How to convert TStrings to String?

[paule32]

I know that is ALL Text in TStringList.
But I mean:
- one TStringList.Add
- one Line + Linefeed

and that Line (or one Item) can be set to TLabel.Caption.

But I will not twist with you.
My Question stands under the Code.

[rvk]

But the point, I would know: why TStrings, and not TStringList in variable definition section ?

TStrings is an abstract class. It can be used by other classes to inherit. For example TMemo.Lines. When you don't actually need the properties from the class which are in TStringList, it's best to use TStrings as your parameter so you can also pass one of those other classes.

I agree, when declaring list: TStrings; and creating it as TStringList it doesn't much matter, but if you are going to pass it to a function foo(AStrings: TStrings), which can both handle TStringList and the one from TMemo Lines, it does matter.

The same is the example of TAnimal.
You can do

Code: Pascal  [Select][+][-]

1. var
2.   MyPet: TAnimal;
3. begin
4.   MyPet := TCat.Create; // TCat is inherited from TAnimal
5.   FeedMyAnimal(MyPet); // parameter is of type TAnimal

[CM630]

I am not sure that I understand you, but maybe this is what you need:

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Button1: TButton;
16.     Label1: TLabel;
17.     Label0: TLabel;
18.     LabelAllStringSemicolon: TLabel;
19.     LabelAllStringsComma: TLabel;
20.     procedure Button1Click(Sender: TObject);
21.   private
22.  
23.   public
24.  
25.   end;
26.  
27. var
28.   Form1: TForm1;
29.  
30. implementation
31.  
32. {$R *.lfm}
33.  
34. { TForm1 }
35.  
36. procedure TForm1.Button1Click(Sender: TObject);
37. var
38.   myStringList : TStringList;
39. begin
40.   myStringList := TStringList.Create;
41.   myStringList.add('0') ;
42.   myStringList.add('1') ;
43.   myStringList.add('2') ;
44.   LabelAllStringsComma.caption  := myStringList.DelimitedText  ;
45.   Label0.Caption := myStringList.Strings[0];
46.   Label1.Caption := myStringList.Strings[1];
47.   myStringList.StrictDelimiter :=True;
48.   myStringList.Delimiter := ';';
49.   LabelAllStringSemicolon.caption  := myStringList.DelimitedText  ;
50. end;
51. end.
52.  

Note that .Delimter is a char, which is a pretty good reason not to use TStringList.

[Remy Lebeau]

what are the differences between TStrings and TStringList ?

TStrings is an abstract base class.  TStringList is a concrete descendant of TStrings.  There are other descendants available, such as used by the TMemo.Lines property, the TListBox.Items property, etc.

in my Delphi Times, I had created:
...
But the point, I would know: why TStrings, and not TStringList in variable definition section ?

In your particular example, it make no any difference whatsoever, as there is only 1 descendant involved, and everything is local to the procedure.  So use whichever type you want for the variable declaration.

But, in more complex situations, where multiple list objects/descendants are involved, or the variable is a function parameter, etc, then it makes more sense to declare the variable/parameter as TStrings to offer yourself flexibility in which object/descendant can be used, for example:

Code: Pascal  [Select][+][-]

1. procedure AddToList(AStrings: TStrings);
2. begin
3.   AStrings.Add('foo bar');
4. end;
5.  
6. procedure DoSomethingWithMemo;
7. begin
8.   ...
9.   AddToList(Memo1.Lines);
10.   ...
11. end;
12.  
13. procedure DoSomethingWithListBox;
14. begin
15.   ...
16.   AddToList(ListBox1.Items);
17.   ...
18. end;
19.  
20. procedure DoSomethingWithStringList;
21. var
22.   slList: TStringList;
23. begin
24.   ...
25.   slList := TStringList.Create;
26.   try
27.     ...
28.     AddToList(slList);
29.     ...
30.   finally
31.     slList.Free;
32.   end;
33.   ...
34. end;
35.  
36. procedure DoSomethingWithAlternateLists;
37. var
38.   listToAddTo: TStrings;
39.   slList: TStringList;
40. begin
41.   ...
42.   if SomeCondition then
43.     listToAddTo := Memo1.Lines
44.   else if SomeOtherCondition then
45.     listToAddTo := ListBox1.Items
46.   else begin
47.     slList := TStringList.Create;
48.     listToAddTo := slList;
49.   end;
50.   try
51.     ...
52.     AddToList(listToAddTo);
53.     ...
54.   finally
55.     slList.Free;
56.   end;
57.   ...
58. end;
59.