Delphi: TThreadList sometimes lock program

Question

Sometimes this function locks my program, and it's freezes until i close it. What is wrong here ?

function del_from_list(id:string):boolean;
var i : integer;
begin
  Result := True;
  try
    with global_list.LockList do
    begin
      for i:=0 to Count-1 do
      begin
        if Tthread_list(Items[i]).id = id then
        begin
          Delete(i);
          break;
        end;
      end;
    end;
  finally
    global_list.UnlockList;
  end;
end;

the class

  Tthread_list = class
  public
    id   : string;
    constructor Create(const id: string);
  end;

I'm adding to the list like that:

global_list.Add(Tthread_list.Create('xxx'));

global list is a global variable

var global_list : TThreadList = nil;

Answer 1

You need to call LockList() outside of the try block instead of inside of it, eg:

function del_from_list(const id: string): boolean;
var
  List: TList;
  i : integer;
begin
  Result := False;
  List := global_list.LockList;
  try
    with List do
    begin
      for i :=0 to Count-1 do
      begin
        if Tthread_list(Items[i]).id = id then
        begin
          Delete(i);
          Result := True;
          break;
        end;
      end;
    end;
  finally
    global_list.UnlockList;
  end;
end;

Answer 2

for loop counts in the wrong direction. When deleting members, you MUST count down, not up.