Question

I’ve embedded some C# code into an iphone objective-c app using monotouch. I’m able to call into the C# code and get a return value, but I am not able to invoke a C function from the C# code using PInvoke. I am attempting to get a trivial example working. It is a simple calculator. The objective c code calls into the C# code to add two integers. The C# code should then call back into the objective-c code and give it the updated value. Below is the C# code that handles the calculation:

public class MyClass
{       
    static public void Add(int a, int b)
    {
        updateResult(a + b);
    }

    [DllImport("__Internal", EntryPoint="updateResult")]
    static extern public void updateResult(int result);
}

Here is my code that initializes mono and handles the add method:

@implementation Mono

- (id)init {
self = [super init];

if (self) {
    NSBundle *main = [NSBundle mainBundle];
    NSString *path = [main bundlePath];
    const char *c_path = [path UTF8String];

    [main autorelease];
    [path autorelease];

    chdir (c_path);
    setenv ("MONO_PATH", c_path, 1);
    setenv ("MONO_XMLSERIALIZER_THS", "no", 1);
    setenv ("DYLD_BIND_AT_LAUNCH", "1", 1);
    setenv ("MONO_REFLECTION_SERIALIZER", "yes", 1);

#if defined (__arm__)
    mono_aot_register_module (mono_aot_module_mscorlib_info);
    mono_aot_register_module (mono_aot_module_Math_info);

    mono_jit_set_aot_only (TRUE);
#endif
    mono_jit_init("MonoTouch");
    MonoAssembly *assembly = mono_assembly_open("Math.dll", NULL);
    MonoImage *img = mono_assembly_get_image(assembly);
    MonoClass *cls = mono_class_from_name(img, "Math", "MyClass");
    MonoMethodDesc *methodDesc = mono_method_desc_new("Math.MyClass:Add", TRUE);
    _addMethod = mono_method_desc_search_in_class(methodDesc, cls);
}

return self;
} 

- (void)addA:(int)a plusB:(int)b {
    void *params[] = { &a, &b };
    mono_runtime_invoke(_addMethod, NULL, params, NULL);
}

@end

and here is the definition of the updateResults method:

extern void updateResult(int result) {
    printf("Got Result");
}

When the updateResults is called from the C# side of things I get the following exception:

Unhandled Exception: System.EntryPointNotFoundException: updateResult
at (wrapper managed-to-native) Math.MyClass:updateResult (int)
at Math.MyClass.Add (Int32 a, Int32 b) [0x00000] in <filename unknown>:0 

I can see the symbol in the binary with the following command:

$ nm Calc | grep updateResult
00002b2e t _updateResult

Setting the MONO_LOG_LEVEL environment variable to debug I get the following output when attempting the PInvoke. It looks like it finds the method and then can’t find it:

Mono: DllImport attempting to load: ‘_Internal’.Mono: Searching for ‘updateResult’.Mono: Probing ‘updateResult’.Mono: Probing ‘updateResult’.Mono: Probing ‘updateResultA’.Mono: Probing ‘updateResultA’.Mono: DllImport attempting to load: ‘_Internal’.Mono: Searching for ‘updateResult’.Mono: Probing ‘updateResult’.Mono: Probing ‘updateResult’.Mono: Probing ‘updateResultA’.Mono: Probing ‘updateResultA’.Mono: AOT FOUND AOT compiled code for (wrapper managed-to-native) Math.MyClass:updateResult (int) 0x3bc8 – 0x3d90 0x3dcb

Mono: DllImport attempting to load: ‘__Internal’.Mono: Searching for ‘updateResult’.Mono: Probing ‘updateResult’.Mono: Probing ‘updateResult’.Mono: Probing ‘updateResultA’.Mono: Probing ‘updateResultA’.Unable to resolve pinvoke method ‘Math.MyClass:updateResult (int)’ Re-run with MONO_LOG_LEVEL=debug for more information.

I’ve spent quite some time trying to figure this out. From what I’ve read it seems like this should be pretty trivial, but I can’t get it to work. Any help would be much appreciated.

Answer 1

Would it be better to use Selectors?

That is the default path to follow as suggested by the makers of MonoTouch.