How should "Double-Checked Locking" be implemented in Delphi?

Question

In C#, the following code (from this page) can be used to lazily instantiate a singleton class in a thread safe way:

  class Foo {
        private volatile Helper helper = null;
        public Helper getHelper() {
            if (helper == null) {
                lock(this) {
                    if (helper == null)
                        helper = new Helper();
                }
            }
            return helper;
        }
    }

What would be the equivalent thread safe Delphi code?

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The article also mentions two problems with Double Checked Locking in Java:

• it is possible that the new object is constructed before the helper reference is made to point at the newly created object meaning that two objects are created
• it is possible that the helper reference is made to point at a block of memory while the object is still being created meaning that a reference to an incomplete object will be returned

So while the code of the C# and the Java version in the mentioned article look almost identical, only the C# version works as expected. Which leads to the additional question if these two problems also exist in a Delphi version of Double-Checked Locking?

Answer 1

Use System.TMonitor to lock the object instance in a thread safe way.

function TFoo.GetHelper(): THelper;
begin
  if not Assigned(FHelper) then
  begin
    System.MonitorEnter(Self);
    try
      if not Assigned(FHelper) then
        FHelper := THelper.Create();
    finally
      System.MonitorExit(Self);
    end;
  end;
  Result := FHelper;
end;

For further reference look at Lock my object..., please! from Allen Bauer. In fact, the rep. I gather from this should go to Allen.