Why do we need static private array to initialize other non static private array fields?

Question

If you take a look inside Stack<T> class from .NET 4.0, you will notice that there is an "emptyArray" private static field which is used inside constructors to initialize a real "array" private field.

private T[] array;
private static T[] emptyArray;
private int size;
private int version;

static Stack()
{
    Stack<T>.emptyArray = new T[0];
}

public Stack()
{
    array = Stack<T>.emptyArray;
    size = 0;
    version = 0;
}

Why not just put this.array = new T[0]; ? And also why there are placed initialization strokes for size and version fields, if you omit those lines they will be initialied to default values (0) anyway.

Ronald Wildenberg · Accepted Answer · 2010-08-31 08:56:14Z

1

That's because otherwise, every Stack gets his own instance of a new T[0]. Now they all refer to the same instance: the one that's declared static. Suppose you declare 1000 Stack<string>. These all have a reference to one string[0] object. If you declared the empty array inside the constructor, you would have a 1000 string[0] instances, one for each Stack<string>. So it's for performance reasons.

The other initializers are unnecessary but if you take a look with Reflector through other source files, you see the same pattern everywhere: fields with default values are assigned explicit values inside a constructor.

edited Aug 31, 2010 at 8:56

answered Aug 31, 2010 at 8:44

Ronald Wildenberg

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6 Comments

Grief Coder Over a year ago

Every Stack is already getting his own T[0] when constructor is called array = Stack<T>.emptyArray, isn't it?

Ronald Wildenberg Over a year ago

No, they get a reference (only a pointer) to the same instance.

Ronald Wildenberg Over a year ago

I think it may be a coding convention not to depend on default values anywhere but to make them explicit. Another possibility is that Reflector simply shows it this way because the underlying IL code assigns explicit 'default' values. That would mean that the compiler adds these statements. In the end every variable should have a value so it could be the compiler's job to assign default values. You could test this by writing a small class that doesn't explicitly assign default values, compile it and open it inside Reflector.

atamanroman Over a year ago

No, every stack gets a reference to this single version of an empty array. Theres only one thinkable type of an empty array<T> (which is, surprise, empty), so emptyArray is a singleton due to performance reasons. Same should go for true, false, small integers (at least in java its < 256 or so). This reduces memory usage.

Grief Coder Over a year ago

I've just compiled a testing app, opened it in Reflector.. default values are not initialized there.

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