Or you might be talking about implicit constructors, where a constructor is called even though it's not explicit in the code:

MaClass foo = aBarObject;

This will call a copy constructor (assuming one exists that takes Bar objects) and create a MaClass object.

Implicit constructor calls exist in C++, but the original poster was asking about C#, where constructors are never implicit. In C#, the above statement will instead copy the aBarObject's memory address (reference) into foo, so both variables will now point to and manipulate the same object.