Question

I have a dummy class where I am testing arrays. I’ve noticed that when I want to dynamically allocate size of array at runtime, fields that indicate this size have to be static. I know I should probably use collections for this kind of code, but I’m more interested why do these fields have to be static? Is there any particular reason behind this?

class Foo
{
    private static int x;
    private static int y;

    private int[,] bar = new int[ x, y ];

    public Foo( int a, int b )
    {
        x = a;
        y = b;
    }
}

Answer 1

They don’t really have to be static – it’s just that you can’t refer to other instance variables within instance variable initializers. In other words, it’s a bit like this:

class Foo
{
    private int x;
    private int y = x; // Invalid
}

From section 10.5.5.2 of the C# 3 spec:

A variable initializer for an instance
field cannot reference the instance
being created. Thus, it is a
compile-time error to reference this
in a variable initializer, as it is a
compile-time error for a variable
initializer to reference any instance
member through a simple-name.

I suspect you really want something like this:

class Foo
{
    private int x;
    private int y;

    private int[,] bar;

    public Foo( int a, int b )
    {
        x = a;
        y = b;
        bar = new int[x, y];
    }
}

Of course you don’t really need x and y at all – they’re just convenient to keep each dimension of the array. You could also use bar.GetLength(0) and bar.GetLength(1) to get the two lengths, but that’s not terribly pleasant.

You might want to rename x and y to width and height though, or something similar 🙂