Read-only array field in unsafe struct

Question

This is the original declaration:

[StructLayout(LayoutKind.Explicit, Size = 16)]
public unsafe struct X
{
  [FieldOffset(0)] public ushort a;
  [FieldOffset(2)] public fixed byte b[14];
};

I would like to make the struct read-only, but I don't know how I should write a getter for the array. The only solution I could figure out is a getter method:

[StructLayout(LayoutKind.Explicit, Size = 16)]
public unsafe struct X
{
  [FieldOffset(0)] private ushort a;
  [FieldOffset(2)] private fixed byte b[14];

  public ushort A { get { return a; } }
  public byte B(int i) { fixed (byte* p = b) { return p[i]; } }
};

Is it possible to write a getter property for b instead of a getter method?

=== UPDATE ===

I would also like to handle the case when there are more than one array fields. For example:

[StructLayout(LayoutKind.Explicit, Size = 24)]
public unsafe struct Y
{
  [FieldOffset(0)] private ushort a;
  [FieldOffset(2)] private fixed byte b[14];
  [FieldOffset(16)] private fixed byte c[8];

  public ushort A { get { return a; } }
  public byte B(int i) { fixed (byte* p = b) { return p[i]; } }
  public byte C(int i) { fixed (byte* p = c) { return p[i]; } }
};

Is it possible to write a getter properties for b and c instead of getter methods? I would like to write y.B[i] and y.C[i] instead of y.B(i) and y.C(i).

You should only ever use a declaration like this for unmanaged code interop. At that point, having a structure member read-only just doesn't mean anything anymore. The language you interop with needs to declare it that way, maybe with *const*. — Hans Passant, Aug 12 '13 at 13:08
@HansPassant Yes, I need this for interop code. I have a byte* from 3rd party unmanaged lib and after adding an offset I cast it to pointers to similar unsafe structures. I just want (1) to stop the users of my code overwriting the original unmanaged data, and (2) to modify some fields inside the getter. So I made every field private and added getters. — kol, Aug 12 '13 at 13:25
Never expose unsafe structures to client code. It is unsafe and nobody likes having to turn on the option. — Hans Passant, Aug 12 '13 at 13:27
@HansPassant I needed to do this to achieve the prescribed performance. Actually this question is related to this one: http://stackoverflow.com/questions/17549123/c-sharp-performance-using-unsafe-pointers-instead-of-intptr-and-marshal — kol, Aug 12 '13 at 13:34

Chris · Answer 1 · 2013-08-12T11:21:12.063

3

You can use an indexer propertie to access the array at a particular index. Unfortunately it needs to be defined on the struct itself rather than B, but this should provide what you're looking for:

[StructLayout(LayoutKind.Explicit, Size = 16)]
public unsafe struct X
{
    [FieldOffset(0)]
    private ushort a;
    [FieldOffset(2)]
    private fixed byte b[14];

    public ushort A { get { return a; } }
    public byte this [int i]
    {
        get 
        {
            byte b1;
            fixed (byte* b2 = b)
            {
                b1 = b2[i];
            }
            return b1;
        }
    }
 };

edited Aug 12 '13 at 11:21

answered Aug 12 '13 at 11:15

Chris

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2

note: can be simplified to `fixed (byte* ptr = b) { return ptr[i]; }` - but essentially, yup: this – Marc Gravell Aug 12 '13 at 11:21
+1 OK, but what I would like to index is not X itself, but its field b. Suppose that I have another array field, c. I would like to index both b, like x.B[i], and c, like x.C[i], where x is of type X. – kol Aug 12 '13 at 11:24
@Chris I extended the question to include the case when there are multiple array fields. Thank you for your help! – kol Aug 12 '13 at 11:37
@MarcGravell Thanks for the suggestion, I simplified the code in the question. – kol Aug 12 '13 at 11:38

kol · Answer 2 · 2013-08-12T11:58:18.133

I found a solution, which is based on the idea of indexed properties:

[StructLayout(LayoutKind.Explicit, Size = 14)]
public struct YB
{
  [FieldOffset(0)] private fixed byte b[14];
  public byte this[int i] { get { fixed (byte* p = b) { return p[i]; } } }
}

[StructLayout(LayoutKind.Explicit, Size = 8)]
public struct YC
{
  [FieldOffset(0)] private fixed byte c[8];
  public byte this[int i] { get { fixed (byte* p = c) { return p[i]; } } }
}

[StructLayout(LayoutKind.Explicit, Size = 24)]
public unsafe struct Y
{
  [FieldOffset(0)] private ushort a;
  [FieldOffset(2)] private YB b;
  [FieldOffset(16)] private YC c;

  public ushort A { get { return a; } }
  public YB B { get { return b; } }
  public YC C { get { return c; } }
};

Read-only array field in unsafe struct

2 Answers2