How can I create a public const Size in C#?

Question

I am trying to write the following code:

public const Size ImageSize = new Size() { Width = 28, Height = 28 };

But I get the error that Width and Height are read-only.

What is the recommended way to do this?

Answer 1

const is restricted to primitives that the compiler can directly write as IL directly. readonly should suffice here if Size is treated as immutable, i.e.

public static readonly Size ImageSize = new Size(28,28);

Note that if Size is a mutable struct, bad things can happen; I would recommend a property rather than a field to prevent a number of confusing side-effects.

Answer 2

The fundamental problem is that you cannot declare an object of type System.Drawing.Size as const. That indicates that the symbol is to be replaced at compile-time with the value of the constant.

Instead, you should use readonly. This is also a "constant" value in the sense that it cannot be modified once the constructor has run, but the objects are created at run-time instead of compile-time.

The following code compiles just fine:

public static readonly Size ImageSize = new Size() { Width = 28, Height = 28 };