How to P/Invoke "__arglist" function?

Question

Background:
I have written the following function in C++:

extern "C" __declspec(dllexport) int test(const char*, ...);

And I am using P/Invoke to call it from C#:

[DllImport("foo/bar.dll", EntryPoint = "test")]
public static unsafe extern int __unmanaged__test(byte* buffer, __arglist);

Problem:
I need to initialize the __arglist dynamically, meaning that I have an Array<object>, which I have to convert into the __arglist before calling the C++-function.

I have tried the following, but it returns an compiler error:

unsafe {
    byte[] buffer = ....
    object[] args = ....

    // ....

    fixed (byte* ptr = buffer)
        return __unmanaged__test(ptr, __arglist(args)); // <--ERROR
}

Does anyone know a way to solve this problem?

for those who do not know what __arlist is: http://bartdesmet.net/blogs/bart/archive/2006/09/28/4473.aspx

score 1 · Accepted Answer · edited May 23 '17 at 12:14

You need to use the C-style calling convention - the others (including the default Stdecl) don't support varargs:

[DllImport("foo/bar.dll", EntryPoint = "test", 
           CallingConvention = CallingConvention.Cdecl)]
public static unsafe extern int __unmanaged__test(byte* buffer, __arglist);

Another tricky part is that __arglist(...) is a compile-time feature - it simply generates a (virtual) stack push for each of its "arguments". This means that you can't use it with arguments not known at compile-time - in your sample code, you're simply attempting to make a single argument, typed object[].

Building the helper reflection methods isn't too hard - see Calling varargs method via DynamicMethod for an example. I cannot test this, but I'd guess Expression could be used to build this code easily - if you didn't have to pass that byte*, that is (do you really have to? passing byte[] could work just as well).

Thank you, but the error still exists. I think its the conversion between the array and the __arglist — unknown6656, Aug 24 '15 at 06:52
@Unknown6656 Well, yeah, you can't do that. If you need to build `__arglist` dynamically (that is, you don't know the types and amount of arguments in advance), you'll probably have to use reflection. See http://stackoverflow.com/questions/29458616/calling-varargs-method-via-dynamicmethod for example. — Luaan, Aug 24 '15 at 06:54

score 0 · Answer 2 · edited May 23 '17 at 12:14

0

The comment Luuan wrote (see here) had the solution I needed:

quote:

If you need to build __arglist dynamically (that is, you don't know the types and amount of arguments in advance), you'll probably have to use reflection.
See stackoverflow.com/questions/29458616/… for example

edited May 23 '17 at 12:14

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answered Aug 24 '15 at 06:59

unknown6656

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How to P/Invoke "__arglist" function?

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