CODEWITHSUNDEEP
c#.net
InfZero
InfZero

Reputation: 3045

Why storage overhead generates waste in C# data types?

In subtopic Storage Overhead (on Chapter) -C# 5.0 in a Nutshell book- there is this general note that says:

enter image description here

Now, I'm wondering why the fields in struct A generates a waste of space? Or, what is the author's point with the entire note?

Upvotes: 5

Views: 417

Answers (4)

Matthew Watson
Matthew Watson

Reputation: 109587

There are two reasons for this default for structs:

1) Performance, as others have already explained. Placing the field members at multiples of their size allows faster transfers.

2) To be the same as the default for most C/C++ compilers. This makes interop with C/C++ (including all the Windows API) a bit easier.

Note that if you don't need to marshal your structures, you can use [StructLayout(LayoutKind.Auto)]:

[StructLayout(LayoutKind.Auto)]
struct A
{
    byte b;
    long l;
}

With that, the following code

unsafe
{
    Console.WriteLine(sizeof(A));
}

prints 12, which indicates better packing.

If you use [StructLayout(LayoutKind.Sequential, Pack = 1)] you can get the size down to 9.

Upvotes: 4

Curt
Curt

Reputation: 5722

They are generally aligned at processor word boundaries, so that retrieving them is a simple one-cycle operation. Otherwise, the CLR would have to pick up the whole address, and XOR/shift the struct field to reference it.

Upvotes: 2

Douglas
Douglas

Reputation: 54887

Each byte field occupies 1 byte, whilst each long field occupies 8 bytes. This means that, whilst b could be placed anywhere in memory, l needs to be placed at an address that is a multiple of 8. It cannot be placed at address 0 since that is already occupied by b; thus, it must be placed at the next available multiple of 8, which is 8, causing the 7 bytes of intervening space to be wasted.

---------------------------------------------------------------------------------
|  0 |  1 |  2 |  3 |  4 |  5 |  6 |  7 |  8 |  9 | 10 | 11 | 12 | 13 | 14 | 15 |
---------------------------------------------------------------------------------
<--b->                                  <------------------l-------------------->
      <--------------waste------------->

Upvotes: 9

Lukas H&#228;fliger
Lukas H&#228;fliger

Reputation: 526

Just look at the alignment. A Long must be at position 0, 8, 16,...

But if we have first the byte it looks like that:

b-------llllllll

With b beeing the byte b and l beeing the long l. The - are the "wasted space" So as you can see the struct uses a whole 16 bytes but only 9 bytes are used thus 7 bytes are wasted

Upvotes: 5

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