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// Read-only fields can be assigned in ctors,

// but nowhere else.

public readonly double PI;

public MyMathClass ()

{

PI = 3.14;

}

}

Again, any attempt to make assignments to a field marked readonly outside the scope of a con-

structor results in a compiler error:

class MyMathClass

{

public readonly double PI;

public MyMathClass ()

{

PI = 3.14;

}

// Error!

public void ChangePI()

{ PI = 3.14444;}

}

C H A P T E R   5  

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D E F I N I N G   E N C A P S U L AT E D   C L A S S  T Y P E S

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Static Read-Only Fields

Unlike a constant field, read-only fields are not implicitly static. Thus, if you wish to expose PI from

the class level, you must explicitly make use of the static keyword. If you know the value of a static

read-only field at compile time, the initial assignment looks very similar to that of a constant:

class MyMathClass

{

public static readonly double PI = 3.14;

}

class Program

{

static void Main(string[] args)

{

Console.WriteLine("***** Fun with Const *****");

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

Console.ReadLine();

}

}

However, if the value of a static read-only field is not known until runtime, you must make use

of a static constructor as described earlier in this chapter:

class MyMathClass

{

public static readonly double PI;

static MyMathClass()

{ PI = 3.14; }

}

Now that we have examined the role of constant data and read-only fields, we can return to the

Employee example and put the wraps on this chapter.

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Source Code

The ConstData project is included under the Chapter 5 subdirectory.

Understanding Partial Types

Classes and structures can be defined with a type modifier named partial that allows you to define

a type across multiple *.cs files. Earlier versions of the language required all code for a given type

be defined within a single *.cs file. Given the fact that a production-level C# class may be hundreds

of lines of code (or more), this can end up being a mighty lengthy file indeed.

In these cases, it may be beneficial to partition a type’s implementation across numerous *.cs

files in order to separate code that is in some way more important from other aspects of the type

definition. For example, using the partial class modifier, you could place all of the Employee con-

structors and properties into a new file named Employee.Internals.cs: