C#

Difference Between Constant And ReadOnly

Constants

  • Constants are static by default.
  • They must have a value at compilation-time (you can have e.g. 3.14 * 2 + 5, but cannot call methods).
  • Could be declared within functions.
  • These are copied into every assembly that uses them (every assembly gets a local copy of values).
  1. using System;   
  2. namespace ConstantExample  
  3. {   
  4.     class ConstantExample  
  5.     {          
  6.         private const int PI = 3.14;   
  7.         public static void Main()  
  8.         {  
  9.             //You have to initialize Const variables while declaration  
  10.             const int RollNo = 1284;   
  11.             //Valid scenario  
  12.             const int Age = 10 + 13;   
  13.             const string Name = "Satish Kumar";  
  14.    
  15.             // Reassigning a const variable is not allowed.  
  16.             RollNo = 1405;   // Will result compile time error.  
  17.             Age++;    // Will result compile time error.   
  18.             Console.WriteLine(Name);  
  19.    
  20.             Console.Read();  
  21.         }  
  22.     }  
  23. }  
Readonly
  • Must have set value, by the time constructor exits.
  • Are evaluated when instance is created.
  • You can use static modifier for readonly fields.
  • readonly modifier can be used with reference types.
  • readonly modifier can be used only for instance or static fields, you cannot use readonly keyword for variables in the methods. 
  1. using System;  
  2.    
  3. namespace ReadOnlyExample  
  4. {  
  5.    class ReadOnlyTest   
  6.    {    
  7.       //You have to initilize readonly varabiles while declaration or in constructor  
  8.       readonly int RollNo = 1284;  
  9.    
  10.       //Valid scenario  
  11.       readonly  int Age;  
  12.    
  13.       readonly  string Name = "Satish Kumar";  
  14.   
  15.       //readonly fields can be initlized in constructor  
  16.       public ReadOnlyTest (string name)  
  17.       {  
  18.           Age = 23;  
  19.           Name = name;     
  20.       }  
  21.   
  22.       public changeName(string newName)  
  23.       {  
  24.           Name = "Satish Kumar Vadlavalli"; ;   // Will result error.  
  25.       }  
  26.    }  
  27.    
  28.    class ReadOnlyExample  
  29.    {   
  30.         public static void Main()  
  31.         {    
  32.            ReadOnlyTest obj = new ReadOnlyTest("Satish");     
  33.            Console.Read();    
  34.         }  
  35.    }  
  36. }  
Difference between const and readonly

  • const fields has to be initialized while declaration only, while readonly fields can be initialized at declaration or in the constructor.
  • const variables can declared in methods ,while readonly fields cannot be declared in methods.
  • const fields cannot be used with static modifier, while readonly fields can be used with static modifier.
  • A const field is a compile-time constant, the readonly field can be used for run time constants.
A blog without comments is not a blog at all, but do try to stay on topic. Please comment if you have any questions or leave your valuable feedback. Happy Learning.
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