8.4 Automatic memory management(cont')

public void Push(object o) {
first = new Node(o, first);
}
class Node
{
public Node Next;
public object Value;
public Node(object value): this(value, null) {}
public Node(object value, Node next) {
Next = next;
Value = value;
}
}
}
shows a Stack class implemented as a linked list of Node instances. Node
instances are created in the Push
method and are garbage collected when no longer needed. A Node instance
becomes eligible for garbage
collection when it is no longer possible for any code to access it. For
instance, when an item is removed
from the Stack, the associated Node instance becomes eligible for garbage
collection.
The example
class Test
{
static void Main() {
Stack s = new Stack();
for (int i = 0; i < 10; i++)
s.Push(i);
s = null;
}
}
shows code that uses the Stack class. A Stack is created and initialized
with 10 elements, and then
assigned the value null. Once the variable s is assigned null, the Stack
and the associated 10 Node
instances become eligible for garbage collection. The garbage collector is
permitted to clean up immediately,
but is not required to do so.
The garbage collector underlying C# may work by moving objects around in
memory, but this motion is
invisible to most C# developers. For developers who are generally content
with automatic memory
management but sometimes need fine-grained control or that extra bit of
performance, C# provides the
ability to write .unsafe. code. Such code can deal directly with pointer
types and object addresses, however,
C# requires the programmer to fix objects to temporarily prevent the
garbage collector from moving them.
This .unsafe. code feature is in fact a .safe. feature from the perspective
of both developers and users.
Unsafe code must be clearly marked in the code with the modifier unsafe, so
developers can’t possibly use
unsafe language features accidentally, and the compiler and the execution
engine work together to ensure
that unsafe code cannot masquerade as safe code. These restrictions limit
the use of unsafe code to situations
in which the code is trusted.
The example
using System;
class Test
{
static void WriteLocations(byte[] arr) {
unsafe {
fixed (byte* pArray = arr) {
byte* pElem = pArray;
for (int i = 0; i < arr.Length; i++) {
byte value = *pElem;
Console.WriteLine("arr[{0}] at 0x{1:X} is {2}",
i, (uint)pElem, value);
pElem++;
}
}
}
}
static void Main() {
byte[] arr = new byte[] {1, 2, 3, 4, 5};
WriteLocations(arr);
}
}
shows an unsafe block in a method named WriteLocations that fixes an array
instance and uses pointer
manipulation to iterate over the elements. The index, value, and location
of each array element are written to
the console. One possible example of output is:
arr[0] at 0x8E0360 is 1
arr[1] at 0x8E0361 is 2
arr[2] at 0x8E0362 is 3
arr[3] at 0x8E0363 is 4
arr[4] at 0x8E0364 is 5
but, of course, the exact memory locations may be different in different
executions of the application.