Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

Property Access
Every property has a read specifier, a write specifier, or both. These are called access specifiers and they have the
form
read fieldOrMethod
write fieldOrMethod
where fieldOrMethod is the name of a field or method declared in the same class as the property or in an ancestor
class.
If fieldOrMethod is declared in the same class, it must occur before the property declaration. If it is declared in
an ancestor class, it must be visible from the descendant; that is, it cannot be a private field or method of an
ancestor class declared in a different unit.
If fieldOrMethod is a field, it must be of the same type as the property.
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If fieldOrMethod is a method, it cannot be dynamic and, if virtual, cannot be overloaded. Moreover, access
methods for a published property must use the default register calling convention.
In a read specifier, if fieldOrMethod is a method, it must be a parameterless function whose result type is the
same as the property's type. (An exception is the access method for an indexed property or an array property.)
In a write specifier, if fieldOrMethod is a method, it must be a procedure that takes a single value or const
parameter of the same type as the property (or more, if it is an array property or indexed property).
For example, given the declaration
property Color: TColor read GetColor write SetColor;
the 
GetColor
 method must be declared as
function GetColor: TColor;
and the 
SetColor
 method must be declared as one of these:
procedure SetColor(Value: TColor); 
procedure SetColor(const Value: TColor);
(The name of 
SetColor
's parameter, of course, doesn't have to be 
Value
.)
When a property is referenced in an expression, its value is read using the field or method listed in the read specifier.
When a property is referenced in an assignment statement, its value is written using the field or method listed in the
write specifier.
The example below declares a class called 
TCompass
 with a published property called 
Heading
. The value of
Heading
 is read through the 
FHeading
 field and written through the 
SetHeading
 procedure.
type 
   THeading = 0..359; 
   TCompass = class(TControl) 
     private 
        FHeading: THeading; 
        procedure SetHeading(Value: THeading); 
     published 
        property Heading: THeading read FHeading write SetHeading; 
        ... 
    end;
Given this declaration, the statements
if Compass.Heading = 180 then GoingSouth; 
Compass.Heading := 135;
correspond to
if Compass.FHeading = 180 then GoingSouth; 
Compass.SetHeading(135);
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In the 
TCompass
 class, no action is associated with reading the 
Heading
 property; the read operation consists of
retrieving the value stored in the 
FHeading
 field. On the other hand, assigning a value to the 
Heading
 property
translates into a call to the 
SetHeading
 method, which, presumably, stores the new value in the 
FHeading
 field
as well as performing other actions. For example, 
SetHeading
 might be implemented like this:
procedure TCompass.SetHeading(Value: THeading); 
begin 
            if FHeading <> Value then 
             begin 
               FHeading := Value; 
               Repaint;    // update user interface to reflect new value 
             end; 
end;
A property whose declaration includes only a read specifier is a read-only property, and one whose declaration
includes only a write specifier is a write-only property. It is an error to assign a value to a read-only property or use
a write-only property in an expression.