Size of A Class (object) in .NET

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The size of a class instance is determined by:

  • The amount of data actually stored in the instance
  • The padding needed between the values
  • Some extra internal data used by the memory management

So, typically a class containing a string property needs (on a 32 bit system):

  • 8 bytes for internal data
  • 4 bytes for the string reference
  • 4 bytes of unused space (to get to the minimum 16 bytes that the memory manager can handle)

And typically a class containing an integer property needs:

  • 8 bytes for internal data
  • 4 bytes for the integer value
  • 4 bytes of unused space (to get to the minimum 16 bytes that the memory manager can handle)

As you see, the string and integer properties take up the same space in the class, so in your first example they will use the same amount of memory.

The value of the string property is of course a different matter, as it might point to a string object on the heap, but that is a separate object and not part of the class pointing to it.

For more complicated classes, padding comes into play. A class containing a boolean and a string property would for example use:

  • 8 bytes for internal data
  • 1 byte for the boolean value
  • 3 bytes of padding to get on an even 4-byte boundary
  • 4 bytes for the string reference

Note that these are examples of memory layouts for classes. The exact layout varies depending on the version of the framework, the implementation of the CLR, and whether it’s a 32-bit or 64-bit application. As a program can be run on either a 32-bit or 64-bit system, the memory layout is not even known to the compiler, it’s decided when the code is JIT:ed before execution.

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