What is the difference between const and readonly in C#?
There is a gotcha with consts! If you reference a constant from another assembly, its value will be compiled right into the calling assembly. That way when you update the constant in the referenced assembly it won't change in the calling assembly!
Apart from the apparent difference of
- having to declare the value at the time of a definition for a
const
VSreadonly
values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen. const
's are implicitlystatic
. You use aClassName.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' theAssemblyB
's IL. This means that if tomorrow I updateI_CONST_VALUE
to 20,AssemblyB
would still have 2 till I recompile it. - in the case of the
readonly
value, it is like aref
to a memory location. The value is not baked intoAssemblyB
's IL. This means that if the memory location is updated,AssemblyB
gets the new value without recompilation. So ifI_RO_VALUE
is updated to 30, you only need to buildAssemblyA
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