Marshaling a union with a pointer

    Date: 08/05/10 (C Sharp)    Keywords: linux

    I am using Mono on Linux, and want to marshal the following unmanaged structure to a native one:

    int 	type
    union {
       guint32   u
       gint32   i
       gboolean   b
       gdouble   d
       gchar *   s
    } 	value

    According to Mono's excellent page on native-library interop, the way to deal with unions is to use StructLayoutAttribute with LayoutKind.Explicit and specify a FieldOffsetAttribute that is the same for all union members. So you might do something similar to this:

    public struct Rpc {
     [FieldOffset(0)] public int type;
     [FieldOffset(4)] public uint value_u;
     [FieldOffset(4)] public int value_i;
     [FieldOffset(4)] public bool value_b;
     [FieldOffset(4)] public double value_d;
     [FieldOffset(4)] public string value_s;

    The problem however is this, the Mono runtime crashes when this code is run, complaining that: "you cannot have a reference field that is the same offset as another field" in an [ExplicitLayout] structure. I assume this is referring to the last member of the union which is a pointer. Thing is, I'm not sure how else to do this.

    I thought about just having a "public object value;" member of the structure, and then casting this as appropriate in managed code, but the runtime apparently cannot marshal to a vanilla object this way.



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