Read Introduction in C# - how to protect against it?

An article in MSDN Magazine discusses the notion of Read Introduction and gives a code sample which can be broken by it.

public class ReadIntro {
  private Object _obj = new Object();
  void PrintObj() {
    Object obj = _obj;
    if (obj != null) {
      Console.WriteLine(obj.ToString()); // May throw a NullReferenceException
    }
  }
  void Uninitialize() {
    _obj = null;
  }
}

Notice this "May throw a NullReferenceException" comment - I never knew this was possible.

So my question is: how can I protect against read introduction?

I would also be really grateful for an explanation exactly when the compiler decides to introduce reads, because the article doesn't include it.