Why does a System.Boolean take 4 bytes? It just stores one state, either true or false, which could be stored in less space than 4 bytes.
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6 Answers
A bool is actually only 1 byte, but alignment may cause 4 bytes to be used on a 32-bit platform, or even 8 bytes on a 64-bit platform. For example, the Nullable<bool> (aka bool?) type uses a full 32 or 64 bits—depending on platform—even though it's comprised of just two bools. EDIT: As pointed out by Jon Skeet, padding for alignment isn't always present. As an example, an array of Nullable<bool>s takes only 2 bytes per object instead of 4 or 8.
But even 8 bits to represent a bool can be considered wasteful if you have many of them to store. For this reason, if you create a type that has many bools as members, (or uses many Nullable<> types), and users of your class might create many instances of it, you might consider using a BitVector32 instead. The framework itself uses this technique to reduce the memory footprint of many of the Windows Forms controls, for instance.
3 Comments
Jon Skeet
Even Nullable<bool> will only use up two bytes in some cases - for instance if you've got an array of them.
P Daddy
Yes, you're right. I would have expected it to align array indices for performance's sake, but it doesn't seem to.
Drunken Code Monkey
This is an artifact of the C standard library and compiler. Most compilers on 32-bits and 64-bits platforms where memory is not really an issue optimize for performance, not code size. Bools are not packed by the compiler into bitfields, unless done explicitly in a struct. This is to keep adequate performance when passing pointers, otherwise you not only have to dereference but also mask on every access.
Because it's fast.
A 32-bit processor typically works with 32-bit values. Working with smaller values involves longer instructions, or extra logic.
Comments
The first result on a Google search for System.Boolean size told me that it's to do with memory alignment. It's faster to push around a four-byte Int32 than it is to work with individual bytes/bits.
1 Comment
Blaisorblade
Today's first Google answer is this question. Since questions are archived, please add also the specific link in the future.
I think it's only for performance, 32 bit values are much more efficient to manipulate.
answered Nov 17, 2008 at 4:56
Christian C. Salvadó
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Comments
Where'd you get that? System.Boolean takes only 1 byte.
Just try:
Console.WriteLine( sizeof( System.Boolean ).ToString() );
4 Comments
Christian C. Salvadó
Check the size after the type has been marshaled: Console.Write(System.Runtime.InteropServices.Marshal.SizeOf(new System.Boolean()));
arul
Which is what you shouldn't do. Most of the unmanaged API's use int for BOOL values, Marhsal.SizeOf() tells you size of an unmanaged type.
JaredPar
Marshalling a bool is slighly more complicated than that. I did a blog post on this subject which covers the various sizes: blogs.msdn.com/jaredpar/archive/2008/10/14/…
John Fairbanks
I used the GC.GetTotalMemory to check it... my post was too long to fit here in the replies but I added it as a response to this question for anyone who is interested.
I used the following code to create a couple arrays and test them. float?[100000] uses twice as much memory as float[100000]. This is because the bool that accompanies the float in the float? case gets aligned to 32 bits (at least on my machine ;-) )
long startMem = 0, stopMem = 0;
DateTime startTime = DateTime.Now;
private void StartMemoryUsage()
{
GC.Collect();
GC.Collect();
startMem = GC.GetTotalMemory(true);
startTime = DateTime.Now;
}
private void StopMemoryUsage()
{
GC.Collect();
GC.Collect();
stopMem = GC.GetTotalMemory(true);
Console.WriteLine("---> {0} sec. Using {1} KB.", (DateTime.Now - startTime).TotalSeconds, ((stopMem - startMem) / 1000).ToString());
}