Differences between const and readonly

Posted Date: 20 Jun 2008	Resource Type: Articles	Category: .NET Framework
Author: Gaurav Agrawal	Member Level: Silver
Rating:	Points: 10

This article explains the differences between a "const" and a "readonly" variable with examples (in C#)

A Const field can only be initialized at the declaration of the field.

For Example:


public class class1
        { 
          const int valu11 = 5;   // Declared a Const and assigned a value to it
        }

But when we try to do something, as in this code sample


        public class class1
        { 
          const int valu11 = 5;
          public class1           // Constructor
          {
             value1 = 10;  // Error  
          }
        }

In the above example, we have assigned a new value to constant and .NET does not allow this because we cannot change the value of the constant field throughout the program.

A const field is a compile-time constant while a readonly field has two point at which it can be - initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used.

Example:


public class class1
      { 
       readonly int valu11 = 5;// Declared a readonly and assigned a value to it
      }

        public class class1
        { 
          readonly int valu11 = 5;
          public class1           // Constructor
          {
             value1 = 10; // Allowed
          }
          public void method1()
          {
              value1 = 15  // Error
          }
        }

We can change the value of readonly field as many times as we want but only inside a constructor and if we try to change it outside the constructor (in any method or any other location)then program will not compile and will show a error like that a readonly field can not be assigned to(execpt in a constructor or a variable intializer.)

The readonly field can be used for runtime constants also.

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