C#.Net对象内存模型及堆/栈数据结构详解 (一)

           public int AddFive(int pValue)
          {
                int result;
                result = pValue + 5;
                return result;
          }

Here''s what happens at the very top of the stack.  Keep in mind that what we are looking at is placed on top of many other items already living in the stack:

Once we start executin ghte method, the method''s parameters are placed on the stack (we''ll talk more about passing parameters later).

NOTE : the method does not live on the stack and is illustrated just for reference.

Next, control (the thread executing the method) is passed to the instructions to the AddFive() method which lives in our type''s method table, a JIT compilation is performed if this is the first time we are hitting the method.

As the method executes, we need some memory for the "result" variable and it is allocated on the stack.

The method finishes execution and our result is returned.

And all memory allocated on the stack is cleaned up by moving a pointer to the available memory address where AddFive() started and we go down to the previous method on the stack (not seen here).

In this example, our "result" variable is placed on the stack.  As a matter of fact, every time a Value Type is declared within the body of a method, it will be placed on the stack.

Now, Value Types are also sometimes placed on the Heap.  Remember the rule, Value Types always go where they were declared?  Well, if a Value Type is declared outside of a method, but inside a Reference Type it will be placed within the Reference Type on the Heap.

Here''s another example.

If we have the following MyInt class (which is a Reference Type because it is a class):

          public class MyInt
          {         
             public int MyValue;
          }

and the following method is executing:

          public MyInt AddFive(int pValue)
          {
                MyInt result = new MyInt();
                result.MyValue = pValue + 5;
                return result;
          }

Just as before, the thread starts executing the method and its parameters are placed on sthe thread''s stack.

Now is when it gets interesting...

Because MyInt is a Reference Type, it is placed on the Heap and referenced by a Pointer on the Stack.

After AddFive() is finished executing (like in the first example), and we are cleaning up...

we''re left with an orphaned MyInt in the heap (there is no longer anyone in the Stack standing around pointing to MyInt)!

This is where the Garbage Collection (GC) comes into play.  Once our program reaches a certain memory threshold and we need more Heap space, our GC will kick off.  The GC will stop all running threads (a FULL STOP), find all objects in the Heap that are not being accessed by the main program and delete them.  The GC will then reorganize all the objects left in the Heap to make space and adjust all the Pointers to these objects in both the Stack and the Heap.  As you can imagine, this can be quite expensive in terms of performance, so now you can see why it can be important to pay attention to what''s in the Stack and Heap when trying to write high-performance code.

Ok... That great, but how does it really affect me?

Good question. 

When we are using Reference Types, we''re dealing with Pointers to the type, not the thing itself.  When we''re using Value Types, we''re using the thing itself.  Clear as mud, right?

Again, this is best described by example.

If we execute the following method:

          public int ReturnValue()
          {
                int x = new int();
                x = 3;
                int y = new int();
                y = x;      
                y = 4;         
                return x;
          }

We''ll get the value 3.  Simple enough, right?

However, if we are using the MyInt class from before

     public class MyInt
          {
                public int MyValue;
          }

and we are executing the following method:

          public int ReturnValue2()
          {
                MyInt x = new MyInt();
                x.MyValue = 3;
                MyInt y = new MyInt();
                y = x;                 
                y.MyValue = 4;              
                return x.MyValue;
          }

What do we get?...    4!

Why?...  How does x.MyValue get to be 4?... Take a look at what we''re doing and see if it makes sense:

In the first example everything goes as planned:

          public int ReturnValue()
          {
                int x = 3;
                int y = x;    
                y = 4;
                return x;
          }

In the next example, we don''t get "3" because both variables "x" and "y" point to the same object in the Heap.

          public int ReturnValue2()
          {
                MyInt x;
                x.MyValue = 3;
                MyInt y;
                y = x;                
                y.MyValue = 4;
                return x.MyValue;
          }

Hopefully this gives you a better understanding of a basic difference between Value Type and Reference Type variables in C# and a basic understanding of what a Pointer is and when it is used.  In the next part of this series, we''ll get further into memory management and specifically talk about method parameters.