General Info: Member Field

Also known as a Class Field.

A class variable defined with a specific class visibility, usually private visibility. A member property is different than a member field. A member property uses a member field to store values through accessor methods (getters and setters). For example, it is common to use a private member field to store the current value of a property. The current values of all the class member fields is the current state of the object.

Languages Focus: Member Field

What modifiers apply to member fields, if any? Typical member field modifiers include scope modifiers (private, protected, etc.) and read-only. Can you initialize the value of a member field when declared ensuring a default value?

Cyborg = class(System.Object)
private
  FSerialNumber: String:="A100";
public
  FCyborgName: String;
  FCyborgAge: Integer:=0;
 
  class SeriesID: Integer:=100; readonly;
end;

A Simple Example

The following example uses our class above. Notice for the static member, you can use either an instance reference (a variable), or the type name (the class name. That's why static members are sometimes referred to as class members. Static members belong to the class and you can reference by the class name.

Note: In Prism, you can refer to static members using an instance or class type. In VB.Net you get a warning and in C# you get a compiler error. My preferred usage is to stick with the class type (the class name).

namespace CR_StaticMembers;

interface

uses
  System.Drawing,
  System.Collections,
  System.Collections.Generic,
  System.Linq,
  System.Windows.Forms,
  System.ComponentModel;

type
  /// 

  /// Summary description for MainForm.
  /// 

  MainForm = partial class(System.Windows.Forms.Form)
  private
    method button1_Click(sender: System.Object; e: System.EventArgs);
  protected
    method Dispose(disposing: Boolean); override;
  public
    constructor;
  end;

  Cyborg = class(System.Object)
  private
    FSerialNumber: String:="A100";
  public
    FCyborgName: String;
    FCyborgAge: Integer:=0;

    class SeriesID: Integer:=100; readonly;
  end;

implementation

{$REGION Construction and Disposition}
 

constructor MainForm;
begin
  //
  // Required for Windows Form Designer support
  //
  InitializeComponent();

  //
  // TODO: Add any constructor code after InitializeComponent call
  //
end;

method MainForm.Dispose(disposing: Boolean);
begin
  if disposing then begin
    if assigned(components) then
      components.Dispose();

    //
    // TODO: Add custom disposition code here
    //
  end; 

  inherited Dispose(disposing);
end; 

{$ENDREGION}

method MainForm.button1_Click(sender: System.Object; e: System.EventArgs);
begin
  //Read static member field BEFORE we create our object.
  //Notice the use of the class name, not an object name.
  MessageBox.Show("We will now build a series " + Cyborg.SeriesID + " robot.");

  var MyRobot := New Cyborg;

  MyRobot.FCyborgName := "John";
  MyRobot.FCyborgAge := 34;

  MessageBox.Show("We created a " + MyRobot.FCyborgAge + " year old robot named " + MyRobot.FCyborgName + ".");

  //You can refer to static members using an instance.
  //(in VB.Net you get a warning, in C# you get a compiler error).
  MessageBox.Show("A series " + MyRobot.SeriesID + " robot.");

  //However, my preferred usage is to stick with the class name.
  MessageBox.Show("A series " + Cyborg.SeriesID + " robot.");
end;

end.

Note: The public member fields FCyborgName and FCyborgAge are used here for demonstration only. You normally want to make them private and access them via a member property. Also note that SeriesID starts with an uppercase "S" instead of the customary "F" because SeriesID behaves similar to a read-only properity.