Order of event handler execution

You can change ordering by detaching all handlers, and then re-attaching in desired order.

public event EventHandler event1;

public void ChangeHandlersOrdering()
{
    if (event1 != null)
    {
        List<EventHandler> invocationList = event1.GetInvocationList()
                                                  .OfType<EventHandler>()
                                                  .ToList();

        foreach (var handler in invocationList)
        {
            event1 -= handler;
        }

        //Change ordering now, for example in reverese order as follows
        for (int i = invocationList.Count - 1; i >= 0; i--)
        {
            event1 += invocationList[i];
        }
    }
}

The invocation list of a delegate is an ordered set of delegates in which each element of the list invokes exactly one of the methods invoked by the delegate. An invocation list can contain duplicate methods. During an invocation, a delegate invokes methods in the order in which they appear in the invocation list.

From here: Delegate Class

The order is arbitrary. You cannot rely on the handlers being executed in any particular order from one invocation to the next.

Edit: And also - unless this is just out of curiosity - the fact that you need to know is indicative of a serious design problem.

Currently, they are executed in the order they are registered. However, this is an implementation detail, and I would not rely on this behavior staying the same in future versions, since it is not required by specifications.