How to decide or calculate size of class

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how to calculate size of class.

What I have tried:

C#

class A
{

   int a;
   float f;

  public void cal();
  public void cal2();

}

what is size of this class?
how to calculate it?

is memory allocated for function ?

is this declaration allocate a memory or

C#

A a=new A();

will allocate a memory?

Posted 2-Dec-16 0:57am

Kishor-KW

Updated 2-Dec-16 3:00am

CPallini

v2

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OriginalGriff · Answer 1 · 2016-12-02T01:21:00

Solution 1

Methods take no space in a class instance (although they do take space in the EXE file).
You can get the size of the class quote easily, but ... you don;t need to, except when you talk to unmanaged code - and the value you get is probably rather confusing.
But...

C#

int size = Marshal.SizeOf(typeof(Myclass));

Will tell you.
Run on your class:

C#

[StructLayout(LayoutKind.Sequential)]
class Myclass
    {
    int a;
    float f;
    public void cal() { }
    public void cal2() { }
    }

It will give you "8" because an int is 4 bytes, and a float is 4 bytes. But it won't tell you what you think!
If I add to your class:

C#

[StructLayout(LayoutKind.Sequential)]
class Myclass
    {
    int a;
    float f;
    int[] array = new int[100000];
    public void cal() { }
    public void cal2() { }
    }

How big do you think it will give you now?
The answer is "16" - the array takes 8 bytes! How? Because the array is a reference, and like all class instances all a variable contains is a reference to the actual data - and a reference on a 64 bit system is always 8 bytes, regardless of the size of the data it references.

Posted 2-Dec-16 1:21am

OriginalGriff

Comments

Kishor-KW 2-Dec-16 7:34am

so there is only reference of 8 byte is created for a class. and if any other reference type declared in it then it will be add 8 in it.

class Myclass
{
int a;
float f;
char c;
public void cal() { }
public void cal2() { }
}

if my class will still occupy 8 byte ?

and if memory is not allocated for a function then how they are called ? are there reference is stored on stack ? and just reference address is fetches at the time of run. if reference is stored on stack then there will a some memory is allocated it's an address then is should be 8 byte, right?

and for each instance memory is allocated, if my class size is 8 byte and i create two instance of it then it will take 16 bytes of memory ?

OriginalGriff 2-Dec-16 7:58am

"Memory" isn't allocated for a method because you only need one copy of it - all instances of the class use the same code.
(This is a lie-to-children, as the code you write is converted into MSIL by the compiler, which is converted to native code at run time by the JIT on a line-by-line basis, which all takes memory - but that's the advanced stuff and you really don't want to get into that yet!)

No, your class will not occupy 8 bytes if you add a char to it - it will go up from 8 to 12, because a char value occupies 2 bytes (and data is always aligned to boundaries which are a multiple of 4 for a huge number of reasons I don't want to go into just now (this is only a little textbox!).

A reference to the data is not the data itself - that's why it's called a "reference"! A reference "points at" the real data on the heap. All classes are reference types, unlike structs which are value types.
This will probably not help much:
https://www.codeproject.com/Articles/728836/Using-struct-and-class-whats-that-all-about
But it does try to explain some of what a reference is - and it's important that you know! Have a read - it's pretty simple to start with.

If your class is 8 bytes, and you create two instances of it, each instance will be a separate 8 bytes, so it will take 16 bytes to store them both, yes. (And that's another lie-to-children - there is a lot of stuff going on behind the scenes that you don't need to know yet, if ever, and that takes more memory).

CPallini 2-Dec-16 8:59am

5.

Kishor-KW 3-Dec-16 5:33am

lie-to-children, Thanks :) 5

OriginalGriff 3-Dec-16 5:53am

You're welcome!
"Lies-to-children" are when the teacher doesn't tell you the whole truth - just "all you can understand at the moment". Like physics at school - they tell you the atom is the smallest particle, then the next year it's "well, that's not the whole truth, atoms are made of protons, neutrons, and electrons - but they are the smallest", then the following year quarks start to appear, and then electrons aren't really particles, and they don't really orbit, and...
"Lies-to-children" :laugh:
Teachers get quite upset when you use that phrase and prefer things like "age appropriate approximations", but that is exactly what they are: lies. ;)

Richard Deeming 2-Dec-16 11:13am

You missed out all the fun you can have with the default packing size! :)

OriginalGriff 2-Dec-16 11:25am

More lies-to-children, I'm afraid! :laugh: