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POC# P/Invoke: Marshalling structures containing function pointers
 

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  1. POC# P/Invoke: Marshalling structures containing function pointers
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    <p>Sorry for the verbose introduction that follows. I need insight from someone knowing P/Invoke internals better than I do.</p> <p>Here is how I'm marshalling structures containing function pointers from C to C#. I would like to know whether it's the cleanest and/or most efficient way of doing it.</p> <p>I'm interfacing with a native DLL coded in C that provides the following entry point:</p> <pre><code>void* getInterface(int id); </code></pre> <p>You have to pass <code>getInterface(int)</code> one of the following enum values:</p> <pre><code>enum INTERFACES { FOO, BAR }; </code></pre> <p>Which returns a pointer to a structure containing function pointers like:</p> <pre><code>typedef struct IFOO { void (*method1)(void* self, int a, float b); void (*method2)(void* self, int a, float b, int c); } IFoo; </code></pre> <p>And here is how you use it in C:</p> <pre><code>IFoo* interface = (IFoo*)getInterface(FOO); interface-&gt;method1(obj, 0, 1.0f); // where obj is an instance of an object // implementing the IFoo interface. </code></pre> <p>In C# I have a <code>Library</code> class that maps the <code>getInterface(int)</code> entry point using P/Invoke.</p> <pre><code>class Library { [DllImport("MyDLL"), EntryPoint="getInterface", CallingConvention=CallingConvention.Cdecl)] public static extern IntPtr GetInterface(int id); }; </code></pre> <p>Then I defined:</p> <pre><code>struct IFoo { public M1 method1; public M2 method2; [UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate void M1(IntPtr self, int a, float b); [UnmanagedFunctionPointer(CallingConvention.Cdecl)] public delegate void M2(IntPtr self, int a, float b, int c); } </code></pre> <p>And I'm using it this way:</p> <pre><code>IntPtr address = Library.GetInterface((int)Interfaces.FOO); IFoo i = (IFoo)Marshal.PtrToStructure(address, typeof(IFoo)); i.method1(obj, 0, 1.0f): // where obj is an instance of an object // implementing the IFoo interface. </code></pre> <p>I have the following questions:</p> <ol> <li><p>Is mapping the whole structure less efficient than mapping a single pointer inside the structure using <code>Marshal.GetDelegateForFunctionPointer()</code>?</p> <p>Since I mostly don't need all the methods exposed by an interface, I can do (tested and works): </p> <pre><code>unsafe { IntPtr address = Library.GetInterface(id); IntPtr m2address = new IntPtr(((void**)address.toPointer())[1]); M2 method2 = (M2)Marshal.GetDelegateForFunctionPointer(m2address, typeof(M2)); method2(obj, 0, 1.0f, 1); } </code></pre></li> <li><p>When mapping the whole structure at once using <code>Marshal.PtrToStructure()</code>, is there a less verbose way than what I described? I mean less verbose than having to define the delegate types for every methods etc?</p></li> </ol> <hr> <p>EDIT: For the sake of clarity and completeness, in the code snippets above, <code>obj</code> is an instance obtained with the <code>void* createObject(int type)</code> entry point.</p> <hr> <p>EDIT2: One advantage of method 1) is that <code>Marshal.GetDelegateForFunctionPointer()</code> is only available starting from .NET Framework 2.0. However, <code>Marshal.PrtToStructure()</code> has always been available. That said, I'm not sure it's worth ensuring 1.0 compatibility nowadays.</p> <hr> <p>EDIT3: I tried to inspect the generated code using <a href="http://www.red-gate.com/products/reflector/" rel="noreferrer">Reflector</a> but it doesn't give much information since all the interesting details are done in helper functions like <code>PtrToStructureHelper</code> and are not exposed. Then, even if I could see what's done in the framework internals, then the runtime has the opportunity to optimize things away and I don't know exactly what, why and when :)</p> <p>However, I benchmarked the two approaches described in my question. The <code>Marshal.PtrToStructure()</code> approach was slower by a factor around 10% compared to the <code>Marshal.GetDelegateForFunctionPointer()</code> approach; that whith a structure containing <code>IntPtr</code>s for all the functions that are not of interest.</p> <p>I also compared the <code>Marshal.GetDelegateForFunctionPointer()</code> with my own rolled marshaller: I align a <code>struct</code> representing the call stack, pin it in memory, pass its address to the native side where I use a trampoline coded in asm so that the call function uses the memory area as its parameter stack (this is possible since the <code>cdecl</code> x86 calling convention passes all the function parameters on the stack). Timings were equivalent.</p>
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