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
– Hans Passant, Commented Aug 12, 2013 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
– kol, Commented Aug 12, 2013 at 13:25
Never expose unsafe structures to client code. It is unsafe and nobody likes having to turn on the option. — Hans Passant
– Hans Passant, Commented Aug 12, 2013 at 13:27
@HansPassant I needed to do this to achieve the prescribed performance. Actually this question is related to this one: stackoverflow.com/questions/17549123/… — kol
– kol, Commented Aug 12, 2013 at 13:34

Chris · Accepted Answer · 2013-08-12 11:21:12Z

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, 2013 at 11:21

answered Aug 12, 2013 at 11:15

Chris

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4 Comments

Marc Gravell Over a year ago

note: can be simplified to fixed (byte* ptr = b) { return ptr[i]; } - but essentially, yup: this

kol Over a year ago

+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 Over a year ago

@Chris I extended the question to include the case when there are multiple array fields. Thank you for your help!

kol Over a year ago

@MarcGravell Thanks for the suggestion, I simplified the code in the question.

kol · Accepted Answer · 2013-08-12 11:58:18Z

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; } } };

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