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CHAPTER 18 MANAGED AND UNMANAGED CODE INTEROPERATION
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// Obj = Activator::CreateInstance(Typ); ldloc Typ call instance object [mscorlib]System.Activator::CreateInstance( class [mscorlib]System.Type) stloc Obj ... // Ret = (int)Typ->InvokeMember("Baz",BindingFlags::InvokeMethod, // NULL,Obj,NULL); ldloc Typ ldstr "Baz" ldc.i4 0x100 // System.Reflection.BindingFlags::InvokeMethod ldnull // Reflection.Binder don't need it ldloc Obj ldnull // Parameter array don't need it call instance object [mscorlib]System.Type::InvokeMember(string, valuetype [mscorlib]System.Reflection.BindingFlags, class [mscorlib]System.Reflection.Binder, object, object[]) unbox valuetype [mscorlib]System.Int32 stloc Ret ... An RCW converts the HRESULT returns of COM methods to managed exceptions. The only problem with this is that the RCW throws exceptions only for failing HRESULT values, so subtleties such as S_FALSE go unnoticed. The only way to deal with this situation is to set the implementation flag preservesig on the methods that might return S_FALSE and forgo the automated HRESULT to exception transformation. Another problem arises when the COM method has a variable-length array as one parameter and the array length as another. The type library carries no information about which parameter is the length, and the runtime is thus unable to marshal the array correctly. In this case, the signature of the method must be modified to include explicit marshaling information. Yet another problem requiring manual intervention involves unions with overlapped reference types. Perfectly legal in the unmanaged world, such unions are outlawed in managed code. Therefore, these unions are converted into value types with .pack and .size parameters specified but without the member fields. The manual intervention mentioned usually involves disassembling the interop assembly, editing the text, and reassembling it. Since the interop assemblies don t contain embedded native code, this operation can easily be performed.
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All thunks and wrappers provide data conversions between managed and unmanaged data types, which is referred to as marshaling. Marshaling information is kept in the FieldMarshal metadata table, which is described in 9. The marshaling information can be associated with Field and Param metadata records.
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CHAPTER 18 MANAGED AND UNMANAGED CODE INTEROPERATION
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One significant subset of managed data types directly corresponds to unmanaged types, requiring no data conversion across managed and unmanaged code boundaries. These types, which are referred to as blittable, include pointers (not references), function pointers, signed and unsigned integer types, and floating-point types. Formatted value types (the value types having sequential or explicit class layout) that contain only blittable elements are also blittable. The nonblittable managed data types that might require conversion during marshaling because of different or ambiguous unmanaged representation are as follows: bool (1-byte, true = 1, false = 0) can be converted either to native type bool (4-byte, true = 1, false = 0) or to variant bool (2-byte, true = 0xFFFF, false = 0). char (Unicode character, unsigned 2-byte integer) can be converted either to int8 (an ANSI character) or to unsigned int16 (a Unicode character). string (class System.String) can be converted either to an ANSI or a Unicode zeroterminated string (an array of characters) or to bstr (a Unicode Visual Basic style string). object (class System.Object) can be converted either to a structure or to a COM interface (CCW/RCW) pointer. class can be converted either to a COM interface pointer or, if the class is a delegate, to a function pointer. valuetype (nonblittable) is converted to a structure with a fixed layout. An array and a vector can be converted to a safe array or a C-style array. The references (managed pointers) are marshaled as unmanaged pointers. The managed objects and interfaces are references in principle, so they are marshaled as unmanaged pointers as well. Consequently, references to the objects and interfaces (class IFoo&) are marshaled as double pointers (IFoo**). All object references passed to the unmanaged code must be pinned; otherwise, the GC subsystem might move them during the call to an unmanaged method.
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