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C# - What is the Difference between Const and Readonly

Oct 31, 2012
Introduction:

Here I will explain what are the differences between const and readonly in c#.net and vb.net.

Description:

In previous posts I explained convert.tostring() VS .tostring(), web application VS website, difference between datareader, dataadapter and dataset, difference between tinyint, smallint, int, bigint and many articles relating to asp.net, SQL Server. Now I will explain the difference between const and readonly in c#.net and vb.net.

Const:

      1.    Const can only be initialized at the time of declaration of the field.
      2.    Const values will evaluate at compile time only.
      3.    Const value can’t be changed these will be same at all the time.
      4.    This type of fields are required when one of the field values remains constant throughout the system like Pi will remain same in your Maths Class.

Read-only:

     1.    The value will be initialized either declaration time or the constructor of the class allowing you to pass the value at run time.
      2.    Read only values will evaluate at runtime only.
        
Example

public class Const_VS_Readonly
{
public const int I_CONST_VALUE = 2;
public readonly int I_RO_VALUE;
public Const_VS_Readonly()
{
I_RO_VALUE = 3;
}
}
Suppose if you want the value of the constant won't change use a const or if you have a constant that may change or when in doubt, use a readonly.  I hope it helps.

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11 comments :

vipin pandey said...

hi suresh ji! you r doing great job.thanks for you articles.

sir,its my request to u ,please teach us MVC and WCF.
highly thankful to you. one of the most valuable sites for .net

Harsha said...

i always get confused with these two keywords, i faced this question many times in interviews. thank you for sharing.

Harsha said...

one more difference is, readonly variables can only be declared as global variables, but const variables can also be declared in a method or constructor.

Anonymous said...

nice sir...but i have a small confusion that is what is the difference b/w static and const and read only?

shoeb said...

thanks sir,
can you please explain static is well

Anonymous said...

thanks sir,it is really a nice site for .net

Vipin Kushwaha said...

THANKS SIR,,,,,

Vipin Kushwaha said...

THANKS SIR,,,,,

sureshdangeti said...

hi suresh, u done great job, i am unable to post or
send feedback,

Krishan Kumar said...

thank sir
Please MVC and WCF article need, help me

Anonymous said...

really a nice site for .net learners.....

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