CODEWITHSUNDEEP
delphigenericsdelphi-xe7
Makla
Makla

Reputation: 10459

Delphi (XE7) add Generic class to generic TDictionary

In the following code:

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, System.Generics.Collections;

type
  TForm1 = class(TForm)
  private
    { Private declarations }
  public
    { Public declarations }
  end;

  TMyClass<T: TForm> = class
  public
    constructor Create;
  end;

var
  Form1: TForm1;
  List: TDictionary<integer, TMyClass<TForm>>;

implementation

{$R *.dfm}


{ TMyClass<T> }

constructor TMyClass<T>.Create;
begin
  List.Add(1, self);
end;

end.

I get error:

[dcc32 Error] Unit1.pas(35): E2010 Incompatible types: 'Unit1.TMyClass' and 'Unit1.TMyClass.T>'

in line where I am trying to add Self to the TDictionary. How can I add generic class to the TDictionary, where second parameter takes generic object?

Upvotes: 1

Views: 377

Answers (1)

Stefan Glienke
Stefan Glienke

Reputation: 21713

While your constraint is ensuring that T can only be of TForm the compiler does not support what is called covariance.

What you can do is hardcast Self to TMyClass<TForm> to add it.

Upvotes: 3

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