Implementing List Enumerator OfType in Delphi

Question

I am using Delphi XE to implement an enumerator that allows filtering the elements of the list by type. I have quickly assembled a test unit as follows:

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Answer 1

A worked version using system.IEnumerable<T> and system.IEnumerator<T>

unit uTestList;

interface

uses Generics.Collections;

type
  TListItemBase = class(TObject)
  end; { TListItemBase }

  TListItemChild1 = class(TListItemBase)
  end;

  TListItemChild2 = class(TListItemBase)
  end;

  TTestList<T : TListItemBase> = class;

  TOfTypeEnumerator<T : class; TFilter : class> = class(TInterfacedObject, IEnumerator<TFilter>, IEnumerator)
  private
    FTestList : TList<T>;
    FIndex : Integer;
  protected
    constructor Create(Owner : TList<T>); overload;

    function GetCurrent: TObject;
    function GenericGetCurrent : TFilter;
    function MoveNext : Boolean;
    procedure Reset;

    function IEnumerator<TFilter>.GetCurrent = GenericGetCurrent;
  end;

  TOfTypeEnumeratorFactory<T : class; TFilter : class> = class(TInterfacedObject, IEnumerable<TFilter>, IEnumerable)
  private
    FTestList : TList<T>;
  public
    constructor Create(Owner : TList<T>); overload;
    function GetEnumerator : IEnumerator;
    function GenericGetEnumerator : IEnumerator<TFilter>;
    function IEnumerable<TFilter>.GetEnumerator = GenericGetEnumerator;
  end;

  TTestList<T : TListItemBase> = class(TList<T>)
  public
    function OfType<TFilter : TListItemBase>() : IEnumerable<TFilter>;
  end; { TTestList }


implementation

{ TOfTypeEnumerator<T, TFilter> }

constructor TOfTypeEnumerator<T, TFilter>.Create(Owner: TList<T>);
begin
  inherited Create;
  FTestList := Owner;
  FIndex := -1;
end;

function TOfTypeEnumerator<T, TFilter>.GenericGetCurrent: TFilter;
begin
  Result := TFilter(TObject(FTestList[FIndex]));
end;

function TOfTypeEnumerator<T, TFilter>.GetCurrent: TObject;
begin
  Result := TObject( FTestList[FIndex] );
end;

function TOfTypeEnumerator<T, TFilter>.MoveNext: Boolean;
begin
  repeat
    Inc(FIndex);
  until (FIndex >= FTestList.Count) or FTestList[FIndex].InheritsFrom(TFilter);
  Result := FIndex < FTestList.Count;
end;

procedure TOfTypeEnumerator<T, TFilter>.Reset;
begin
  FIndex := -1;
end;

{ TOfTypeEnumeratorFactory<T, TFilter> }

constructor TOfTypeEnumeratorFactory<T, TFilter>.Create(Owner: TList<T>);
begin
  inherited Create;
  FTestList := Owner;
end;

function TOfTypeEnumeratorFactory<T, TFilter>.GetEnumerator: IEnumerator;
begin
  Result := GenericGetEnumerator;
end;

function TOfTypeEnumeratorFactory<T, TFilter>.GenericGetEnumerator: IEnumerator<TFilter>;
begin
  Result := TOfTypeEnumerator<T,TFilter>.Create(FTestList);
end;

{ TTestList<T> }

function TTestList<T>.OfType<TFilter>: IEnumerable<TFilter>;
begin
  Result := TOfTypeEnumeratorFactory<T,TFilter>.Create(self);
end;

end.

