How do I create an array of structs within a struct in C#?

Question

How do I initialize an array of structs within a struct in C#?

public const int MAX_AXIS = 10;

struct realprm
{     /* real parameter */
    long prm_val;             /* value of variable */
    long dec_val;             /* number of places of decimals */
};

[StructLayout(LayoutKind.Explicit)]
unsafe struct iodbpsd
{
    [FieldOffset(0)] short datano;               /* parameter number */
    [FieldOffset(2)] short type;                 /* upper byte:type */
    
    /* lower byte:axis */{
    [FieldOffset(4)] char cdata;           /* bit/byte parameter */
    [FieldOffset(4)] short idata;           /* word parameter */
    [FieldOffset(4)] long ldata;           /* 2-word parameter */
    [FieldOffset(4)] realprm rdata;           /* real parameter */
    [FieldOffset(4)] fixed char cdatas[MAX_AXIS];/*bit/byte parameter with axis*/
    [FieldOffset(4)] fixed short idatas[MAX_AXIS];/* word parameter with axis */
    [FieldOffset(4)] fixed long ldatas[MAX_AXIS];/* 2-word parameter with axis */
    [FieldOffset(4)] realprm rdatas[MAX_AXIS];/* real parameter with axis */
};

The last entry, realprm, throws compiler errors:

CS0270 Array size cannot be specified in a variable declaration (try initializing with a 'new' expression)

CS0650 Bad array declarator: To declare a managed array the rank specifier precedes the variable's identifier. To declare a fixed size buffer field, use the fixed keyword before the field type.

The 'fixed' keyword won't work for structs, and C# does not allow 'new' keyword initialization:

CS0573 Field declaration: cannot have instance field initializers in structs

Is there any elegant way to initialize an array like this in C#?

Try defining as fixed byte rdatas[MAX_AXIS * sizeof(realprm)], accessing to such array elements can be done via helper methods. — Svyatoslav Danyliv
– Svyatoslav Danyliv, Commented Jun 18 at 17:28

Guru Stron · Accepted Answer · 2025-06-18 17:07:35Z

2

From the language specification, section 23.8.2 Fixed-size buffer declarations:

The buffer element type shall be one of the predefined types sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, or bool.

Hence you can't define a fixed array of custom structs.

You can look into using the inline arrays added with C# 12:

[System.Runtime.CompilerServices.InlineArray(MAX_AXIS)]
public struct BufferMAX_AXIS<TT>
{
    private TT _element0;
}

And then use it in your type, but this depends on what actually is needed.

Also it is a bit strange that there is a lot of fields starting on the same offset (i.e. [FieldOffset(4)]).

answered Jun 18 at 17:07

Guru Stron

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1 Comment

cncmang Jun 18 at 17:21

Thank you. I am trying to communicate with industrial equipment, and depending on the data to be retrieved, it will be of different lengths, but all starting at the offset +4, hence the multiple items at 4.

It all makes cents · Accepted Answer · 2025-06-19 17:03:05Z

1

You can create a new instance of an array within a constructor. Try the following:

public const int MAX_AXIS = 10;

struct RealPrm
{
    long prmVal;
    long decVal;
}

[StructLayout(LayoutKind.Explicit)]
unsafe struct IoDbPsd
{
    [FieldOffset(0)] short dataNo;
    [FieldOffset(2)] short type;
    [FieldOffset(4)] char cData;
    [FieldOffset(4)] long lData;
    [FieldOffset(4)] RealPrm rData;
    [FieldOffset(4)] fixed char cDatas[MAX_AXIS];
    [FieldOffset(4)] fixed short iDatas[MAX_AXIS];
    [FieldOffset(4)] fixed long lDatas[MAX_AXIS];
    [FieldOffset(4)] RealPrm[] rDatas;

    public IoDbPsd()
    {
        rDatas = new RealPrm[MAX_AXIS];
    }
}

Resources:

answered Jun 19 at 17:03

It all makes cents

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1 Comment

cncmang Jun 20 at 13:13

This was helpful. Apparently you can't use objects or structs in a de facto C# struct union. If I remove the RealPrm members, the data sitting at +4 will play nicely.

cncmang · Accepted Answer · 2025-06-20 14:39:36Z

0

    public class IODBPSD_3
    {
        [FieldOffset(0)]
        public short datano;    /* data number */
        [FieldOffset(2)]
        public short type;      /* axis number */
        [FieldOffset(4),
        MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_AXIS)]
        public byte[] cdatas = new byte[MAX_AXIS];
        [FieldOffset(4),
        MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_AXIS)]
        public short[] idatas = new short[MAX_AXIS];
        [FieldOffset(4),
        MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_AXIS)]
        public int[] ldatas = new int[MAX_AXIS];
    }

The equipment manufacturer had this buried in their code.

edited Jun 20 at 14:39

answered Jun 20 at 13:35

cncmang

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