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What is the difference between const and readonly and do you use one over the other?
asked by (1.1k points) 13 28 42

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Apart from the apparent diff 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 ctor 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() 
= 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 tomorrow if I update I_CONST_VALUE to 20 in the future. Assembly B 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, Assembly B gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA. 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; 
answered by (1.9k points) 1 2 7

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