What is the difference between const
and readonly
in C#?
When would you use one over the other?
The first, const, is initialized during compile-time and the latter, readonly, initialized is by the latest run-time. The second difference is that readonly can only be initialized at the class-level. Another important difference is that const variables can be referenced through "ClassName.
readonly keyword is used to define a variable which can be assigned once after declaration either during declaration or in constructor. const keyword is used to define a constant to be used in the program.
Constant and ReadOnly keyword is used to make a field constant which value cannot be modified. The static keyword is used to make members static that can be shared by all the class objects.
ReadOnly is a runtime constant. Const is a compile time constant. The value of readonly field can be changed. The value of the const field can not be changed.
Apart from the apparent difference of
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:
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.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
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!
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