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";
  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);
  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;
  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;
  10. //Valid scenario
  11. readonly int Age;
  13. readonly string Name = "Satish Kumar";
  15. //readonly fields can be initlized in constructor
  16. public ReadOnlyTest (string name)
  17. {
  18. Age = 23;
  19. Name = name;
  20. }
  22. public changeName(string newName)
  23. {
  24. Name = "Satish Kumar Vadlavalli"; ; // Will result error.
  25. }
  26. }
  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.
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