What is difference between Readonly and Constant Keyword??

Priyanka Singh

6y969Visual Basic .NET

What is difference between Readonly and Constant Keyword in c#?

5 Replies

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Rajanikant Hawaldar6y ago
Accepted answer
Hey Priyanka, However, both the keywords are very different from each other. Const is a reserved word used to represent constant values and it specifies that a value cannot be modified after compile-time, whereas readonly is a modifier that can be used on fields and the value of which can be changed during runtime.

'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'll update I_CONST_VALUE to 20 in the future. 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. 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;
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Rajanikant Hawaldar6y ago
@Prinyank yes now your clear on this
+1
Bhavna Chaudhri6y ago
@Sachin Singh, what a cool definition, now I have no doubt about this, thank you so much.
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Sachin Singh6y ago
hello priyanka ,after this, in your whole life you will never ever be confused with const and readonly.

Yes both are constant (which can not be changed) but now listen carefully

1.Const is a compile time constant , meaning when you declare a const you are bound to assign a value to it.
you cant do const int meterToCm; you are bound to assign it like const int meterToCm=100; otherwise compilar will shout at you.

2.Readonly is a run time constant meaning you are free to assign value at run time, enjoy you can save a value in config file then read it and assign it to readonly field in constructor of a class , don't worry this time compilar will not shout at you.

the more interesting fact that i use,
const is 100%, without any doubt, crorepati wala lock kar diya jaaye type constant like meterToCm it will always be 100 in every area of world so it is logical to assign value at compile time only.

while Read only is half constant meaning the value can be different for different reasons for example Pi , somebody can say i will take 3.14 but some like me can say, no i am irfan of life of pi , and i will take the value of Pi till 8 precision so its better to assign value at run time by any config file.

Don't be confused after assigning the value whether at compile time or run time you can not change it, arre yaar these are constant by EOD.
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Priyanka Singh6y ago
Hello Rajanikant Hawaldar

Thanks for giving the answer
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;
}
}
I just want to clear that in the example you explained me that const value cannot be changed until we compliled it. Like as you said if i assigned I_CONST_VALUE = 2o; and before compile its value remain I_CONST_VALUE = 2; and after we complie constant value changes,and for readonly its value changes at run time,like if we assigned
I_RO_VALUE = 30; at runtime so its value changes.I am right or still confused.

Please let me know.
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