Question

I’m trying to embed mono in an application i’m writing, but mono_field_set_value doesn’t behave like I think it should. Here is an example:

monotest.c:

#include <mono/jit/jit.h>
#include <mono/metadata/assembly.h>
#include <mono/metadata/debug-helpers.h>
#include <stdio.h>

#define TRUE 1

int main(){
        MonoDomain *domain;
        MonoAssembly *assembly;
        MonoImage *image;
        MonoClass *klass;
        MonoObject *object;
        MonoClassField *field;
        MonoMethodDesc *desc;
        MonoMethod *method;
        int val;

        domain = mono_jit_init ("Domain");
        assembly = mono_domain_assembly_open (domain, "T.DLL");;
        image = mono_assembly_get_image(assembly);
        klass = mono_class_from_name(image, "", "T");
        mono_class_init (klass);
        object = mono_object_new (domain, klass);

        val=11;
        field = mono_class_get_field_from_name(klass,"i");
        mono_field_set_value(object,field,&val);
        //mono_field_set_value((MonoObject*)((void*)object-8),field,&val);                                                                                                                                                                                                                                                     

        printf("val = %d\n", val);

        desc = mono_method_desc_new (":Write()",TRUE);
        method = mono_method_desc_search_in_class (desc, klass);
        mono_runtime_invoke(method,object,NULL,NULL);

        return 0;
}

T.cs

public struct T
{

  public int i;
  public void Write()
  {

    System.Console.WriteLine(i);
  }
}

When run, i expect the output:

val = 11
11

but the second number (printed by the mono runtime) is random. What am I doing wrong?

Answer 1

The argument to mono_runtime_invoke() is incorrect, it needs to be the unboxed pointer, since the type T is a struct.
From the documentation at http://docs.go-mono.com/index.aspx?link=xhtml%3Adeploy%2Fmono-api-methods.html:

obj is the ‘this’ pointer, it should be NULL for static methods, a
MonoObject* for object instances and a pointer to the value type for
value types.