Why can Array.Length and Array.Count() not be assigned to const values?

Question

I wanted to follow some excellent C++ advice of calculating the array length once and then using a value without having to call a function like so:

Instead of:

for( int i = 0; i < arr.length; ++i )

I write

const int size = arr.length; // or arr.Count()
for( int i = 0; i < size; ++i )

After reading a different thread (Is it costly to do array.length or list.count in a loop) I noticed that the performance gain is moot and that this is C# and not C++.

The second reason for this was to initialize my array with a value:

const int size = arr.Length;

int[] primes_ = new int[size];

So my question is this:

Why can't I declare this anyway? It spits me the error:

Error 2 The expression being assigned to 'length' must be constant

Which is very confusing because my value IS constant. However, when I remove the const, poof goes the message. ...What?

The initialization of Length reads:

public int Length {get;} from MSDN. Which is especially confusing because my last question (Get-Set Accessor functionality differs on existence of get-set keyword) explicitly gave me the answer of "It's not possible to declare only get and leave set absent."

It's not clear to my why I can declare int size = arr.Length, but not const int size = arr.Length. Why is this so?

Answer 1

Definition with const in C# requires an initializer that can be evaluated at compile time.

Look here.

You can use readonly

Answer 2

The const keyword requires the value to be known at compile time - the .Net compiler cannot determine the value of arr.length when you compile the code, so you can't assign it to a const.

With regard to your last question about getters and setters - you must declare both only if you use the short cut syntax: i.e. private long n { get; private set; }. If you use the full, verbose property declaration with a local field, you can declare just a get:

private long _n = 0;
private long N { get { return _n; } }