[Mono-dev] using unsafe structs as opaque pointers...

jeske davidj at gmail.com
Thu Aug 16 16:35:33 UTC 2012

I have a C# wrapper for a native DLL which exists on both windows and UNIX.
However, it doesn't work on Mono because of a behavior difference between
Mono and MS.NET.

Specifically, I use "unsafe struct pointers" to manage pointers to opaque
data (rather than IntPtr). I do this to get typechecking, because there are
a bunch of types of pointers, and using IntPtr is dangerously
non-type-checked.  (I honestly don't undertand why IntPtr is allowed, rather
than always requiring the use of typed unsafe-struct-pointers.)


My code looks something like:

// opaque types
public unsafe struct HDF {};

// NEOERR* hdf_set_value (HDF *hdf, char *name, char *value)
private static unsafe extern NEOERR* hdf_set_value(HDF *hdf,
      string name,
      string value);

This works fine in MS.NET, but Mono complains with the following error:

Unhandled Exception:
System.Runtime.InteropServices.MarshalDirectiveException: Can not marshal
'parameter #1': Pointers can not reference marshaled structures. Use byref
  at (wrapper managed-to-native) Clearsilver.Hdf:hdf_set_value
  at Clearsilver.Hdf.setValue (System.String name, System.String value)
  at CSTest.Main (System.String[] argv) [0x00000]

My questions:

(1) Why is Mono complaining about this? In the CLR, an unsafe struct pointer
is not a marshalled structure, it's a value type.

(2) Why would it suggest using ByRef? ByRef is for managed pointers to
managed data. This is a value-type pointer value.  AFAIK, ByRef is not
capable of doing anything here, because the pointer is just a "typed" IntPtr
value type.

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