A test procedure:

var
  MyElem: TListItemBase;
  MyElem1: TListItemChild1;
  MyElem2: TListItemChild2;
begin
  Memo1.Clear;
  for MyElem in FTestList.OfType<TListItemBase>() do
  begin
    Memo1.Lines.Add('----------');
  end;
  for MyElem1 in FTestList.OfType<TListItemChild1>() do
  begin
    Memo1.Lines.Add('==========');
  end;
  for MyElem2 in FTestList.OfType<TListItemChild2>() do
  begin
    Memo1.Lines.Add('++++++++++');
  end;

Answer 2

You don't want to use the IEnumerator<T> from System.pas, trust me. That thing brings along so much trouble because it inherits from IEnumerator and so has that GetCurrent method with different results (TObject for IEnumerator and T for IEnumerator<T>).

Better define your own IEnumerator<T>:

IEnumerator<T> = interface
  function GetCurrent: T;
  function MoveNext: Boolean;
  procedure Reset;
  property Current: T read GetCurrent;
end;

Same with IEnumerable. I would say define your own IEnumerable<T>:

IEnumerable<T> = interface
  function GetEnumerator: IEnumerator<T>;
end;

If you use that in your TOfTypeEnumerator<T, TFilter> you can remove the method resolution clauses causing the ICE.

When you do that you will start seeing other compiler errors E2008, E2089 and some more.

• calling just inherited in your constructor tries to call the constructor with the same signature in your ancestor class which does not exist. So change it to inherited Create.

• don't use IEnumerable but use IEnumerable<TFilter> because that is what your want to enumerator over

• don't use methods and casts that are only allowed for objects or specify the class constraint on T and TFilter

• MoveNext needs a Result

Here is the compiling unit. Did a quick test and it seems to work:

unit uTestList;

interface

uses
  Generics.Collections;

type
  IEnumerator<T> = interface
    function GetCurrent: T;
    function MoveNext: Boolean;
    property Current: T read GetCurrent;
  end;

  IEnumerable<T> = interface
    function GetEnumerator: IEnumerator<T>;
  end;

  TOfTypeEnumerator<T: class; TFilter: class> = class(TInterfacedObject, IEnumerator<TFilter>)
  private
    FTestList: TList<T>;
    FIndex: Integer;
  protected
    constructor Create(Owner: TList<T>); overload;

    function GetCurrent: TFilter;
    function MoveNext: Boolean;
  end;

  TOfTypeEnumeratorFactory<T: class; TFilter: class> = class(TInterfacedObject, IEnumerable<TFilter>)
  private
    FTestList: TList<T>;
  public
    constructor Create(Owner: TList<T>); overload;
    function GetEnumerator: IEnumerator<TFilter>;
  end;

  TTestList<T: class> = class(TList<T>)
  public
    function OfType<TFilter: class>: IEnumerable<TFilter>;
  end;

implementation

{ TOfTypeEnumerator<T, TFilter> }

constructor TOfTypeEnumerator<T, TFilter>.Create(Owner: TList<T>);
begin
  inherited Create;
  FTestList := Owner;
  FIndex := -1;
end;

function TOfTypeEnumerator<T, TFilter>.GetCurrent: TFilter;
begin
  Result := TFilter(TObject(FTestList[FIndex]));
end;

function TOfTypeEnumerator<T, TFilter>.MoveNext: Boolean;
begin
  repeat
    Inc(FIndex);
  until (FIndex >= FTestList.Count) or FTestList[FIndex].InheritsFrom(TFilter);
  Result := FIndex < FTestList.Count;
end;

{ TOfTypeEnumeratorFactory<T, TFilter> }

constructor TOfTypeEnumeratorFactory<T, TFilter>.Create(Owner: TList<T>);
begin
  inherited Create;
  FTestList := Owner;
end;

function TOfTypeEnumeratorFactory<T, TFilter>.GetEnumerator: IEnumerator<TFilter>;
begin
  Result := TOfTypeEnumerator<T, TFilter>.Create(FTestList);
end;

{ TTestList<T> }

function TTestList<T>.OfType<TFilter>: IEnumerable<TFilter>;
begin
  Result := TOfTypeEnumeratorFactory<T,TFilter>.Create(self);
end;

end.