as every object that is allocated within the .NET environment is in explicit observation of GC. When we start an application, it creates its own address space where the memory used by the application would be stored. The runtime maintains a pointer which points to the base object of the heap. Now as the objects are created, the runtime first checks whether the object can be created within the reserved space, if it can it creates the object and returns the pointer to the location, so that the application can maintain a Strong Reference to the object. I have specifically used the term Strong Reference for the object which is reachable from the application. Eventually the pointer shifts to the next base address space.

When GC strikes with the assumption that all objects are garbage, it first finds all the Strong References that are global to the application, known as Application Roots and go on object by object. As it moves from object to object, it creates a Graph of all the objects that it finds from the application Roots, such that every object in the Graph is unique. When this process is finished, the Graph will contain all the objects that are somehow reachable to the application. Now as the GC already identified the objects that are not garbage to the application, it goes on Compaction. It linearly traverses to all the objects and shifts the objects that are reachable to non reachable space which we call as Heap Compaction. As the pointers are moved during the Heap compaction, all the pointers are reevaluated again so that the application roots are pointing to the same reference again.

WeakReference as an Exception

On each GC cycle, a large number of objects are collected to release the memory pressure of the application. As I have already stated, that it finds all the objects that are somehow reachable to the Application Roots. The references that are  not collected during the Garbage Collection are called StrongReference, as by the definition of StrongReference, the objects that are reachable to the GC are called StrongReference objects.

This creates a problem. GC is indeterminate. It randomly starts deallocating memory. So say if one have to work with thousand bytes of data at a time, and after it removes the references of the object it had to rely on the time when GC strikes again and removes the reference. You can use GC.Collect to request the GC to start collecting, but this is also a request.

Now say you have to use the large object once again, and you removed all the references to the object and need to create the object again. Here comes huge memory pressure. So in such situation you have :

1. Already removed all references of the object.
2. Garbage collection didnt strike and removed the address allocated.
3. You need the object again.

In such a case, even though the object still in the application memory area, you still need to create another object.  Here comes the use of WeakReference.

Download Sample Application - 27 KB

Design Patterns in C#

As I am doing a lot of architecture stuffs, lets discuss the very basics of designing a good architecture. To begin with this, you must start with Design patterns.

What is Design Patterns ?

Design patterns may be said as a set of probable solutions for a particular problem which is tested to work best in certain situations. In other words, Design patterns, say you have found a problem. Certainly, with the evolution of software industry, most of the others might have faced the same problem once. Design pattern shows you the best possible way to solve the recurring problem.

Uses of Design Patterns

While creating an application, we think a lot on how the software will behave in the long run. It is very hard to predict how the architecture will work for the application when the actual application is built completely. There might issues which you cant predict and may come while implementing the software. Design patterns helps you to find tested proven design paradigm. Following design pattern will prevent major issues to come in future and also helps the other architects to easily understand your code.

History of Design Patterns

When the word design pattern comes into mind, the first thing that one may think is the classical book on Design Pattern "Gangs of Four" which was published by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. In this book, it is first discussed capabilities and pitfalls of Object oriented programming, and later on it discusses about the classic Design Patterns on OOPS.

Types of Design Pattern

Design patterns can be divided into 3 categories.

1. Creational Patterns : These patterns deals mainly with creation of objects and classes.
2. Structural Patterns : These patterns deals with Class and Object Composition.
3. Behavioural Patterns : These mainly deals with Class - Object communication. That means they are concerned with the communication between class and objects.

In this article, I am going to discuss few examples of these patterns.

You can Read the entire article from
http://www.dotnetfunda.com/articles/article889-design-pattern-implementation-using-csharp-.aspx

or

Memory Management in .NET

In .NET memory is managed through the use of Managed Heaps. Generally in case of other languages, memory is managed through the Operating System directly. The program is allocated with some specific amount of memory for its use from the Raw memory allocated by the Operating system and then used up by the program. In case of .NET environment, the memory is managed through the CLR (Common Language Runtime) directly and hence we call .NET memory management as Managed Memory Management.

Allocation of Memory

Generally .NET is hosted using Host process, during debugging .NET creates a process using VSHost.exe which gives the programmer the basic debugging facilities of the IDE and also direct managed memory management of the CLR. After deploying your application, the CLR creates the process in the name of its executable and allocates memory directly through Managed Heaps.

When CLR is loaded, generally two managed heaps are allocated; one is for small objects and other for Large Objects. We generally call it as SOH (Small Object Heap) and LOH (Large Object Heap). Now when any process requests for memory, it transfers the request to CLR, it then assigns memory from these Managed Heaps based on their size. Generally, SOH is assigned for the memory request when size of the memory is less than 83 KBs( 85,000 bytes). If it is greater than this, it allocates memory from LOH. On more and more requests of memory .NET commits memory in smaller chunks.

