Memory Barrier by lock statement

I read recently about memory barriers and the reordering issue and now I have some confusion about it.

Consider the following scenario:

private object _object1 = null;    
private object _object2 = null;
private bool _usingObject1 = false;

private object MyObject
{
    get 
    {
        if (_usingObject1)
        {
            return _object1;
        }
        else
        {
            return _object2;
        }
    }
    set 
    {
        if (_usingObject1)
        {
           _object1 = value;
        }
        else
        {
           _object2 = value;
        }
    }
}

private void Update()
{
    _usingMethod1 = true;
    SomeProperty = FooMethod();
    //..
    _usingMethod1 = false;
}

1. At Update method; is the _usingMethod1 = true statement always executed before getting or setting the property? or due to reordering issue we can not guarantee that?

2. Should we use volatile like

   private volatile bool _usingMethod1 = false;
   

3. If we use lock; can we guarantee then every statement within the lock will be executed in order like:

   private void FooMethod()
   {
       object locker = new object();
       lock (locker)
       {
           x = 1;
           y = a;
           i++;
       }
   }