The const keyword

C# offers the const keyword to define variables with a fixed, unalterable value (constant data). Once the value of a constant has been established, it can never change, and any attempt to alter it results in a compiler error.

Note: The declaration and initialization of a constant field must be simultaneous. This is because of the fact that the value of constant data must be known at compile time.

Ex:

class MyMathClass

{

public const double PI = 3.14;

}

class Program

{

static void Main(string[] args)

{

Console.WriteLine("The value of PI is: {0}", MyMathClass.PI);

}

}

Notice that the constant data defined by MyMathClass is referenced using a class name prefix (i.e., MyMathClass.PI). This is due to the fact that constant fields of a class are implicitly static.

The value assigned to a constant variable must be known at compile time, and therefore a constant member cannot be assigned to an object reference (whose value is computed at runtime). Constant fields are implicitly static. However, it is permissible to define and access a local constant variable within a type member.

Ex:

static void LocalConstStringVariable()

{

// A local constant data point can be directly accessed.

const string fixedStr = "Fixed string Data";

Console.WriteLine(fixedStr);

}

Unlike in C++, in C# the const keyword cannot be used to qualify parameters or return values, and is reserved for the creation of local or instance-level data.

Related Post: Read Only Fields in C#