mono/marshal: SafeHandles missing MANAGED_CONV_RESULT\n

Error message

mono/marshal: SafeHandles missing MANAGED_CONV_RESULT\n

What it means

Third unimplemented branch in emit_marshal_safehandle_ilgen: MARSHAL_ACTION_MANAGED_CONV_RESULT covers the case where a reverse P/Invoke (native->managed) managed method returns a SafeHandle that must be converted to a native handle for the caller. Mono prints this to stderr and emits nothing, so the return-value conversion is missing.

Source

Thrown at src/mono/mono/component/marshal-ilgen.c:2046

		/* Set the return.handle to the value, am using ldflda, not sure if thats a good idea */
		cb_to_mono->methodBuilder.emit_ldloc (mb, 3);
		cb_to_mono->methodBuilder.emit_ldflda (mb, MONO_STRUCT_OFFSET (MonoSafeHandle, handle));
		cb_to_mono->methodBuilder.emit_ldloc (mb, intptr_handle_slot);
		cb_to_mono->methodBuilder.emit_byte (mb, CEE_STIND_I);
		break;
	}

	case MARSHAL_ACTION_MANAGED_CONV_IN:
		fprintf (stderr, "mono/marshal: SafeHandles missing MANAGED_CONV_IN\n");
		break;

	case MARSHAL_ACTION_MANAGED_CONV_OUT:
		fprintf (stderr, "mono/marshal: SafeHandles missing MANAGED_CONV_OUT\n");
		break;

	case MARSHAL_ACTION_MANAGED_CONV_RESULT:
		fprintf (stderr, "mono/marshal: SafeHandles missing MANAGED_CONV_RESULT\n");
		break;
	default:
		printf ("Unhandled case for MarshalAction: %d\n", action);
	}
	return conv_arg;
}

static int
emit_marshal_handleref_ilgen (EmitMarshalContext *m, int argnum, MonoType *t,
			MonoMarshalSpec *spec, int conv_arg,
			MonoType **conv_arg_type, MarshalAction action)
{
	MonoMethodBuilder *mb = m->mb;

	MonoType *int_type = cb_to_mono->get_int_type ();
	switch (action){
	case MARSHAL_ACTION_CONV_IN: {
		conv_arg = cb_to_mono->methodBuilder.add_local (mb, int_type);

View on GitHub (pinned to 60108ba66e)

Solutions

Change the managed callback's return type from SafeHandle to IntPtr, returning handle.DangerousGetHandle(), and let the native caller treat it as a raw handle.
Return an integer id and expose a separate managed API to resolve it to a SafeHandle on demand.
Keep all SafeHandle lifetime inside managed code and never return one across a reverse-P/Invoke boundary.
Target a runtime that implements MANAGED_CONV_RESULT for SafeHandle.

Example fix

// before
public delegate SafeFileHandle OpenCallback(string path);

// after
public delegate IntPtr OpenCallback(string path);
// managed helper wraps the returned IntPtr into a SafeFileHandle when needed

Defensive patterns

Strategy: validation

Validate before calling

// Flag reverse-P/Invoke delegates that return SafeHandle (Mono MANAGED_CONV_RESULT unsupported).
static IEnumerable<MethodInfo> CheckReturningSafeHandles(Assembly asm) {
    foreach (var t in asm.GetTypes())
      foreach (var m in t.GetMethods(BindingFlags.Public|BindingFlags.NonPublic|BindingFlags.Static|BindingFlags.Instance)) {
        if (m.GetCustomAttribute<UnmanagedFunctionPointerAttribute>() == null) continue;
        if (typeof(SafeHandle).IsAssignableFrom(m.ReturnType))
          yield return m;
      }
}

Prevention

Return IntPtr, not SafeHandle, from delegates called by native code.
Expose a managed helper to wrap the returned handle later.
Scan interop assemblies for SafeHandle-returning reverse delegates at build time.
Target a runtime with full SafeHandle managed-conv support if unavoidable.

When it happens

Trigger: A managed delegate invoked from native code returns a SafeHandle; the marshaller needs to emit IL extracting/returning the raw handle but this case is a no-op.

Common situations: Reverse P/Invoke whose managed handler returns a SafeHandle (e.g. returning a file/socket handle to native); COM interop with SafeHandle return values on Mono; code that behaves on CoreCLR but hits the unimplemented Mono path.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/9d2e84d0f65fdf87. Report an issue: GitHub.