what is the difference between const, readonly and get in a static class [duplicate]

Question

I have a question. Lately I have caught myself using 3 different lines of code which upon closer inspection looks and feels the same.

public static class constant
{
    public static readonly int val1 = 5;
    public const int val2 = 5;
    public static int val3 { get { return 5; } }
}

my question is, are they the same and should one be used over another? if so. When?

also as a extra question in visual studio why are they all represented differently in intellisense?

Answer 1

The member declared as readonly gives the possibility to be changed in the (static) constructor of the class, while the const member cannot be changed at runtime.

Declaring a field as const makes it automatically static, quoting from §10.3.7:

When a field, method, property, event, operator, or constructor declaration includes a static modifier, it declares a static member. In addition, a constant or type declaration implicitly declares a static member.

The third is just a read-only property which happens to always return 5.

You should never use such a property and prefer const members where possible in order to allow the compiler and/or the jitter to perform their optimizations and to help other people reading your code (that property is kind of a WTF to me). The static readonly member has to be used if it is required a constant value initialized during the program start-up (like, for example, the number of cores of a machine).

This is a great example from the C# specs (§10.5.2.1):

A static readonly field is useful when a symbolic name for a constant value is desired, but when the type of the value is not permitted in a const declaration, or when the value cannot be computed at compile-time. In the example

public class Color
{
    public static readonly Color Black = new Color(0, 0, 0);
    public static readonly Color White = new Color(255, 255, 255);
    public static readonly Color Red = new Color(255, 0, 0);
    public static readonly Color Green = new Color(0, 255, 0);
    public static readonly Color Blue = new Color(0, 0, 255);
    private byte red, green, blue;
    public Color(byte r, byte g, byte b) {
        red = r;
        green = g;
        blue = b;
    }
}

the Black, White, Red, Green, and Blue members cannot be declared as const members because their values cannot be computed at compile-time. However, declaring them static readonly instead has much the same effect.

And yet another difference (§10.5.2.2):

Constants and readonly fields have different binary versioning semantics. When an expression references a constant, the value of the constant is obtained at compile-time, but when an expression references a readonly field, the value of the field is not obtained until run-time.

So, summing it up, they are very different even if at a first glance they might look similar and you should use the one which best suits your intent.