Delphi - Is TDictionary thread safe

Question

My idea is to use TDictionary to manage client connections on IdTCPServer. Here is a simple example code (not tested) for understanding purposes:

var
  Dic: TDictionary<string, TIdContext>;

procedure TfrmMain.FormCreate(Sender: TObject);
begin
  Dic := TDictionary<string, TIdContext>.Create;
end;

procedure TfrmMain.FormDestroy(Sender: TObject);
begin
  Dic.Free;
end;

procedure TfrmMain.TCPServerConnect(AContext: TIdContext);
var
  Hostname: string;
begin
  Hostname := UpperCase(GStack.HostByAddress(AContext.Binding.PeerIP));
  if not Dic.ContainsKey(Hostname) then Dic.Add(Hostname, AContext);
end;

procedure TfrmMain.TCPServerDisconnect(AContext: TIdContext);
var
  Hostname: string;
begin
  Hostname := UpperCase(GStack.HostByAddress(AContext.Binding.PeerIP));
  if Dic.ContainsKey(Hostname) then
  begin
    Dic[Hostname].Free;
    Dic.Remove(Hostname);
  end;
end;

Is this code thread safe?

Answer 1

In a word: No.

If you inspect the source of TDictionary you should quickly realise that there is no provision for thread-safety in the implementation itself. Even if it were, by having discrete calls to a Dic instance you have potential race conditions to contend with:

  if Dic.ContainsKey(Hostname) then
  begin

    // In theory the Hostname key may be removed by another thread before you 
    //  get a chance to do this : ...

    Dic[Hostname].Free;
    Dic.Remove(Hostname);
  end;

You need to make your own use of Dic thread safe, and fortunately in this sort of example this is easily achieved using a monitor on the object itself:

MonitorEnter(Dic);
try
  if not Dic.ContainsKey(Hostname) then 
    Dic.Add(Hostname, AContext);

finally
  MonitorExit(Dic);
end;


// ....


MonitorEnter(Dic);
try
  if Dic.ContainsKey(Hostname) then
  begin
    Dic[Hostname].Free;
    Dic.Remove(Hostname);
  end;

finally
  MonitorExit(Dic);
end;

If you are not familiar with monitors in Delphi, in simple terms you can think of a monitor as a ready-to-use critical section supported by every TObject descendant (in older versions of Delphi which did not support these monitors you could have achieved the same thing with an explicit critical section).