How to do for in TObjectList?

Question

I am trying to use for in to iterate a TObjectList:

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.SysUtils, Contnrs;

var
    list: TObjectlist;
    o: TObject;
begin
    list := TObjectList.Create;
    for o in list do
    begin
        //nothing
    end;
end.

And it fails to compile:

[dcc32 Error] Project1.dpr(15): E2010 Incompatible types: 'TObject' and 'Pointer'

It seems as though Delphi's for in construct does not handle the untyped, undescended, TObjectList an as enumerable target.

How do i enumerate the objects in a TObjectList?

What i do now

My current code is:

procedure TfrmCustomerLocator.OnBatchDataAvailable(BatchList: TObjectList);
var
   i: Integer;
   o: TObject;
begin
   for i := 0 to BatchList.Count-1 do
   begin
      o := BatchList.Items[i];

      //...snip...where we do something with (o as TCustomer)
   end;
end;    

For no good reason, i was hoping to change it to:

procedure TfrmCustomerLocator.OnBatchDataAvailable(BatchList: TObjectList);
var
   o: TObject;
begin
   for o in BatchList do
   begin
      //...snip...where we do something with (o as TCustomer)
   end;
end;    

Why use an enumerator? Just cause.

Answer 1

How do i enumerate the objects in a TObjectList?

To answer the specific question, this is an example of introducing an enumerator. Note that you will have to create a descendant of TObjectList in order to add the GetEnumerator function. You could do without subclassing with a class helper, but I leave that as an exercise for the interested reader.

type

TObjectListEnumerator = record
private
  FIndex: Integer;
  FList: TObjectList;
public
  constructor Create(AList: TObjectList);
  function GetCurrent: TObject;
  function MoveNext: Boolean;
  property Current: TObject read GetCurrent;
end;

constructor TObjectListEnumerator.Create(AList: TObjectList);
begin
  FIndex := -1;
  FList := AList;
end;

function TObjectListEnumerator.GetCurrent;
begin
  Result := FList[FIndex];
end;

function TObjectListEnumerator.MoveNext: Boolean;
begin
  Result := FIndex < FList.Count - 1;
  if Result then
    Inc(FIndex);
end;

//-- Your new subclassed TObjectList

Type

TMyObjectList = class(TObjectList)
  public
    function GetEnumerator: TObjectListEnumerator;
end;

function TMyObjectList.GetEnumerator: TObjectListEnumerator;
begin
  Result := TObjectListEnumerator.Create(Self);
end;

This implementation of an enumerator uses a record instead of a class. This has the advantage of not allocating an extra object on the heap when doing for..in enumerations.

procedure TfrmCustomerLocator.OnBatchDataAvailable(BatchList: TObjectList);
var
   o: TObject;
begin
   for o in TMyObjectList(BatchList) do // A simple cast is enough in this example
   begin
      //...snip...where we do something with (o as TCustomer)
   end;
end;   

---

As others have noted, there is a generics class that is a better option to use, TObjectList<T>.