Common Type System—Object Layout
An object is completely self-describing. Areference to it is the size of a machine pointer—that is, 32 bits on a 32-bit machine, 64 bits on a 64-bit—that points into the GC heap. The target of the pointer is actually another pointer, which refers to an internal CLR data structure called a method table. The method table facilitates method calls and is also used to obtain an object’s type dynamically. The double word before that (fancy name for 4 bytes, or 32 bits) makes up the so-called sync-block, which is used to store such miscellaneous things as locking, COM interoperability, and hash code caching (among others). After these come the actual values that make up instance state of the object.
The sum of this is that there is roughly a quad word (8-byte, 64-bit) overhead per object. This is, of course, on a 32-bit machine; for 64-bit machines, the size would be slightly larger. The exact number is an implementation detail and can actually grow once you start using certain parts of the runtime. For example, the sync-block points at other internal per-object runtime data structures that can collect dust over time as you use an object.