Why variables are declared as TStrings and created as TStringList?

delphi tstringlist
Fabio Vitale picture Fabio Vitale · Feb 21, 2012 · Viewed 19.9k times · Source

Why variables are declared as TStrings and created as TStringList?

eg: the var sl is declared as TStrings but created as TStringList

var
  sl : TStrings;
begin
  sl := TStringList.Create;

  // add string values...
  sl.Add( 'Delphi' );
  sl.Add( '2.01' );

  // get string value using its index
  // sl.Strings( 0 ) will return
  //   'Delphi'
  MessageDlg(
    sl.Strings[ 0 ],
    mtInformation, [mbOk], 0 );

  sl.Free;
end;

Answer

Chris picture Chris · Feb 21, 2012

TStrings is an abstract type that doesn't have all methods implemented.

TStringList is a descendant of TStrings and implements all functions. In your code, you could declare your variable also as TStringList.

However e.g. on function definitions it makes sense to accept a TStrings parameter instead of a TStringList:

procedure doSomething(lst: TStrings);

This enables the function to work with all implementations of TStrings, not only TStringList.

Related questions

Delphi: StringList Delimiter is always a space character even if Delimiter is set

I am having trouble with the delimiter in the TStringList Class. Take a look: var s: string; sl: TStringList; begin sl := TStringList.Create; s := 'Users^foo bar^bar foo^foobar^barfoo'; sl.Delimiter := '^'; sl.DelimitedText := s; ShowMessage(sl[1]); …

Read More
How do I store and load a list of key-value pairs in a string?

I have a list of strings and the values they are to be replaced with. I'm trying to combine them in a list like 'O'='0',' .'='.', ... so it's easy for me to edit it and …

Read More
Converting TStringlist to string with delimiter

I have a list of strings stored in TStringList, i want to convert it into string seperated by commas and i use the following code channelList: TStringList; aCurrentChannel := Stringreplace(channelList.Text,Char(13)+Char(10),',',[rfReplaceAll]); but the last character …

Read More