Question

When compiling this unit:

unit Test;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
  private
    class var StartDate, EndDate: TDateTime;   // Line 12
    fTest: TNotifyEvent;
  public
    property OnTest: TNotifyEvent read fTest;  // Line 15.
  end;

implementation

{$R *.dfm}

end.

I get the following compiler error:

[DCC Error] Test.pas(15): E2356 Property accessor must be an instance field or method

But if I comment out line 12 it compiles fine. Can someone explain why? I need the dates as class variables to store a date interval.

Answer 1

That error should be self-explanatory. Let’s try to de-construct it.

Property accessor must be an instance field or method.

The property accessor is the expression following the read. If your property was writeable, then the expression following the write would also be a property accessor.

Therefore, in your code the property accessor is fTest.

An instance field is a normal field of the class. So, class fields do not qualify. Similarly, an instance method is a plain method of the class. A class method is not an instance method. In fact, any method that is not a class method are instance methods.

The error therefore indicates that fTest is not an instance field.

And that is correct. It is a class field.

private
  class var StartDate, EndDate: TDateTime;   
  fTest: TNotifyEvent; // class var applies to fTest also

I guess you don’t mean for fTest to be a class field. You need to write the class like this:

TForm1 = class(TForm)
private
  class var StartDate, EndDate: TDateTime;   
private
  fTest: TNotifyEvent;
public
  property OnTest: TNotifyEvent read fTest;  
end;

Or perhaps:

TForm1 = class(TForm)
private
  class var
    StartDate, EndDate: TDateTime;   
  var 
    fTest: TNotifyEvent;
public
  property OnTest: TNotifyEvent read fTest;  
end;

I favour the former as it gives a much clearer distinction between class and instance fields.