Passing C# enum from C++ using mono

Question

Why can't I get the MonoMethod* for the method that takes an enum,
and is there a way to list the methods that mono knows about?

Here's a reduced example of what I'm trying to do. I tried different combinations of what (Windows) ildasm reported but no success.

// enum.cs
namespace EnumTest
{
    class EnumClass
    {
        public enum HelloType { Hi, Hey, Howdy, Greetings, Aloha };

        public static void SetDefault() { type_ = HelloType.Hi; }

        public static void SetType(HelloType h) { type_ = h; }

        public static HelloType type_;
    }
}

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

int main(int argc, char* argv[]) {
  MonoDomain* domain = mono_jit_init("enum.dll");
  MonoAssembly* assembly = mono_domain_assembly_open(domain, "enum.dll");
  MonoImage* image = mono_assembly_get_image(assembly);
  MonoClass* enumClass = mono_class_from_name(image, "EnumTest", "EnumClass");

  MonoMethodDesc* defaultDesc = mono_method_desc_new("EnumTest.EnumClass:SetDefault()", false);
  MonoMethod* defaultMethod = mono_method_desc_search_in_class(defaultDesc, enumClass);

  printf("defaultDesc from mono_method_desc_new() is %p\n", defaultDesc);
  printf("defaultMethod from mono_method_desc_search_in_class() is %p\n\n", defaultMethod);


  MonoMethodDesc* typeDesc = mono_method_desc_new("EnumTest.EnumClass:SetType(HelloType)", false);
  MonoMethod* typeMethod = mono_method_desc_search_in_class(typeDesc, enumClass);

  printf("typeDesc from mono_method_desc_new() is %p\n", typeDesc);
  printf("typeMethod from mono_method_desc_search_in_class() is %p      <------\n", typeMethod);


  mono_jit_cleanup(domain);
}

$ mcs /nologo /warn:4 /debug:pdbonly /o  /out:enum.dll /target:library enum.cs
$ gcc enum.cpp -g3 `pkg-config --cflags --libs mono-2` -o enum
$ ./enum
defaultDesc from mono_method_desc_new() is 0x1443920
defaultMethod from mono_method_desc_search_in_class() is 0x13dce40

typeDesc from mono_method_desc_new() is 0x1444590
typeMethod from mono_method_desc_search_in_class() is (nil)      <------
$ uname -a
Linux U14-OOXML 3.16.0-37-generic #51~14.04.1-Ubuntu SMP Wed May 6 15:23:14 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Try iterating all of the class methods with mono_class_get_methods - does the method show up there? — cbr
– cbr, Commented May 21, 2015 at 7:50
The method in question (SetType) shows up when iterating with mono_class_get_methods(). And using mono_method_signature(), mono_signature_get_params() and mono_type_get_name() shows that internally mono know its parameter type to be EnumTest.EnumClass.HelloType So it would seem that my above code is correct. — Howard Rubin
– Howard Rubin, Commented May 21, 2015 at 16:32

cbr · Accepted Answer · 2015-05-21 18:10:05Z

1

Looks like the parameter type requires the full type name.

I used mono_method_full_name with the second argument signature true to get the signature (I used my own test class with identical structure but different names):

MonoDomain* domain = mono_jit_init(CSHARP_LIB);
MonoAssembly* assembly = mono_domain_assembly_open(domain, CSHARP_LIB);
MonoImage* image = mono_assembly_get_image(assembly);
MonoClass* sandboxClass = mono_class_from_name(image, "CSharp_ClassLibrary", "Sandbox");

void* ptr = 0;
MonoMethod* iter;

while ((iter = mono_class_get_methods(sandboxClass, &ptr)) != NULL)
{
    printf("--------------------------------\n");
    const char* name = mono_method_get_name(iter);
    MonoMethodDesc* methodDesc = mono_method_desc_from_method(iter);

    const char* paramNames = "";
    mono_method_get_param_names(iter, &paramNames);

    printf("Name: %s\n", name);
    printf("Full name: %s\n", mono_method_full_name(iter, true));
    printf("    namespace: %s\n", methodDesc ->namespace);
    printf("    class:     %s\n", methodDesc ->klass);
    printf("    name:      %s\n", methodDesc ->name);
    printf("    args:      %s\n", methodDesc ->args);
    printf("    num_args:  %d\n", methodDesc ->num_args);
    printf("    paramnames:%s\n", paramNames);
}

The output I received was this:

--------------------------------
Name: SetDefault
Full name: CSharp_ClassLibrary.Sandbox:SetDefault ()
    namespace: CSharp_ClassLibrary
    class:     Sandbox
    name:      SetDefault
    args:      (null)
    num_args:  0
    paramnames:
--------------------------------
Name: SetType
Full name: CSharp_ClassLibrary.Sandbox:SetType (CSharp_ClassLibrary.Sandbox/HelloType)
    namespace: CSharp_ClassLibrary
    class:     Sandbox
    name:      SetType
    args:      (null)
    num_args:  0
    paramnames:h
--------------------------------
Name: .ctor
Full name: CSharp_ClassLibrary.Sandbox:.ctor ()
    namespace: CSharp_ClassLibrary
    class:     Sandbox
    name:      .ctor
    args:      (null)
    num_args:  0
    paramnames:

So to fix your code, you'd just add the class name before the parameter type:

MonoMethodDesc* typeDesc = mono_method_desc_new("EnumTest.EnumClass:SetType(EnumClass/HelloType)", false);
MonoMethod* typeMethod = mono_method_desc_search_in_class(typeDesc, enumClass);

This, at least for me, works just fine!

edited May 21, 2015 at 18:10

answered May 21, 2015 at 17:16

cbr

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5 Comments

boo Over a year ago

your example code is not Not exactly correct, mono_method_get_param_names. your code will cause stack crash. try:const char* paramNames[128] = { 0 }; mono_method_get_param_names(iter, paramNames);

awattar Over a year ago

Accessing methodDesc members gives compiler error - member access into incomplete type.

cbr Over a year ago

@awattar This answer was written in 2015. The Mono API has probably had major changes since then.

awattar Over a year ago

I've assumed that but also have searched back in the history of the github.com/mono/mono/blob/main/mono/metadata/debug-helpers.h and didn't find MonoMethodDesc struct be ever fully defined public.

cbr Over a year ago

I see. There's probably corresponding functions in mono/metadata/class.h for getting the fields. Feel free to suggest an edit or leave a new, up to date answer.

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