C Bit fields in C#

Question

I need to translate a C struct to C# which uses bit fields.

typedef struct foo  
{  
    unsigned int bar1 : 1;
    unsigned int bar2 : 2;
    unsigned int bar3 : 3;
    unsigned int bar4 : 4;
    unsigned int bar5 : 5;
    unsigned int bar6 : 6;
    unsigned int bar7 : 7;
    ...
    unsigned int bar32 : 32;
} foo;

Anyone knows how to do this please?

Brainwash

Already answered in Bit fields in C#. – Melissamelisse

I hope you are aware of that you are allocating 1+2+3.. +32 bits = 528 bits = 66 bytes. – Ornithischian

@Lundin: It is just an example, I just wanted to point out that I have all possible bit field variants. – Brainwash

@Aasmund Eldhuset: Yes I have already seen this post but I don't get it why he converts all uints into one long. – Brainwash

@Brainwash I guess he just wanted to demonstrate that the bit mask ended up as desired, and he assumed that "64 bits should be enough for anyone"... ;-) However, his solution does not save any space (I didn't realize that until now), since each element of his struct is a full uint. If you really need to conserve space or need to communicate with hardware (which I guess are the only defendable reasons for using bit fields in the first place), you should consider using BitArray as suggested by @Nekresh, and perhaps wrap it in properties that get/set bit groups (quite a bit of work, though). – Melissamelisse

Are you trying to marshal the data? If not, you could just write your own BitField class which would be very simple. – Reich

I guess you need to clarify HOW you intent to use this struct first. – Hurlow

Answer 1

As explained in this answer and this MSDN article, you may be looking for the following instead of a BitField

[Flags]
enum Foo
{
    bar0 = 0,
    bar1 = 1,
    bar2 = 2,
    bar3 = 4,
    bar4 = 8,
    ...
}

as that can be a bit annoying to calculate out to 2³², you can also do this:

[Flags]
enum Foo
{
    bar0 = 0,
    bar1 = 1 << 0,
    bar2 = 1 << 1,
    bar3 = 1 << 2,
    bar4 = 1 << 3,
    ...
}

And you can access your flags as you would expect in C:

Foo myFoo |= Foo.bar4;

and C# in .NET 4 throws you a bone with the HasFlag() method.

if( myFoo.HasFlag(Foo.bar4) ) ...

Brubaker

Answer 2

You could use the BitArray class for the framework. Look at the msdn article.

Sealskin

Thanks, but that would be pain in the ass if I have to declare every single bit. – Brainwash

Answer 3

Unfortunately there is no such thing in C#. The closest thing is applying a StructLayout attribute and using FieldOffset attribute on fields. However the field offset is in bytes, not in bits. Here is an example:

[StructLayout(LayoutKind.Explicit)]
struct MyStruct
{
    [FieldOffset(0)]
    public int Foo; // this field's offset is 0 bytes

    [FieldOffset(2)]
    public int Bar; // this field's offset is 2 bytes. It overlaps with Foo.
}

But it is NOT the same as the functionality you want.

If what you need is to decode a bits sequence to a struct you will have to write manual code (using, for example, using classes like MemoryStream or BitConverter).

Picket

>If what you need is to decode a bits sequence to a struct you will have to write manual code (using, for example, using classes like MemoryStream or BitConverter). Well that was what I was looking for. – Brainwash

Answer 4

Are you looking for the FieldOffset attribute? See here: FieldOffsetAttribute Class

Alex