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10 November 2010

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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