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How to force CLR not to handle Exceptions from unmanaged code ?

Posted on 2008-10-27
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Last Modified: 2013-12-17
Hi experts...
I have managed code in C# calling unmanaged code from a DLL written in C++.
Every time an Exception is thrown by unmanaged code, this exception is handled by C# directly.
How to force to handle the exception by the DLL itself, instead of C# ?

example attached below.
   
//C# code
[DllImport("some.dll")]
    public static extern int A();
static int Main()
{
    try {
        A();
    } catch {
        // handle exception by managed code
    }
}
 
// C++ code
extern "C" __declspec(dllexport) int A();
int B(void);
 
int A()
{
    try {
        B();
    } catch (...) {
        // handle exception by unamanged code
        // how to handle the exeption here ???
    }
}
 
int B()
{
    // throw some exception
    throw;
}

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Question by:darkriser
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8 Comments
 
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Expert Comment

by:Jens Fiederer
ID: 22814686
Are you able to rewrite the DLL?

If so, make sure you catch the error.

If not, you could avoid calling the DLL from C#, but only from ANOTHER unmanaged DLL that calls it, catching all errors.

Obviously, you aren't going to change the DLL itself without changing the DLL.
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Author Comment

by:darkriser
ID: 22819880
Yes, of course I can rewrite the DLL. But how to achieve my requirements?

The problem is, that as soon as the C# code detects some exception inside my unmanaged DLL, it immediately skips the catch block inside the DLL and starts the C# catch block. Therefore I'm unable to handle exceptions inside my unmanaged DLL.
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Expert Comment

by:Jens Fiederer
ID: 22821385
Maybe you should be throwing SOMETHING instead of a naked throw?

When I use your code as written, I crash with "application has asked the runtime to terminate it in an unusual way" .

But when I throw (for example) an int,  I get a usable function.

See snippet.
C++
 
extern "C" __declspec(dllexport) int A();
int B(int);
 
int A(int parm)
{
    try {
		if (parm == 1) 
		{
			return B(parm);
		}
    } catch (int exc) {
        return exc;
	} catch (...) {
		return 100;
	}
 
	return parm*2;
}
 
int B(int parm)
{
    // throw some exception
    throw parm*3;
}
 
# C# test program
namespace usemanaged
{
    
    class Program
    {
        [DllImport("unmanageddll.dll", EntryPoint = "A",
        ExactSpelling = false, CallingConvention = CallingConvention.Cdecl)]
        static extern int A(int parm);
 
        static void Main(string[] args)
        {
            while (true)
            {
                int parm = int.Parse(Console.ReadLine());
 
                int z = A(parm);
                Console.WriteLine(z.ToString());
            }
        }
    }
}

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Expert Comment

by:Jens Fiederer
ID: 22821424
'a naked call to "throw;" will terminate the program if not invoked during the execution of a catch-handler.'

according to
http://www.velocityreviews.com/forums/t282888-abort-instead-of-throwing-exceptions.html
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Author Comment

by:darkriser
ID: 22822044
That's a good point.
But my problem is a little bit different (haven't mentioned this in my orig post, my fault).
What if unmanaged DLL fails in accessing dynamically allocated memory (trying to access element outside array's boundary)? Then I receive following error from C# directly and catch(...) block inside the DLL is ignored. How to catch this type of error within the DLL?

An unhandled exception of type 'System.AccessViolationException' occurred in Test.exe

Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
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Accepted Solution

by:
Jens Fiederer earned 250 total points
ID: 22822601
If you are using Visual Studio to compile your C++ dll, there is a project option under C/C++ code generation called "Enable C++ Exceptions".

When I had it set to "Yes (/EHsc)" my C++ did not catch the exception.
But when I had it set to "Yes With SEH Exceptions (/EHa)" my C++ DID catch the exception in the unmanaged code.
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Author Closing Comment

by:darkriser
ID: 31510359
I have missed this option because it is written in very small font :-)
No, seriously, this is what I've been looking for.
Thanks many times.
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LVL 23

Expert Comment

by:Jens Fiederer
ID: 22823594
Glad to help, that was a fun one.
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