Now let’s come to processes. Generally a process can invoke multiple threads, as multi-threading is supported in .NET directly. Now when a process creates a new thread, it creates its own stack, i.e. for the main thread .NET creates a new Stack which keeps track of all informations associated with that particular thread. It keeps informations regarding the current state of the thread, number of nested calls etc. But every thread is using the same Heap for memory. That means, Heaps are shared through all threads.

Upon request of memory from a thread say, .NET allocates its memory from the shared Heap and moves its pointer to the next address location. This is in contrast to all other programming languages like C++ in which memory is allocated in linked lists directly managed by the Operating system, and each time memory requests is made by a process, Operating system searches for the big enough block. Still .NET win32 application has the limitation of maximum 2GB memory allocation for a single process.

32 bit processors have 32 bits of address space for locating a single byte of data. This means each 2^32 unique address locations that each byte of data can locate to, means 4.2 billion unique addresses (4GB). This 4GB memory is evenly distributed into two parts, 2 GB for Kernel and 2 GB for application usage.

De- Allocation of Memory

De - allocation of memory is also different from normal Win32 applications..NET has a sophisticated mechanism to de-allocate memory called Garbage Collector. Garbage Collector creates a thread that runs throughout the runtime environment, which traces through the code running under .NET. .NET keeps track of all the accessible paths to the objects in the code through the Graph of objects it creates. The relationships between the Object and the process associated with that object are maintained through a Graph. When garbage collection is triggered it deems every object in the graph as garbage and traverses recursively to all the associated paths of the graph associated with the object looking for reachable objects. Every time the Garbage collector reaches an object, it marks the object as reachable. Now after finishing this task, garbage collector knows which objects are reachable and which aren’t. The unreachable objects are treated as Garbage to the garbage collector. Next, it releases all the unreachable objects and overwrites the reachable objects with the Unreachable ones during the garbage collection process. All unreachable objects are purged from the graph. Garbage collection is generally invoked when heap is getting exhausted or when application is exited or a process running under managed environment is killed.

Garbage collector generally doesn’t take an object as Garbage if it implements Finalize method. During the process of garbage collection, it first looks for the object finalization from metadata. If the object has implemented Finalize(), garbage collector doesn’t make this object as unreachable, but it is assigned to as Reachable and a reference of it is placed to the Finalization queue. Finalize is also handled by a separate thread called Finalizer thread which traces through the finalizer queue and calls the finalize of each of those objects and then marks for garbage collection. Thus, if an object is holding an expensive resource, the finalize should be used. But there is also a problem with this, if we use finalize method, the object may remain in memory for long even the object is unreachable. Also, Finalize method is called through a separate thread, so there is no way to invoke it manually when the object life cycle ends.

Because of this, .NET provides a more sophisticated implementation of memory management called Dispose, which could be invoked manually during object destruction. The only thing that we need is to write the code to release memory in the Dispose and call it manually and not in finalize as Finalize() delays the garbage collection process.

Cost of Finalize in your Program:

Now let us talk about the cost that you have to bear if you have implemented indeterministic approach of .NET and included Finalize in your class. To make it clear you must know how GC works in CLR:

Generation 0 object means the objects that we have declared after last garbage collection is invoked. 1st Generation objects means which is persisting for last 1 GC cycle. Likewise 2nd Generation objects and so on. Now GC does imposes 10 examinies for 0 to 1 generation objects before doing actual Garbage Collection. For 1 to 2 Generation objects it does 100 examinees before collecting.

Now lets think of Finalize, an object that implemented Finalize will remain 9 cycle more than it would actually collected. If it still not finalized, it would move to Geeration 2 and have to go through 100 examinees to be collected. Thus use of Finalize is generally very expensive in your program.

IDisposable implementation:

For Deterministic approach of resource deallocation, microsoft introduced IDisposable interface to clear up all the resources that may be expensive.

Let us take an example :

Protected virtual void Dispose(bool isDisposing)
{
if(IsDisposed) return;
if(isDisposing)
{
// Dispose all Managed Resources
}
IsDisposed = true;
GC.SuppressFinalize(this);
}

Now let us explain,
The first line indicates an if condition statement, Here I have checked if the object is already disposed or not. This is very essential, as in code one can call dispose a multiple times, we need to always check whether the object is already disposed or not. Then we did the disposing, and then made IsDisposed to true.
Now GC.SuppressFinalize will suppress the call to finalize if it is there. This is because, if the user already disposed the object and cleared up all the expensive resources using deterministic approach of deallocation, we dont need the GC to wait to call Indeterministic Finalize method during the Garbage Collection process.

For local objects, we can call dispose directly after using the object. We can also make use of Using block or try/catch block for automatic disposal of objects.

Note: In case of USING, you must remember it works only with the objects that Implements IDisposable. If you use object that dont have implemented IDisposable interface in using block, .NET will through error.