6 important .NET concepts: - Stack, heap, Value types, reference types, boxing and Unboxing.

Introduction

This article will explain 6 important concepts Stack , heap , value types , reference types , boxing and unboxing. This article starts first explaining what happens internally when you declare a variable and then it moves ahead to explain 2 important concepts stack and heap. Article then talks about reference types and value types and clarifies some of the important fundamentals around them.

Finally the article concludes by demonstrating how performance is hampered due to boxing and unboxing with a sample code.

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What goes inside when you declare a variable?

When you declare a variable in a .Net application, it allocates some chunk of memory in to the RAM. This memory has 3 things first the name of the variable, second data type of the variable and finally the value of the variable.

That was a simple explanation of what happens in the memory, but depending on what kind of data type your variable is allocated on that type of memory. There are two types of memory allocation stack memory and heap memory. In the coming sections we will try to understand these two types of memory in more details.

Stack and Heap

In order to understand stack and heap, let’s understand what actually happens in the below code internally.

public void Method1()
{
// Line 1
int i=4;

// Line 2
int y=2;

//Line 3
class1 cls1 = new class1();
}

It’s a 3 line code so let’s understand line by line how things execute internally.
Line 1:- When this line is executed compiler allocates a small amount of memory in to memory type called as stack. Stack is responsible of keeping track of running memory needed in your application.

Line 2:- Now the execution moves to the next step. As the name says stack it stacks this memory allocation on the top of the first memory allocation. You can think about stack as series of compartment or boxes put on top of each other.
Memory allocation and de-allocation is done using LIFO (Last in first out) logic. In other words memory is allocated and de-allocated at only one end of the memory i.e. top of the stack.

Line 3:- In line 3 we have a created an object. When this line is executed it creates a pointer on the stack and the actual object is stored in a different type of memory location called as ‘Heap’. ‘Heap’ does not track
running memory it’s just pile of objects which can reached at any moment of time. Heap is used for dynamic memory allocation.

One more important point to note here is reference pointers are allocated on stack. The statement, Class1 cls1; does not allocate memory for an instance of Class1, it only allocates a stack variable cls1 (and sets it to null). The time it hits the new keyword it allocates on "HEAP".
Exiting the method (The fun):- Now finally the execution control starts exiting the method. When it passes the end control it clears all the memory variables which are assigned on stack. In other words all variables which are related to ‘int’ data type are de-allocated in ‘LIFO’ fashion from the stack.
The BIG catch – It did not de-allocate the heap memory. This memory will be later de-allocated by “GARBAGE COLLECTOR”.


Now many of our developer friends must be wondering why two types of memory, can’t we just allocate everything on just one memory type and we are done.
If you look closely primitive data types are not complex, they hold single
values like ‘int i = 0’. Object data types are complex, they reference other
objects or other primitive data types. In other words they hold reference to other multiple values and each one of them must be stored in memory. Object types need dynamic memory while primitive needs static type memory. If the requirement is of dynamic memory it’s allocated on a heap or else it goes on a stack.
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Value types and reference types

Now that we have understood the concept of ‘Stack’ and ‘Heap’ it’s time to understand the concept of value types and reference types.
Value types are types which hold both data and the memory on the same location. While a reference type has a pointer which points to the memory location.
Below is a simple integer data type with name ‘i’ whose value is assigned to an other integer data type with name ‘j’. Both these memory values are allocated on the stack.
When we assign the ‘int’ value to the other ‘int’ value it creates a complete different copy. In other word if you change either of them the other does not change. These kinds of data types are called as ‘Value types’.



When we create an object and when we assign one object to the other object, they both point to the same memory location as show in the below code snippet. So when we assign ‘obj’ to ‘obj1’ they both point to the same memory location.
In other words if we change one of them the other object is also affected this is termed as ‘Reference types’.

So which data types are ref type and value type?

In .NET depending on data types the variable is either assigned on the stack or on the heap. ‘String’ and ‘Objects’ are reference types and any other .NET primitive data types are assigned on the stack. Below figure explains the same in a more detail manner.

Boxing and Unboxing

WOW, you have given so much knowledge, so what’s the use of it in actual
programming. One of the biggest implications is to understand the performance hit which is incurred due to data moving from stack to heap and vice versa.
Consider the below code snippet. When we move a value type to reference type the data is moved from the stack to the heap. When we move reference type to a value type the data is moved from the heap to the stack.
This movement of data from the heap to stack and vice-versa creates a
performance hit.

When the data moves from value types to reference types its termed as ‘Boxing’ and the vice versa is termed as ‘UnBoxing’.



If you compile the above code and see the same in ILDASM you can see in the IL code how ‘boxing’ and ‘unboxing’ looks, below figure demonstrates the same.

Performance implication of Boxing and unboxing

In order to see how the performance is impacted we ran the below two
functions 10,000 times. One function has boxing and the other function is simple. We used a stop watch object to monitor the time taken.
The boxing function was executed in 3542 MS while without boxing the code was executed in 2477 MS. In other words try to avoid boxing and unboxing. In project you always need boxing and unboxing , use it when it’s absolutely necessary.
With the same article the sample code is attached which demonstrates this performance implication.



Currently I have not put a source code for unboxing but the same hold true for the same. You can write the same and experiment it by using stopwatch class.

ASP.NET application and page life cycle
Introduction
In this article we will try to understand what are the different events which takes place right from the time the user sends a request, until the time request is rendered on the browser. So we will first try to understand the two broader steps of an ASP.NET request and then we will move in to different events emitted from ‘HttpHandler’, ‘HttpModule’ and ASP.NET page object. As we move in this event journey we will try to understand what kind of
logic should go in each every of these events.
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The Two step process

From 30,000 feet level ASP.NET request processing is a 2 step process as shown below. User sends a request to the IIS:-
• ASP.NET creates an environment which can process the request. In other words it creates the application object, request, response and context objects to process the request.

• Once the environment is created the request is processed through series of events which is processed by using modules, handlers and page objects. To keep it short lets name this step as MHPM (Module, handler, page and Module event), we will come to details later.


In the coming sections we will understand both these main steps in more details.Creation of ASP.NET environment
Step 1:- The user sends a request to IIS. IIS first checks which ISAPI extension can serve this request. Depending on file extension the request is processed. For instance if the page is an ‘.ASPX page’ then it will be passed to ‘aspnet_isapi.dll’ for processing.
Step 2:- If this the first request to the website then a class called as ‘ApplicationManager’ creates an application domain where the website can run. As we all know application domain creates isolation between two web applications hosted on the same IIS. So in case there is issue in one app domain it does not affect the other app domain.
Step 3:- The newly created application domain creates hosting environment i.e. the ‘HttpRuntime’ object. Once the hosting environment is created necessary core ASP.NET objects like ‘HttpContext’ , ‘HttpRequest’ and ‘HttpResponse’ objects are created.
Step 4:- Once all the core ASP.NET objects are created ‘HttpApplication’ object is created to serve the request. In case you have a ‘global.asax’ file in your system then object of the ‘global.asax’ file will be created. Please note
‘global.asax’ file inherits from ‘HttpApplication’ class.
Note: The first time an ASP.NET page is attached to an application, a new instance of ‘HttpApplication’ is created. Said and done to maximize performance, ‘HttpApplication’ instances might be reused for multiple requests.Step 5:- The ‘HttpApplication’ object is then assigned to the core ASP.NET objects to process the page.
Step 6:- ‘HttpApplication’ then starts processing the request by http module events , handlers and page events. It fires the MHPM event for request processing.
Note: - For more details


http://msdn.microsoft.com/en-us/library/ms178473.aspx



Below image explains how the internal object model looks like for an ASP.NET request. At the top level is the ASP.NET runtime which has creates an ‘Appdomain’ which in turn has ‘HttpRuntime’ with ‘request’, ‘response’ and ‘context’ objects.



Process request using MHPM events fired

Once ‘HttpApplication’ is created it starts processing request it goes through 3 different sections ‘HttpModule’ , ‘Page’ and ‘HttpHandler’. As it moves through these sections it invokes different events which the developer can extend and add customize logic to the same.Before we move ahead lets understand what are ‘HttpModule’ and ‘HttpHandlers’. They help us to inject custom logic before and after the ASP.NET page is processed. The main differences between both of them are:-
• If you want to inject logic based in file extensions like ‘.ASPX’ , ‘.HTML’ then you use ‘HttpHandler’. In other words ‘HttpHandler’ is an extension based processor.


• If you want to inject logic in the events of ASP.NET pipleline then you use ‘HttpModule’. ASP.NET . In other word ‘HttpModule’ is an event based processor.




You can read more about the differences from http://tinyurl.com/yyfuc9r
Below is the logical flow of how the request is processed. There are 4 important steps MHPM as explained below :-
Step 1(M à HttpModule):- Client request processing starts. Before the ASP.NET engine goes and creates the ASP.NET ‘HttpModule’ emits events which can be used to inject customized logic. There are 6 important events which you can utilize before your page object is created ‘BeginRequest’,’AuthenticateRequest’,’AuthorizeRequest’,’ResolveRequestCache’,’AcquireRequestState’
and ‘PreRequestHandlerExecute’.
Step 2 (H à ‘HttpHandler’ ) :- Once the above 6 events are fired , ASP.NET engine will invoke ‘ProcessRequest’ event if you have implemented ‘HttpHandler’ in your project.
Step 3 (P – ASP.NET page):- Once the ‘HttpHandler’ logic executes the ASP.NET page object is created. While the ASP.NET page object is created many events are fired which can help us to write our custom logic inside those page events. There are 6 important events which provides us placeholder to write
logic inside ASP.NET pages ‘Init’ , ‘Load’ , ‘validate’ , ‘event’ , ‘render’ and ‘unload’. You can remember the word ‘SILVER’ to remember the events S – Start ( does not signify anything as such just forms the word ) , I – (Init) , L ( Load) , V ( Validate) , E ( Event) and R ( Render).
Step4 (M à HttpModule):- Once the page object is executed and unloaded from memory ‘HttpModule’ provides post page execution events which can be used to inject custom post-processing logic. There are 4 important post-processing events ‘PostRequestHandlerExecute’, ‘ReleaserequestState’, ‘UpdateRequestCache’ and ‘EndRequest’.
Below figure shows the same in a pictorial format.


