Why Object always contain 4 byte extra memory.

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please explain the memory mapping concept of c# with example.

class A{
int x;
int y;
}

now if i create the object of A class then total memory of class is
A a1=new A();

int x.....contain 4 byte
int y.....contain 4 byte
total 8 byte

a1 always contain extra memory 4 byte

now total memory consumption is 12 byte?

so finally my question is why a1(object of A class) contain 4 byte extra memory while only 2 variable declare in A Class?

can you please explain the memory consumption concept of C# Code with example

Posted 3-Apr-14 18:41pm

Member 10723360

Updated 16-Feb-15 23:39pm

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[no name] 4-Apr-14 0:57am

You may wish to rephrase your question. Suggest you read this http://www.codeproject.com/Questions/177604/Size-of-a-class-in-c

Sergey Alexandrovich Kryukov 4-Apr-14 1:23am

The question is clear enough; I basically answered.

Still, the question is not very appropriate. I suggest OP creates some code samples and ask some questions on the observed size in different instruction-set architecture.

—SA

Sergey Alexandrovich Kryukov 4-Apr-14 2:00am

Not always. This is simply not true.
—SA

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Sergey Alexandrovich Kryukov · Accepted Answer · 2014-04-03T19:22:00

This is related to word alignment. Memory allocated by the structure/class members may be not continuously aligned, but aligned by some chunks, say, of 32-bit. You can control this alignment using attributes (see the links below), by if you don't, the default is used.

The difference between 64-bit and 32-bit default layout is harder to explain, but the reason is pretty obvious. The thing is: all the types use the same sizes in both kinds of architectures, except the size of the managed pointer: it is 64 or 32 bits, respectively. Therefore, structures or classes having reverence-type instance fields are aligned differently.

Please see:
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.structlayoutattribute.aspx[^],
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.fieldoffsetattribute.aspx[^].

Please, find out the samples adding 4 or 8 extra bytes to the size of a data type by yourself. If you fail to do this exercise, show what have you tried and what were the results. Then we will be able to discuss it.

—SA