CRM 2011 Plug-in error: Object reference not set to an instance of an object.

Problem/Error:

When I create an instance of object from the sdk generated code by using the default constructor like:

email.regardingobjectid = new CrmEntityReference();

I will find it return a null object:

This is not the result I wanted because the object does not have correct structure and when I try to assign a value to the object (in this case is regardingobjectid) it will throw a NullReferenceException error like this:

Solution

The solution is relatively simple by using a different constructor and it will give a correct object:

email.regardingobjectid = new CrmEntityReference("contact", contact.contactid);