C# – Static Readonly vs. Const

c++constants

I've read around about const and static readonly fields. We have some classes which contain only constant values. They are used for various things around in our system. So I am wondering if my observation is correct:

Should these kind of constant values always be static readonly for everything that is public? And only use const for internal/protected/private values?

What do you recommend? Should I maybe even not use static readonly fields, but rather use properties maybe?

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public static readonly fields are a little unusual; public static properties (with only a get) would be more common (perhaps backed by a private static readonly field).

const values are burned directly into the call-site; this is double edged:

it is useless if the value is fetched at runtime, perhaps from config
if you change the value of a const, you need to rebuild all the clients
but it can be faster, as it avoids a method call...
...which might sometimes have been inlined by the JIT anyway

If the value will never change, then const is fine - Zero etc make reasonable consts ;p Other than that, static properties are more common.

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C# Constants – Difference Between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits. After that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision) or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

C# Constants – Difference Between Const and Static

const fields can only hold value types or System.String. They must be immutable and resolvable at compile-time.

static readonly fields can and generally do hold reference types, which (other than strings) can only be created at runtime. These can (but shouldn't) be mutable types; the only thing that cannot change is the reference itself.

If you need to maintain a "constant" set of instances that are reference types, you generally do it with a set of public static readonly fields, such as the members of System.Drawing.SystemColors.

Last but not least, initialization of a readonly field can be deferred until the execution of a constructor, which means that it even though it can only be written to once, it does not always have to be initialized with the exact same value. True constants declared with const can only ever have a single value (specified at compile time).

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C# Constants – Difference Between const and readonly in C#

C# Constants – Difference Between Const and Static

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