In What event we should do what?
The million dollar question is in which events we should do what? . Below is the table which shows in which event what kind of logic or code can go.

A sample code for demonstration
With this article we have attached a sample code which shows how the events actually fire. In this code we have created a ‘HttpModule’ and ‘Httphandler’ in this project and we have displayed a simple response write in all events , below is how the output looks like.Below is the class for ‘HttpModule’ which tracks all event s and adds it to a
global collection.

public class clsHttpModule : IHttpModule

{

......

void OnUpdateRequestCache(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnUpdateRequestCache");

}

void OnReleaseRequestState(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnReleaseRequestState");

}

void OnPostRequestHandlerExecute(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnPostRequestHandlerExecute");

}

void OnPreRequestHandlerExecute(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnPreRequestHandlerExecute");

}

void OnAcquireRequestState(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnAcquireRequestState");

}

void OnResolveRequestCache(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnResolveRequestCache");

}

void OnAuthorization(object sender, EventArgs a)

{
objArrayList.Add("httpModule:OnAuthorization");

}

void OnAuthentication(object sender, EventArgs a)

{


objArrayList.Add("httpModule:AuthenticateRequest");
}

void OnBeginrequest(object sender, EventArgs a)

{


objArrayList.Add("httpModule:BeginRequest");

}

void OnEndRequest(object sender, EventArgs a)

{

objArrayList.Add("httpModule:EndRequest");

objArrayList.Add("<hr>");

foreach (string str in objArrayList)

{
httpApp.Context.Response.Write(str + "<br>") ;

}

}

} 

Below is the code snippet for ‘HttpHandler’ which tracks ‘ProcessRequest’ event.




public class clsHttpHandler : IHttpHandler

{

public void ProcessRequest(HttpContext context)

{

clsHttpModule.objArrayList.Add("HttpHandler:ProcessRequest");

context.Response.Redirect("Default.aspx");

}
}



We are also tracking all the events from the ASP.NET page.



public partial class _Default : System.Web.UI.Page 
{
protected void Page_init(object sender, EventArgs e)
{

clsHttpModule.objArrayList.Add("Page:Init");
}
protected void Page_Load(object sender, EventArgs e)
{
clsHttpModule.objArrayList.Add("Page:Load");
}
public override void Validate()
{
clsHttpModule.objArrayList.Add("Page:Validate");
}
protected void Button1_Click(object sender, EventArgs e)
{
clsHttpModule.objArrayList.Add("Page:Event");
}
protected override void Render(HtmlTextWriter output)
{
clsHttpModule.objArrayList.Add("Page:Render");
base.Render(output);
}
protected void Page_Unload(object sender, EventArgs e)
{
clsHttpModule.objArrayList.Add("Page:UnLoad");
}}




Below is how the display looks like with all events as per the sequence discussed in the previous section.





Zooming ASP.NET page events


In the above section we have seen the overall flow of events for an ASP.NET page request.
One of the most important section is the
ASP.NET page, we have not discussed the same in detail. So let’s take some luxury to describe
the ASP.NET page events in more detail in this section.

Any ASP.NET page has 2 parts one is the page which is displayed on the browser which has HTML tags ,
hidden values in form of viewstate and data on the HTML
inputs. When the page is posted these HTML tags are created in to ASP.NET controls with viewstate
and form data tied up together on the server.
Once you get these full server controls on the behind code
you can execute and write your own login on the same and render the page back to the browser.







Now between these HTML controls coming live on the server as ASP.NET controls, the ASP.NET page emits out lot of events
which can be consumed to inject logic. Depending on what task / logic
you want to performwe need to put these logics appropriately in those events.


Note: - Most of the developers directly use the ‘page_load’ method for everything, which is not a good thought. So it’s either
populating the controls, setting view state, applying themes etc everything happens on the page load. So if we can put logic in proper events as per the
nature of the logic that would really make your code clean.










References

I am not so smart to write this article by myself ;-) , lot of things I have plugged from the below articles.

Intercepting filters:-http://msdn.microsoft.com/en-us/library/ms998536.aspx

Explains how to implement Httphandlers and modules:- http://msdn.microsoft.com/en-us/library/system.web.httpapplication.aspx


Httphandlers and Httpmodules :-http://www.15seconds.com/Issue/020417.htm

Implementing security using modules and handlers :-http://joel.net/articles/asp.net2_security.aspx

Difference between Httpapplication and global.asax :-http://codebetter.com/blogs/karlseguin/archive/2006/06/12/146356.aspx