Marshalling ArgIterator to va_list

问题

So, I got an idea to try to p/invoke C functions that take va_list. I know how to p/invoke classic varargs functions (with __arglist), and as it seemed to me that va_list works just like an ArgIterator, I thought it might be possible to just pass it to the method:

[DllImport("msvcrt.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern int vswprintf(StringBuilder str, int length, string format, ArgIterator args);

public static int vswprintf(StringBuilder str, int length, string format, __arglist)
{
    return vswprintf(str, length, format, new ArgIterator(__arglist));
}

int len = 100;
StringBuilder sb = new StringBuilder(len);
vswprintf(sb, len, "%s, %d", __arglist("test", 2));

It turns out that .NET actually recognizes this, because the following exception is thrown:

System.NotSupportedException
   in System.StubHelpers.StubHelpers.MarshalToUnmanagedVaListInternal(IntPtr va_list, UInt32 vaListSize, IntPtr pArgIterator)
   in Program.vswprintf(StringBuilder str, Int32 length, String format, ArgIterator args)
   in Program.vswprintf(StringBuilder str, Int32 length, String format)

So, apparently, .NET knows what I am trying to do and has something that should do it:

[MethodImpl(MethodImplOptions.InternalCall), ForceTokenStabilization]
internal static extern void MarshalToUnmanagedVaListInternal(IntPtr va_list, uint vaListSize, IntPtr pArgIterator); //Reflector

The names of the arguments hint at the purpose, but the exception confuses me. Do I do the p/invoke wrong, or is this .NET marshalling method not finished?

Edit:
After a bit of a research, I've found the implementation of this method and it seems it isn't possible to pass a normal string. So I've modified the code:

vswprintf(sb, len, "%d, %d", __arglist(1, 2));

But this throws an AccessViolationException (my favourite) instead! What's wrong now?

来源：https://stackoverflow.com/questions/29675018/marshalling-argiterator-to-va-list

标签

interop

variadic-functions