TStringList: The Swiss Army Knife You're Probably Underusing¶

If you've been writing Delphi code for any length of time, you've used TStringList. It's the go-to container for managing lists of strings. But here's the thing: most developers only scratch the surface of what this versatile class can do.

Let me show you three powerful features that often fly under the radar: duplicate handling, built-in sorting, and the surprisingly useful CommaText property.

Duplicate Handling: Keep Your Lists Clean¶

Did you know TStringList can automatically prevent duplicates? The Duplicates property gives you fine-grained control over how duplicate entries are handled.

var
  List: TStringList;
begin
  List := TStringList.Create;
  try
    List.Duplicates := dupIgnore;  // Silently ignore duplicates
    List.Add('Apple');
    List.Add('Banana');
    List.Add('Apple');  // This won't be added

    ShowMessage(List.Text);  // Shows only "Apple" and "Banana"
  finally
    List.Free;
  end;
end;

The Duplicates property has three options:

dupIgnore: Silently rejects duplicate entries (my personal favorite for building unique lists)
dupAccept: Allows duplicates freely (the default behavior)
dupError: Raises an EStringListError exception when you try to add a duplicate

Here's a practical example - collecting unique email domains from a user list:

procedure GetUniqueDomains(Users: TStringList; Domains: TStringList);
var
  Email, Domain: string;
  AtPos: Integer;
begin
  Domains.Clear;
  Domains.Duplicates := dupIgnore;
  Domains.Sorted := True;  // Must be sorted for dupIgnore to work!

  for Email in Users do
  begin
    AtPos := Email.IndexOf('@');
    if AtPos > 0 then
    begin
      Domain := Email.Substring(AtPos + 1).ToLower;
      Domains.Add(Domain);
    end;
  end;
end;

// Usage:
var
  Users, Domains: TStringList;
begin
  Users := TStringList.Create;
  Domains := TStringList.Create;
  try
    Users.Add('john@company.com');
    Users.Add('jane@company.com');
    Users.Add('bob@example.org');
    Users.Add('alice@company.com');

    GetUniqueDomains(Users, Domains);
    ShowMessage(Domains.Text);  // company.com, example.org
  finally
    Users.Free;
    Domains.Free;
  end;
end;

Important gotcha: Duplicate detection only works when Sorted is True. The duplicate check relies on binary search, which requires a sorted list.

Built-in Sorting: No Need to Reinvent the Wheel¶

Speaking of sorting, TStringList has built-in sorting capabilities that are criminally underused.

var
  Names: TStringList;
begin
  Names := TStringList.Create;
  try
    Names.Add('Zimmerman');
    Names.Add('Anderson');
    Names.Add('Baker');

    Names.Sort;  // Manual sort
    ShowMessage(Names.Text);  // Anderson, Baker, Zimmerman
  finally
    Names.Free;
  end;
end;

But here's where it gets interesting - you can enable automatic sorting:

Names.Sorted := True;  // Automatically maintains sort order
Names.Add('Davis');    // Inserted in correct position automatically

When Sorted is True, every Add operation automatically places the string in the correct sorted position. This is perfect for maintaining alphabetically ordered lists without manual intervention.

Custom Sorting with OnCompare¶

Need case-insensitive sorting? Or maybe you want to sort by string length? Use the CustomSort method:

var
  List: TStringList;
begin
  List := TStringList.Create;
  try
    List.Add('zebra');
    List.Add('Apple');
    List.Add('banana');

    // Case-insensitive sort
    List.CustomSort(
      function(List: TStringList; Index1, Index2: Integer): Integer
      begin
        Result := CompareText(List[Index1], List[Index2]);
      end
    );

    ShowMessage(List.Text);  // Apple, banana, zebra
  finally
    List.Free;
  end;
end;

Or sort by string length:

List.CustomSort(
  function(List: TStringList; Index1, Index2: Integer): Integer
  begin
    Result := Length(List[Index1]) - Length(List[Index2]);
  end
);

The CustomSort comparison function works exactly like other comparison functions in Delphi - it should return a negative value if the first item should come before the second, zero if they're equal, and a positive value if the first item should come after the second. For example, CompareText(List[Index1], List[Index2]) returns -1 when Index1's string is alphabetically before Index2's string, 0 when they're equal, and 1 when Index1 comes after Index2. You can use simple arithmetic for numeric comparisons like Length(List[Index1]) - Length(List[Index2]) which naturally produces the correct negative/zero/positive result for sorting by string length.

