Understading Stack and Heap

The stack is the memory set aside as scratch space for a thread of execution.

When a function is called, a block is reserved on the top of the stack for local variables and some bookkeeping data. When that function returns, the block becomes unused and can be used the next time a function is called.

The stack is always reserved in a LIFO order; the most recently reserved block is always the next block to be freed. This makes it really simple to keep track of the stack; freeing a block from the stack is nothing more than adjusting one pointer.

The heap is memory set aside for dynamic allocation. 

Unlike the stack, there's no enforced pattern to the allocation and deallocation of blocks from the heap; you can allocate a block at any time and free it at any time. This makes it much more complex to keep track of which parts of the heap are allocated or free at any given time; there are many custom heap allocators available to tune heap performance for different usage patterns.

Each thread gets a stack, while there's typically only one heap for the application (although it isn't uncommon to have multiple heaps for different types of allocation).

To what extent are they controlled by the OS or language runtime?

The OS allocates the stack for each system-level thread when the thread is created. Typically the OS is called by the language runtime to allocate the heap for the application.

What is their scope?

The stack is attached to a thread, so when the thread exits the stack is reclaimed. The heap is typically allocated at application startup by the runtime, and is reclaimed when the application (technically process) exits.

What determines the size of each of them?

The size of the stack is set when a thread is created. The size of the heap is set on application startup, but can grow as space is needed (the allocator requests more memory from the operating system).

What makes one faster?

The stack is faster because the access pattern makes it trivial to allocate and deallocate memory from it (a pointer/integer is simply incremented or decremented), while the heap has much more complex bookkeeping involved in an allocation or free. Also, each byte in the stack tends to be reused very frequently which means it tends to be mapped to the processor's cache, making it very fast.

Understanding Generics in C#

Generics allow you to delay the specification of the data type of programming elements in a class or a method, until it is actually used in the program. In other words, generics allow you to write a class or method that can work with any data type.

When the compiler encounters a constructor for the class or a function call for the method, it generates code to handle the specific data type.

Generics is a technique that enriches your programs in the following ways:

• It helps you to maximize code reuse, type safety, and performance.
• You can create generic collection classes. The .NET Framework class library contains several new generic collection classes in the System.Collections.Genericnamespace. You may use these generic collection classes instead of the collection classes in the System.Collections namespace.
• You can create your own generic interfaces, classes, methods, events, and delegates.
• You may create generic classes constrained to enable access to methods on particular data types.
• You may get information on the types used in a generic data type at run-time by means of reflection.

Simple example on Generics -

// Declare the generic class.

public class GenericList<T>

{

    void Add(T input) 

   {

      //Do something here

   }

}

class TestGenericList

{

    private class ExampleClass 

   { }

   

    static void Main()

    {

        // Declare a list of type int.

        GenericList<int> list1 = new GenericList<int>();

        // Declare a list of type string.

        GenericList<string> list2 = new GenericList<string>();

        // Declare a list of type ExampleClass.

        GenericList<ExampleClass> list3 = new GenericList<ExampleClass>();

    }

}