Author: Tomas Rutkauskas

How do I create a Collection myself? I need a Collection of Items with ArticelNr, 
Name, Price, etc.. Each item should be in the collection of ResellItems.

Answer:

First, you need to create your ResellItem class based upon TCollectionItem:


1   TResellItem = class(TCollectionItem)
2   private
3     fArticleNr: integer;
4     fName: string;
5     fPrice: double;
6   public
7     property ArticleNr: integer read fArticleNr write fArticleNr;
8     property Name: string read fName write fName;
9     property Price: double read fPrice write fPrice;
10  end;



The next step is optional although I generally find it useful (self-documents for a 
start): create a descendent of TCollection:


11  TResellCollection = class(TCollection)
12  end;
13  
14  
15  //Now you need to declare an instance somewhere:
16  
17  
18  var
19    ResellCollection: TResellCollection;
20  
21  
22  //The constructor for TCollection takes a TCollectionItem class parameter:
23  
24  
25  ResellCollection: TResellCollection.Create(TResellItem);
26  
27  
28  //Having created our collection (don't forget to free it afterwards) then adding 
29  items becomes:
30  
31  
32  var
33    item: TResellItem;
34  
35    item := ResellCollection.Add as TResellItem;
36    item.ArticleNr := aNumber;
37    item.Name := aName;
38    item.Price := aPrice;



I generally wrap the above code into a TResellCollection method, say AddResellItem, 
thus allowing:


ResellCollection.AddResellItem(aNumber, aName, aPrice);


Accessing items afterwards is done like this:


item := ResellCollection.Items[index] as TResellItem;


The syntax above can be simplified by adding a new default array property to 
TResellCollection, thus encapsulating the messy casting:


property ResellItem[index: integer]: TResellItem read GetResellItem; default;


The GetResellItem method required looks like this:


39  function TResellCollection.GetResellItem(index: integer): TResellItem;
40  begin
41    Result := Items[index] as TResellItem;
42  end;
43  
44  
45  //Now you can say things like:
46  
47  
48  item := ResellCollection[0];
49  ResellCollection[1].Name := newname;



Just in case you have got a bit lost with the changes, the interface and 
implementation of TResellCollection now look like this:


50  TResellCollection = class(TCollection)
51  private
52    function GetResellItem(index: integer): TResellItem;
53  public
54    procedure AddResellItem(aNumber: integer; const aName: string; aPrice: Double);
55    property ResellItem[index: integer]: TResellItem read GetResellItem;
56    default;
57  end;
58  
59  procedure TResellCollection.AddResellItem(aNumber: integer; const aName: string;
60    aPrice: Double);
61  var
62    item: TResellItem;
63  begin
64    item := Add as TResellItem;
65    item.ArticleNr := aNumber;
66    item.Name := aName;
67    item.Price := aPrice;
68  end;
69  
70  function TResellCollection.GetResellItem(index: integer): TResellItem;
71  begin
72    Result := Items[index] as TResellItem;
73  end;