TBitmap.Canvas suffer from same issue that I have when trying to use 
ShowMessage() - TForm.Canvas - in a multithread application.
This piece of code in dialog.pas causes randomly AV's, 'cause the Canvas DC 
get's freed by main thread.


function GetAveCharSize(Canvas: TCanvas): TPoint;
var
  I: Integer;
  Buffer: array[0..51] of Char;
begin
  for I := 0 to 25 do Buffer[I] := Chr(I + Ord('A'));
  for I := 0 to 25 do Buffer[I + 26] := Chr(I + Ord('a'));
  GetTextExtentPoint(Canvas.Handle, Buffer, 52, TSize(Result));
  Result.X := Result.X div 52;
end;


My current solution, I just change the code to:


Function GetAveCharSize(Canvas: TCanvas): TPoint;
Var
  I: Integer;
  Buffer: Array[0..51] Of Char;
Begin
  For I := 0 To 25 Do
    Buffer[I] := Chr(I + Ord('A'));
  For I := 0 To 25 Do
    Buffer[I + 26] := Chr(I + Ord('a'));
  If IsMultiThread Then
    Canvas.Lock;
  Try
    GetTextExtentPoint(Canvas.Handle, Buffer, 52, TSize(Result));
  Finally
    If IsMultiThread Then
      Canvas.Unlock;
  End;
  Result.X := Result.X Div 52;
End;


So, it's detect's via "IsMultiThread" function, if there is another thread, 
doing the lock to prevent problems... in normal applications - single 
threaded - the lock is never called, since there is no time to garbage 
collector cut the DC of canvas...