Enumerations

An enumeration is a set of named constants that can apply to a particular operation. For example, when opening 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 specifies this. If this is implemented using an enumerated constant, then the parameter can be one of a specific set of (typically) integer values, for example, 1 = read, 2 = write, 3 = 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 System.Enum base class. The class as a whole represents a set of enumerated constants; each of the constants is represented by a static field within the class.

Using Windows Forms (and its sub-sections) describes the use of System.Windows.Forms to create and manipulate the user interface. The classes in this .NET namespace use enums extensively. For example, the System.Windows.Forms.FormBorderStyle class contains a set of static fields named None, FixedDialog, Sizeable, and so on. These fields have specific integer values, but the values themselves are of no interest to the programmer.

Typically, an enumerated constant would be used as a parameter to a method or to specify the value of a property.

Example

To create a Form with a particular border style, set its BorderStyle property to one of the members of the FormBorderStyle class:

      ⎕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 that can be used instead of the enum itself when such usage is unambiguous. For example, the FormBorderStyle.Fixed3D enum has an underlying value of 2:

     Convert.ToInt32 FormBorderStyle.Fixed3D
2

This means that the scalar value 2 can be used to set the border style of the Form f1 to FormBorderStyle.Fixed3D:

      f1.BorderStyle←2

However, this practice is not recommended as not only does it make the code less clear, but also if a value for a property or a parameter to a method can 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 three parameters, the third parameter can be either a FontStyle enum or a GraphicsUnit enum. If Font is called with a third parameter of 1, APL cannot identify whether this refers to a FontStyle enum or a GraphicsUnit enum, and the call will fail.