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CHAPTER 12 INTEROPERABILITY
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If you use __declspec(dllexport) on a managed function, which causes it to be made available to callers outside the DLL, you create a native entry point that can be used by native callers that can then use __declspec(dllimport) to reference the function.
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How to Avoid Double Thunking
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You should be cognizant of the subtleties involved in native and managed entry points, because of the concern that you could make unnecessary context switches between native and managed code, if you aren t careful to avoid it. The scenario of concern is calling a managed function via a native entry point from managed code, rather than the far more efficient route of calling the managed entry point directly. The existence of the native entry point makes it a possible route that your managed code can use to access the function. If you call a function in this way, you have to first switch to native code to access the native entry point, which is a small piece of code called a thunk, and then you have to switch to managed code to call the managed function, another thunk. All this unnecessary switching between contexts is known as double thunking and can really slow down an application. The compiler will try to get the right entry point; however, it sometimes needs some help. Using the __clrcall calling convention is one way to avoid the problem. You can use the __clrcall calling convention on a managed function as long as you don t need to call your function from native code. If there s no native entry point, then there s no way it can be used improperly. The __clrcall calling convention is only needed when compiling in mixed mode (with /clr), because in pure mode and safe mode, the default is __clrcall and no native entry point is generated anyway. In the other situation where double thunking can occur (exporting a managed function from a DLL), you should avoid using __declspec(dllexport) and __declspec(dllimport) to invoke managed functions from managed code. Instead, you should use #using to reference the managed assembly.
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Managed and Native Exceptions
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You might be wondering, if you have managed code calling into native code, how do errors and exceptions get propagated from native code to managed code In this section you explore this.
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Interop with Structured Exceptions (__try/__except)
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Structured Exception Handling (SEH) is used on the Windows platform in C and C++ for many hardware and software error conditions. Possible error codes are listed in the Windows headers. If an SEH exception is allowed to propagate into managed code, it is wrapped as a .NET exception of some type. Many structured exceptions are mapped to specific .NET exception types. For example, EXCEPTION_INT_DIVIDE_BY_ZERO is mapped to DivideByZeroException. If there is no specific mapping, a System::Runtime::InteropServices::SEHException is generated. In Listing 12-22, two ways of handling structured exceptions are demonstrated. The exception in native code is an integer division by zero. In the first branch, the exception is allowed to propagate to managed code and is caught as an SEHException. In the second branch, it is caught as a native SEH exception in a __try/__catch statement.
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CHAPTER 12 INTEROPERABILITY
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Listing 12-22. Handling Structured Exceptions // try_except.cpp #include <stdio.h> #include <windows.h> // for EXCEPTION_INT_DIVIDE_BY_ZERO #include <excpt.h> using namespace System; using namespace System::Runtime::InteropServices; #pragma unmanaged void generate_SEH_exception() { int i = 0; // Divide by zero generates an SEH exception. int x = 2 / i; } void generate_AV() { int *pn = 0; int n = *pn; // generates an access violation } int filter_div0(unsigned int code, struct _EXCEPTION_POINTERS *ep) { if (code == EXCEPTION_INT_DIVIDE_BY_ZERO) { return EXCEPTION_EXECUTE_HANDLER; } else { return EXCEPTION_CONTINUE_SEARCH; }; } // This must be a native function because __try/__except is not // allowed in the same function as code that uses try/catch. void try_except(bool bThrowUnhandledAV) { __try { if (bThrowUnhandledAV) generate_AV(); else generate_SEH_exception(); }
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