Delphi		Constants must be declared before using in special const blocks: const pi = 3.14159265359; URL = 'http://www.greatis.com'; This block can be both global declared the unit's interface or implementation part and local declared within the function or procedure: procedure Dummy; const // this is local constants StartValue = 0; EndValue = 10; var i: Integer; begin for i:=StartValue to EndValue do // some code here end; There are so-called typed constants, but now this feature is obsolete and you have to use initialized variables instead: // obsolete code: const Start: Integer = 99; // use it instead: var Start: Integer = 99;
C#		There are two related features: const and readonly. Both of them are used as modificators of teh variable declaration. The const keyword is used to modify a declaration of a field or local variable. It specifies that the value of the field or the local variable cannot be modified: class Test { public const double pi = 3.14159265359; public const string URL = "http://www.greatis.com"; void Foo() { const int x = 1; } }; The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class: class Test { public readonly int y = 25; // Initialize a readonly field public Test() { y = 24; // Initialize a readonly instance field } } The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants: class Test { public readonly int y = 25; // Initialize a readonly field public const int z = 30; // Initialize a const field public Test() { // y = 25; // default value } public Test(int var) { y = var; // OK, initialize a readonly field z = var; // Error can't initialize const field } }      Fields can be declarated as readonly. This keyword allows to modify variable value in constructor of the class.