TStringList .add produces duplicates from random function

Having a problem I can't seem to put my finger on. I am trying to gather strings (random code with letters and numbers) from a function call and place into my TStringList variable. Relevant code is below.

If I run a test, the strings repeat for a given amount of time then a new one is produced. If I introduce a sleep(xx) or showmessage command to happen after each time a code is produced (see 'edits' below) it copies/returns to memo fine and everything looks fine. If I remove the 'delay' I get repeats from function again.

The part of function to add to TStringList:

..

 AddToMemo:=TStringList.Create;
 AddToMemo.Clear;
 AddToMemo.Sorted:=False;
 for loop := 1 to totalamount do
  begin
   sResult:=MakeCode(charsp开发者_StackOverflow中文版ercode, cbUpperLowerCase, cbAvoidChars, customchars);

   Sleep(50);
   // (or):
   //ShowMessage(sResult);

   // ^ If I leave a Sleep or ShowMessage in, I can see sResult just fine and
   //   program works fine - results in memo are correct as well. If I remove
   //   it, I get repeated entries.

   AddToMemo.add(sResult+IntToStr(loop));

 // If I remove "sResult+" from AddToMemo.add the ".add"
 // works - shows loop numbers in my updated memo
 // If left in, I see one code (1st one produced) and no
 // appended number at all in Memo produced.

  end;
Result:=AddToMemo; 
end;

Edit: As I mention below if I leave a ShowMessage or Sleep(xx) call in to pause between .add's, it works fine. If I remove it, I get a bunch of duplicate entries in final tmemo. Edit: MakeCode is a function to return a single random string of chars+numbers (A..Z a..z 0..9). It works fine on it's own.

(Edit for Answer 2)

No exceptions showed up.

So if I do not include sleep() it may generate 500 strings but they are all repeats; after a given amount of time it does change. The amount of repeats from the function call decreases as I increase sleep command. At around Sleep(40); it shows up correctly from function. But of course this is time consuming and unacceptable.

The 'guts' of MakeCode()

function MakeCode(CharsPerCode: Integer; bULCase, bAvoidChars: Boolean; sCChars: String): String;


var
  i: integer;
  s: string;


begin
//(misc stuff here)

  begin
    randomize;
    s[0]:=chr(CharsPerCode);
    for i:=1 to CharsPerCode do
    repeat
      s[i]:=chr(random(128));
    until
    (s[i] in ['A'..'Z','a'..'z','0'..'9'])
  end;

Result:=s;
end;

---

This is a behavior of Randomize. The random number generator is initialized by a calculation on the system clock. If you call it in each iteration in a quick loop, it is initialized with the same seed. That's why a Sleep(50) is changing the outcome. Call randomize for once, for instance, before starting to populate the string list.

...
AddToMemo.Clear;
AddToMemo.Sorted:=False;
Randomize;            // <-- possibly here
for loop := 1 to totalamount do

...

function MakeCode(CharsPerCode: Integer; bULCase, bAvoidChars: Boolean; sCChars: String): 

...
  begin
//    randomize;      // <-- not here!
    s[0]:=chr(CharsPerCode);

The below quote is from the Delphi documentation:

Do not combine the call to Randomize in a loop with calls to the Random function. Typically, Randomize is called only once, before all calls to Random.

---

Without seeing what MakeCode() is actually returning in sResult, my guess would be that sResult contains non-printable control characters (null characters in particular) that cause the Memo or even the RTL to skip subsequence characters.

---

You need to show some more Code, evtl. MakeCode. I would try the same with a constant String in sResult without MakeCode, do you get the same? try something like:

for loop := 1 to totalamount do
begin
  try
    sResult:=MakeCode(charspercode, cbUpperLowerCase, cbAvoidChars, customchars);
    AddToMemo.add(sResult+IntToStr(loop)); 
  except on e: exception do
    showmessage(e.message);
  end;
end;

Do you get any exceptions?