Using Attributes and TCustomAttribute descendants

With Delphi 2010 attributes have been added as a language feature. They have been available in Delphi Prism (Targets .NET) and this now adds similar functionality to Win32.

Attributes are a way of associating additional metadata information with a given type or member of a type.

They can be applied in many places, the following code shows several of the places you can place attributes.

  // Declaring an attribute.
  TAttrTest = class(TCustomAttribute)
  end;

  // Places you can use an attribute.
  [TAttrTest]
  TRec = Record
    [TAttrTest]
    value : String;
    [TAttrTest]
    procedure DoThis([TAttrTest]arg1: String);
  End;

  [TAttrTest]
  TMyEnum = (enOne,enTwo,enThree);

  [TAttrTest]
  TMySet = set of TMyEnum;

  [TAttrTest]
  TObj = class(TObject)
  private
    [TAttrTest]
    FID: Integer;
  public
    [TAttrTest]
    FName : String;
    [TAttrTest]
    property Id : Integer read FID write FID;
    [TAttrTest]
    constructor Create;
    [TAttrTest]
    destructor Destroy; override;
    [TAttrTest]
    procedure DoThis;
  end;

var
  [TAttrTest]
  I : Integer;

So how do attributes work in Delphi 2010?

Attributes must descend from TCustomAttribute, if you look at the declaration of TCustomAttribute you will find that there is nothing special.

{ The base class for all custom attributes. Attribute
instances created by the RTTI unit are owned by those
members to which they apply. }
TCustomAttribute = class(TObject)
end;

Passing just the name of the new attribute is only practical in a few cases, usually
you need additional data associated. This is done through the constructor. The following example shows how to setup the call to the constructor in the attribute.

Type
  TAttrTest2 = class(TObject)
  private
     FId : Integer;
  public
     constructor Create(aID : Integer);
     property ID : Integer read FID write FID;
  end;

  [TAttrTest2(123)]
  TMyObject = Class(TObject)
  end;

So its simple to declare an Attribute and decorate your types with them. Accessing the attributes stored in a given type involves using rtti.pas, I covered some of the basics of how this works in the previous post

Anything that can have attributes has an associated .GetAttributes() method that returns
the array of the attributes associated with that code.

The following code shows how to access the attributes.

program Project10;

{$APPTYPE CONSOLE}

uses
  SysUtils, RTTI;
type
  TAttrTest2 = class(TCustomAttribute)
  private
     FId : Integer;
  public
     constructor Create(aID : Integer);
     property ID : Integer read FID write FID;
  end;

  [TAttrTest2(1)]
  [TAttrTest2(2)]
  [TAttrTest2(3)]
  TMyObject = Class(TObject)
  end;

{ TAttrTest2 }

constructor TAttrTest2.Create(aID: Integer);
begin
 FID := aId;
end;

var
 c : TRttiContext;
 t : TRttiType;
 a : TCustomAttribute;
begin
   c := TRttiContext.Create;
   try
     t := c.GetType(TMyObject);
     for a in  t.GetAttributes do
     begin
       Writeln((a as TAttrTest2).ID);
     end;
   finally
     c.Free
   end;
   readln;
end.

Output:

1
2
3

Attributes also have a few other special items that the compiler implements.

If you have an attribute named like this...

type
TestAttribute = class(TCustomAttribute)
end;

It can be refered to in two different ways

[TestAttribute]
TExample = class(Tobject)
end;

[Test]
TExample2 = class(TObject)
end;

The compiler will look for the type, if it is not found it will automatically append "Attribute" to the name and search again. This is done to mimic the .NET behavior.

The compiler also has some special support for types allow you to get TRttiType
easily from the pTypeInfo pointer. The following code segment shows how that pTypeInfo can be interchanged for TRttitype in attributes.

uses
  SysUtils, Rtti;

type
  TestAttribute = class(TCustomAttribute)
  public
    constructor Create(aType : TRttiType);
  end;

  [Test(typeinfo(Integer))]
  TEmployee = class(TObject)
  end;  

There are many practical applications for Attributes, I will explore many of these in later articles.

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