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In the SumC example, arguments are passed by value, but native code often requires data structures to be passed by reference to avoid the cost of copying the entire structure and passing only a pointer to the native data. The ZeroC function resets a complex number whose pointer is passed as an argument: void CINTEROPDLL_API ZeroC(Complex* c) { c->re = 0; c->im = 0; } The F# declaration for the function is the same as the C prototype: [<DllImport("CInteropDLL")>] extern void ZeroC(Complex* c) Now you need a way to obtain a pointer given a value of type Complex in F#. You can use the && operator that is used to indicate a pass by reference and that results in passing the pointer to the structure expected by the C function: let mutable c4 = CInterop.SumC(c1, c2) printf "c4 = SumC(c1, c2) = %f + %fi\n" c4.re c4.im CInterop.ZeroC(&&c4) printf "c4 = %f + %fi\n" c4.re c4.im In C and C++, the notion of objects (or struct instances) and the classes of memory are orthogonal: an object can be allocated on the stack or on the heap and share the same declaration. In .NET, this is not the case; objects are instances of classes and are allocated on the heap, and value types are stored in the stack or wrapped into objects in the heap. Is it possible to pass objects to native functions through PInvoke The main issue with objects is that the heap is managed by the garbage collector, and one possible strategy for garbage collection is copy collection, which is a technique that moves objects in the heap when a collection

Double-click it to launch the LDAP Schema Analyzer tool. 2. In the schema analyzer, choose File->Load Target Schema. This allows us to load the schema from another LDAP server. In this case, we re going to point it to our server running Open Directory. (If you don t have an Open Directory Server anywhere, you can download our Mac OS X Server 10.6.3 Schema from http://mcxbook.com and choose Load LDIF in the Load Target Schema dialog. Really, though, it s best to actually perform this step.)

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occurs Thus, the base address in memory of an object may change over time, which can be a serious problem if the pointer to the object has been marshalled to a native function through a PInvoke invocation The CLR provides an operation called pinning that allows pinning an object and preventing it from moving during a garbage collection Pinned pointers are assigned to local variables, and pinning is released when the function performing the pinning exits It is important to understand the scope of pinning, since if the native code stores the pointer somewhere before returning, the pointer may become invalid but still usable from native code Now let s define an object type for Complex and marshal F# objects to a C function The goal is to marshal the F# object to the ZeroC function.

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In this case, you cannot use the pass-byreference operator, and you must define everything so that the type checker is happy with it You can define another function that refers to ZeroC but with a different signature involving ObjComplex, which is an object type similar to the Complex value type The EntryPoint parameter maps the F# function onto the same ZeroC C function, though in this case the argument is of type ObjComplex rather than Complex: module CInterop = [<StructLayout(LayoutKindSequential)>] type ObjComplex = val mutable re:double val mutable im:double new() as x = { re = 00; im = 00 } new(r:double, i:double) as x = { re = r; im = i } [<DllImport("CInteropDLL", EntryPoint="ZeroC")>] extern void ObjZeroC(ObjComplex c) let oc = CInteropObjComplex(20, 10) printf "oc = %f + %fi\n" ocre ocim CInteropObjZeroC(oc) printf "oc = %f + %fi\n" oc.

re ocim In this case, the object reference is marshalled as a pointer to the C code, and you don t need the && operator in order to call the function; the object is pinned to ensure that it does not move during the function call..

Fill in the IP address of the Open Directory Server (Figure 6-5). Leave the Username and Password fields empty and ensure that the Bind type parameter is set to Simple.

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Platform Invoke defines default behavior for mapping common types used by the Win32 API; Table 17-1 shows the default conversions. Most of the mappings are natural, but it is important to note that there are several entries for strings. This is because strings are represented in different ways in programming language runtimes. To show how strings are marshalled, we start with a simple C function that echoes a string on the console: void CINTEROPDLL_API echo(char* str) { puts(str); }

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Click OK and the utility will import the schema from your Open Directory server. The main window will populate with Classes, Attributes, and Property Sets containers, as shown in Figure 6-6.

unsigned char short unsigned short int unsigned int long long unsigned long unsigned long char char* const char* wchar_t* const wchar_t* Float Double

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