CommaText: The Serialization Shortcut¶

The CommaText property is a hidden gem for quickly serializing and deserializing string lists. It automatically handles comma-separated values with proper quoting for strings that contain spaces or special characters.

var
  Config: TStringList;
begin
  Config := TStringList.Create;
  try
    Config.Add('Server');
    Config.Add('Port');
    Config.Add('User Name');  // Contains a space

    // Serialize to comma-separated text
    ShowMessage(Config.CommaText);  
    // Output: Server,Port,"User Name"

    // Deserialize from comma-separated text
    Config.Clear;
    Config.CommaText := 'Database,Timeout,"Connection String"';
    ShowMessage(Config.Text);
    // Shows three separate lines
  finally
    Config.Free;
  end;
end;

Notice how "User Name" and "Connection String" are automatically quoted because they contain spaces? That's CommaText handling the complexity for you.

Real-World Use Case: Configuration Files¶

Here's a practical example - saving and loading application settings:

procedure SaveRecentFiles(const RecentFiles: TStringList);
var
  IniFile: TIniFile;
begin
  IniFile := TIniFile.Create(ChangeFileExt(ParamStr(0), '.ini'));
  try
    IniFile.WriteString('Settings', 'RecentFiles', RecentFiles.CommaText);
  finally
    IniFile.Free;
  end;
end;

procedure LoadRecentFiles(RecentFiles: TStringList);
var
  IniFile: TIniFile;
begin
  IniFile := TIniFile.Create(ChangeFileExt(ParamStr(0), '.ini'));
  try
    RecentFiles.CommaText := IniFile.ReadString('Settings', 'RecentFiles', '');
  finally
    IniFile.Free;
  end;
end;

Understanding Delimiters¶

You're not stuck with commas. TStringList has Delimiter and DelimitedText properties for custom separators:

var
  List: TStringList;
begin
  List := TStringList.Create;
  try
    List.Delimiter := '|';
    List.StrictDelimiter := True;  // Don't treat spaces as delimiters
    List.DelimitedText := 'Alpha|Beta|Gamma Delta';

    ShowMessage(List[2]);  // "Gamma Delta" - space preserved
  finally
    List.Free;
  end;
end;

The StrictDelimiter property is crucial here - without it, spaces are treated as additional delimiters, which can cause unexpected splitting.

Putting It All Together¶

Here's a real-world example combining all three features - building a unique, sorted tag list from comma-separated input:

function BuildTagList(const Input: string): string;
var
  Tags: TStringList;
begin
  Tags := TStringList.Create;
  try
    Tags.Sorted := True;           // Enable auto-sorting
    Tags.Duplicates := dupIgnore;  // Reject duplicates
    Tags.CommaText := Input;       // Parse input

    Result := Tags.CommaText;      // Return cleaned, sorted, unique list
  finally
    Tags.Free;
  end;
end;

// Usage:
var
  CleanTags: string;
begin
  CleanTags := BuildTagList('delphi, pascal, delphi, programming, Pascal');
  ShowMessage(CleanTags);  // "delphi,pascal,programming"
end;

The Bottom Line¶

TStringList is far more capable than most developers realize. Before you reach for a third-party collection library or write custom sorting code, check if TStringList already does what you need:

Use Duplicates to maintain unique lists automatically
Use Sorted for automatic alphabetical ordering
Use CommaText for quick serialization and parsing
Use CustomSort when you need specialized sorting logic

These features have been in Delphi since the early days, rock-solid and battle-tested. Sometimes the best tools are the ones hiding in plain sight.