Difference Between Const, ReadOnly and Static ReadOnly in C#

These are very common keywords and are quite confusing. So today we will discuss these keywords and try to understand them.

1. Const: Const is nothing but "constant", a variable of which the value is constant but at compile time. And it's mandatory to assign a value to it. By default a const is static and we cannot change the value of a const variable throughout the entire program.
   
   
   Here I have created a class named Variables and defined all three variables, so first let's play with const.
   
   
   
   Here I tried to de-initialize the const variable, it gaves me an error like "A const field requires a value to be provided".  Ok now I initialize a value for this variable and try to change it further in the class.
   
   
   
   Here I have created a static constructor, default constructor, parameterized constructor and a Simple Method. I tried to change the value of the const variable everywhere but once I assign the value, I am unable to change it again since when I do it gives me a compile time error as you can see in the snapshot above.
   
   Now let's on to the readonly keyword.
    
2. Readonly: Readonly is the keyword whose value we can change during runtime or we
   
   can assign it at run time but only through the non-static constructor. Not even a method. Let's see:
   
   
   
   Here first I try to initialize the value in the static constructor. It gives me an error. Which you can see above.
   Now I try to change the value in a method, see what happened:
   
   
   
   Here, it is also giving an error that you can only assign a value either through a variable or a constructor.
   Now try to change the value in the default constructor.
   
   
   
   Now in the snapshot above you can see it's built successfully without an error, warning or messages. Let's check if there is a runtime error. OK.
   
   
   
   Now here we can see that there is not a runtime error and the value was assigned successfully to the Readonly variable.
   Now one gotcha is, now that you have assigned the value, can you change this value again ??? 
   Let's try to change the value again.
   
   
   
   Here I created  a parameterized constructor and created a new object, and passing a value as "Hello Frend'z" and as I built it, it gave me the result "Build Succeeded".  Now let's move ahead and check for a runtime error:
   
   
   
   See guys. There is no runtime error !!  And the value can be changed again and again through a constructor.
   
   Now move ahead to Static Readonly variables.
    
3. Static ReadOnly: A Static Readonly type variable's value can be assigned at runtime or assigned at compile time and changed at runtime. But this variable's value can only be changed in the static constructor. And cannot be changed further. It can change only once at runtime. Let's understand it practically.
   
   
   
   Now in the preceding you can see that I used two variables, one is not assigned and another is assigned, and the static constructor.  Now in the static constructor you can see that the unassigned variable is being assigned and the assigned value is being changed. And there is no compile time error. Further I try to again change this variable's value.  See what happened:
   
   
   
   As you can see in the above, I created Default, Parameterized Constructor and Method and tried to change the value again here. But I am getting a compile time error for all.

Read only and const variable in C#

Posted by Vithal Wadje in Articles | C# Language on November 18, 2013

in this article we will learn about the read only and constant variables in C#

Background

Whenever i took the interview of any candidate then i always asked one question to them what is read only and constant variable then believe me even four plus experienced candidate get confused ,so whatever i learned from my tech gurus about this concept  according to that i will try to explain it in details so beginners and experienced candidate can understand it.

so let us start with definition 

What is Constant variable ?

The variable whose value can not be changed during the execution of the program is called as constant variable.

in the above definition value can not be changed during execution of the program means we can not be assign the values to constant variable at run time it must be assign at compile time at the declaration of constant variable.

constants are of following types

 

In the above diagram ,we can see the Constants are of two types

1. Compile time constants (const)
2. Runtime constants  (Readonly)

•  Compile time constants 

The compile time constants are declared by using the const keyword which value can not be changed during the execution of the program.

Syntax

     int const a=10;

Some key points about const variable

•   Its must to assign value at the time of declaration.

             eg.

             int const a=10;  

•   const only allow constant variables into the expression.

   eg.

          int const a=10;

          int const b=20; // correct

          int const c=a+b; 

          int  d=1;

          int const c=d+b;   //compile time error because d variable is the non constant into expression.

•   const can be declared at class level as wel as inside the method.
•  const can not be declared using static keyword because they are by default static.
•   constants are  absolute constant which value can no be changed or assigned at the run time.
• constant variable are compile time constant variable.

 When to use const

The const is used when their is value is absolute constant such PI values which can not be changed,but according to your requirement you can use it as wish as rather than PI values declaration.

Example of const   variable.

1.  using System;  

2.    

3.  namespace UsingConst  

4.  {  

5.      class Program  

6.      {  

7.          const int a = 10;  

8.             

9.         static void Main(string[] args)  

10.         {  

11.             const int b = 20;  

12.             const int c = b + a;  

13.             Console.WriteLine(c);  

14.             Console.ReadLine();  

15.         }  

16.     }  

17. }  

 the output of the above program is 30,from the above example we clearly see that the const must be assign the value at declaration time and in expression both the variable must be const.

Runtime constants (Readonly)

The Run time constants are declared by using the Readonly keyword which value can not be changed during the execution of the program.

Syntax

int Readonly a; or

 int Readonly a=0;

Some key points about const variable

•  Its not  must to assign value at the time of declaration,we can also assign the value for readonly through constructor.

eg.

int readonly a;

a=0;

•   Readonly allows, readonly  constant as wel as non  readonly constant  variables into the expression.

eg.

int readonly a=10;

int b=1;

int readonly c=a+b;

• Readonly can be declared only at class level not inside the method.
• Readonly can not be declared using static keyword because they are by default static.
• Readonly   constants  value can  be set through reference variable.
• Readonly  constant variable are runtime time constant variable.

When to use Readonly

we can use Readonly,   when value is not  absolute constant  means which can be changed frequently,like dollar vs INR ,in this requirement we can set the value through configuration file or another variable expression so we can avoid to change class file frequently.

Example of Readonly variable.

let us define the value from config file  for readonly constant variable ,which will be set through constructor .

1.  <configuration>  

2.    <appSettings>    

3.      <add key="DollarPrice" value="61.23"/>  

4.    </appSettings>  

5.      

6.  </configuration>  

 now,let us explain it through sample program

1.  using System;  

2.  using System.Configuration;  

3.    

4.  namespace Usingreadonly 

5.  {  

6.      class Program  

7.      {  

8.          readonly int a = 10;  

9.          int b = 30;  

10.         int c;  

11.         readonly string r;  

12.         public Program()  

13.         {  

14.             r = ConfigurationManager.AppSettings["DollarPrice"];  

15.             Console.WriteLine("The dollar value is:"+" "+r);  

16.             Console.WriteLine();  

17.             c = a + b;  

18.             Console.WriteLine("The addition of readonly constant and non Readonly constant is :"+Environment.NewLine+ c);  

19.             Console.ReadLine();  

20.         }  

21.   

22.         static void Main(string[] args)  

23.         {  

24.             Program p = new Program();  

25.              

26.             Console.ReadLine();  

27.         }  

28.     }  

29. }  

 in the above program ,we have assigned the value for readonly constant through constructor which is defined in the config  file,because readonly constant not must to assign the value at the time of declaration.

now run the program the output will be as.

 

let us  outline the differences  between const and readonly variables.

1.  const fields has to be initialized while declaration only, while readonly fields can be initialized at declaration or in the constructor.
2. const variables can declared in methods ,while readonly fields cannot be declared in methods.
3. const fields cannot be used with static modifier, while readonly fields can be used with static modifier.
4.  const field is a compile-time constant, the readonly field can be used for run time constants.