Object Size in SOS Debugging C#.Net

Question

class Book
{
    public int ISBN { get; set; }      
}

void Method() 
{
  Book book = new Book(); 
  // Break and verify in SoS Debugging.
}


   !dumpheap -type Book
   PDB symbol for clr.dll not loaded
   Address       MT     **Size**
   00c6b76c 009b7f2c       **12**      
   total 0 objects
   Statistics:
   MT    Count    TotalSize Class Name
   009b7f2c        1           12 GCTest.Book
   Total 1 objects

How the size of object here is 12 bytes . It contains only one integer property. sizeof(int) = 4 bytes remaining 8 bytes ? (object instantiation). Can anyone shed some light.

Answer 1

Every reference object has two extra field appended :

Object type ptr : 4 bytes (ddress of a memory (AppDomain specific) that contains a structure holding the Method Table of the Reference Type for which the object is instantiated or points to)

Sync block adress : 4 bytes (sync block address and points to a location in a process-wide table that contains structures used for synchronizing access to instances of Reference Types)

More info Check the memory layout in this article

Answer 2

You have on top of the object strucure 8 bytes of information:

4 bytes for object reference

4 bytes for syncblk: a special memory block used for syncronisation. In your specifica case it will be set to 0, as your object doesn't partecipate in any sync operaiton.

In other words:

<--SyncBlock(4bytes)--> <--ObjectReferece(4bytes)--> <--your object data-->

For more information have a look at:

Drill Into .NET Framework Internals to See How the CLR Creates Runtime Objects