Enumerations

An enumeration is a set of named constants that may apply to a particular operation. For example, when you open a file you typically want to specify whether the file is to be opened for reading, for writing, or for both. A method that opens a file will take a parameter that allows you to specify this. If this is implemented using an enumerated constant, the parameter may be one of a specific set of (typically) integer values; for example, 1=read, 2=write, 3=both read and write. However, to avoid using meaningless numbers in code, it is conventional to use names to represent particular values. These are known as enumerated constants or, more simply, as enums.

In the .NET Framework, enums are implemented as classes that inherit from the base class System.Enum. The class as a whole represents a set of enumerated constants; each of the constants themselves is represented by a static field within the class.

The next chapter deals with the use of System.Windows.Forms to create and manipulate the user interface. The classes in this .NET Namespace use enums extensively.

For example, there is a class named System.Windows.Forms.FormBorderStyle that contains a set of static fields named None, FixedDialog, Sizeable, and so forth. These fields have specific integer values, but the values themselves are of no interest to the programmer.

Typically, you use an enumerated constant as a parameter to a method or to specify the value of a property. For example, to create a Form with a particular border style, you would set its BorderStyle property to one of the members of the FormBorderStyle class, viz.

⎕USING←'System'
  ⎕USING,←⊂'System.Windows.Forms,system.windows.forms.dll'
f1←⎕NEW Form
f1.BorderStyle←FormBorderStyle.FixedDialog
FormBorderStyle.⎕NL ¯2 ⍝ List enum members
Fixed3D  FixedDialog  FixedSingle  FixedToolWindow  None 
Sizable  SizableToolWindow

An enum has a value, which you may use in place of the enum itself when such usage is unambiguous. For example, the FormBorderStyle.Fixed3D enum has an underlying value is 2:

     Convert.ToInt32 FormBorderStyle.Fixed3D
2

You could set the border style of the Form f1 to FormBorderStyle.Fixed3D with the expression:

      f1.BorderStyle←2

However, this practice is not recommended. Not only does it make your code less clear, but also if a value for a property or a parameter to a method may be one of several different enum types, APL cannot tell which is expected and the call will fail.

For example, when the constructor for System.Drawing.Font is called with 3 parameters, the 3^rd parameter may be either a FontStyle enum or a GraphicsUnit enum. If you were to call Font with a 3^rd parameter of 1, APL cannot tell whether this refers to a FontStyle enum, or a GraphicsUnit enum, and the call will fail.