Without using "fixed", how do I access values of an array within a struct?

Question

I'm doing C++ --> C# interop stuff and I have a bunch of structs that contain each other like Matryoshka dolls. The problem is that one of these 'nestings' takes the form of a fixed length array:

typedef struct tagBIRDREADING
{
    BIRDPOSITION    position;
    BIRDANGLES      angles;
    BIRDMATRIX      matrix;
    BIRDQUATERNION  quaternion;
    WORD            wButtons;
}
BIRDREADING;

typedef struct tagBIRDFRAME
{
    DWORD           dwTime;
    BIRDREADING     reading[BIRD_MAX_DEVICE_NUM + 1];
}
BIRDFRAME;

Following the hallowed teachings of Eric Gunnerson, I did the following in C#:

[StructLayout(LayoutKind.Sequential, Pack = 0)]
public struct BIRDREADING
{
    public BIRDPOSITION position;
    public BIRDANGLES angles;
    public BIRDMATRIX matrix;
    public BIRDQUATERNION quaternion;
    public ushort wButtons;
}

[StructLayout(LayoutKind.Sequential, Size = 127)]
public struct BIRDREADINGa
{
    public BIRDREADING reading;
}

public struct BIRDFRAME
{
    public uint dwTime;
    public BIRDREADINGa readings; 
}

My question is, how do I access each of the 127 instances of BIRDREADING contained within BIRDREADINGa and therefore BIRDFRAME? Or have I gone terrible wrong?

David Heffernan · Accepted Answer · 2011-04-15 11:02:28Z

4

I think you just want this:

[StructLayout(LayoutKind.Sequential)]
public struct BIRDFRAME
{
    public uint dwTime;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=127)] 
    public BIRDREADING[] readings; 
}

answered Apr 15, 2011 at 11:02

David Heffernan

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4 Comments

ildjarn Over a year ago

You're always really fast with the P/Invoke answers. Every time I go to submit one, yours pops up :-]

David Heffernan Over a year ago

@ildjarn Sorry! I do like P/Invoke questions, even though I never do any P/Invoke for real myself.

ildjarn Over a year ago

No apology necessary, just paying a compliment. :-]

Tom Wright Over a year ago

Thanks for this Dave. I wonder whether you'd enjoy a follow-up question? stackoverflow.com/q/5681647/50151

R. Martinho Fernandes · Accepted Answer · 2011-04-15 11:13:14Z

0

To access all those instances without using an array you need to use an unsafe block to grab the address of the "fake array" you set up, and then use pointer arithmetic. It's going to get ugly:

public struct BIRDREADINGa
{
    public BIRDREADING reading;

    public BIRDREADING GetReading(int index)
    {
        unsafe
        {
            fixed(BIRDREADING* r = &reading)
            {
                return *(r + index);
            }
        }
    }
}

answered Apr 15, 2011 at 11:13

R. Martinho Fernandes

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3 Comments

R. Martinho Fernandes Over a year ago

Hmm, apparently changing the values will not work like this readingA.GetReading(3).wButtons = 3 :(

David Heffernan Over a year ago

Plus it uses fixed which OP would like not to do

R. Martinho Fernandes Over a year ago

@David: it's not the same fixed. I know from chat that he was talking about the fixed that means fixed size arrays, not the fixed that means pin references.

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