add Mono internal call where a string is passed by reference

Question

I'm trying to create a method using mono where a string is passed by reference, here is the test code I have:

C++:

static bool p_TestMethod(int num, MonoString ** response) {

    auto b = mono_string_new(mono_domain_get(), "Test repsonse");
    response = &b;

    return true;
}
//...
mono_add_internal_call("SharpCode.TestClass::Testmethod", p_TestMethod);

C#:

    [MethodImpl(MethodImplOptions.InternalCall)]
    public static extern bool Testmethod(int num, out string response);

    public bool RunTheTest()
    {
        string x;
        Testmethod(0, out x);
        Console.WriteLine("Test: {0}", x);

        return true;
    }

But no response is printed (only Test: )

How do I properly pass a string by reference using Mono?

Regardless of what happens on the Mono side, `response = ...` in the C++ code just alters a local variable, so that can't be right. — , Mar 15 '14 at 19:13
@delnan I tried MonoString * response instead of MonoString ** response too, but that didn't work either. I decided to test a double pointer after reading this: https://github.com/mono/mono/blob/master/docs/internal-calls . But yet no result. — ikkentim, Mar 15 '14 at 19:23
@ikkentim After skimming over that page, I agree that a pointer-to-pointer seems the right parameter type. But that's not what I was trying to say. `response = &b;` is completely conceptually wrong. It does not affect what the parameter points at, it only makes you lose access to what the parameter used to point at; and the address of `b` has an expiration date, it should not outlive the function call. I'd expect `*respone = b;`, though I can **not** vouch for my expectations, as they are solely based on my C++ knowledge. — , Mar 15 '14 at 19:42
@delnan My C++ kwowledge is near zero, but thanks a lot! I've fixed the issue by doing *response = mono_string_new(mono_domain_get(), "Test repsonse"); as you suggested — ikkentim, Mar 15 '14 at 19:48

score 2 · Accepted Answer · edited Mar 15 '14 at 19:58

2

Fixed by doing it like this:

*response = mono_string_new(mono_domain_get(), "Test repsonse");

as suggested by delnan

edited Mar 15 '14 at 19:58

Sudhakar Tillapudi

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answered Mar 15 '14 at 19:49

ikkentim

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add Mono internal call where a string is passed by reference

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