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CHAPTER 13 INTEROPERABILITY
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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 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 13-24, 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/__except statement. If you use structured exceptions, you ll recall that because they are employed in both C and C++, and C doesn t support the try/catch statement, these exceptions are used with the __try/__except statement rather than the try/ catch statement of C++ exceptions. Listing 13-24. 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) {
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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(); } __except( filter_div0(GetExceptionCode(), GetExceptionInformation())) { printf_s("Divide by zero exception caught via SEH __except block."); } } #pragma managed int main(array<String^>^ args) { if (args->Length < 1) { Console::WriteLine("Usage: try_except [NET|SEH|AV]"); return -1; } if (args[0] == "NET") // Demonstrate catching SEH as a .NET Exception { try { generate_SEH_exception(); } catch(DivideByZeroException^ e) { Console::WriteLine(e->ToString()); } }
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else if (args[0] == "SEH") // Demonstrate handling SEH exception natively. { // Call native function with try/except block // and filter out division by zero exceptions. try_except(false); } else if (args[0] == "AV") // Demonstrate filtering of what exceptions to handle // natively and what to allow through. { try { // AVs, however, are not filtered and are allowed // to propagate to managed code. try_except(true); } catch(AccessViolationException^ e) { Console::WriteLine(e->ToString()); } } } The output of Listing 13-22 with the command line try_except NET is System.DivideByZeroException: Attempted to divide by zero. at generate_SEH_exception() at main(String[] args The output with the command line try_except SEH is
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Divide by zero exception caught via SEH __except block.
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And the output with the command line try_except AV is System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt. at try_except(Boolean ) at main(String[] args) Note that you cannot include CLR exception handling in the same function as structured exception handling, since the two mechanisms are not compatible and, if used together, would corrupt the stack. However, as the version with the access violation demonstrates, you can filter on what exceptions you want SEH to handle and handle others in managed code.
CHAPTER 13 INTEROPERABILITY
Interop with Win32 Error Codes
You cannot simply call GetLastError via P/Invoke after a Windows API call and expect to get the error code corresponding to the Win32 function invoked on the last P/Invoke call, because there is no guarantee that between the function call of interest and the call to GetLastError any error value is preserved. The proper way to get at the error code is to call Marshal::GetLastWin32Error, as shown in Listing 13-25. Listing 13-25. Handling Win32 Error Codes // getlasterror.cpp #using "System.dll" using namespace System; using namespace System::ComponentModel; // for Win32Exception using namespace System::Runtime::InteropServices; [DllImport("kernel32.dll", SetLastError=true)] extern bool SetVolumeLabel(String^ lpRootPathName, String^ lpVolumeName);
bool TestGetLastWin32Error() { if (SetVolumeLabel("BAD:\\", "VolumeName")) { System::Console::WriteLine("Success!"); return true; } else { throw gcnew Win32Exception(Marshal::GetLastWin32Error()); } return false; } int main() { try { TestGetLastWin32Error(); } catch(Win32Exception^ e) { Console::WriteLine(e->ToString()); } }
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