What exceptions should I expect from P/Invoke unsafe code?

Question

Hi skilled,

In my solution I've written the following:

[DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern unsafe bool CopyFileEx(string lpExistingFileName,  
 string lpNewFileName, CopyProgressRoutine lpProgressRoutine, IntPtr lpData,  
 Boolean* pbCancel,    CopyFileFlags dwCopyFlags);

...

bool result;

unsafe{
  result = CopyFileEx(sourceFile, targetFile, null, IntPtr.Zero,  
                      null /* pointer */, 0);
}

if(!result)
    Win32Exception exc = new Win32Exception(Marshal.GetLastWin32Error()); 
// parameter could be omitted according to Win32Exception constructor 
// implementation

In the assumption that CopyFileEx was exported with SetLastError = true parameter of DllImport attribute, do I have a chance to get any unhalted exception here?

I'm specifically interested in non-CLR exceptions which are wrapped in RuntimeWrappedException instance. In C++ "throw 1" is a valid construct. So what exceptions should I expect from such P/Invoke calls and where I can obtain such information regarding exceptions (MSDN says nothing regarding exceptions from CopyFileEx)?

Answer 1

If a DLL uses the Win32 structured exception handling system, then any exceptions thrown by the DLL will be translated to OutOfMemoryException, AccessViolationException, NullReferenceException (rarely), or SEHException.

RuntimeWrappedException is only used when managed C++ throws a non-Exception-derived object. It is not used for unmanaged DLLs. If you had a managed C++ DLL that did a throw 1, then you could catch RuntimeWrappedException.

SEHException acts as a catch-all for the unmappable types of SEH exceptions from unmanaged DLLs. This would include any C++ class, or the throw 1 example. AFAIK, there is no way to pull the C++-specific exception info out of SEHException, though I'm sure it's hackable if you try hard enough.

Note that there are a few DLLs that do not use the Win32 structured exception handling system. Most notably Cygwin DLLs. In that case, any C++ exception is likely to crash the process or at least your thread. Naturally, all the Windows DLLs (including the one defining CopyFileEx) do use Win32 structured exception handling, as do any unmanaged DLLs created by Microsoft's C++ compiler.

Also note that the Win32 API (including CopyFileEx) does not normally report errors using Win32 structured exceptions; they report them via GetLastError. The only way to get most Win32 functions to throw a Win32 structured exception is to pass it wildly invalid arguments (such as asking it to write into the middle of ntdll.dll).