Simple harness to print exceptions in an app

Several people have asked how to write something that runs some executable under a harness and then dumps all the exceptions that are thrown.
Back in November, I wrote  a similar harness to dump load module events using MDbg.  You can easily modify that to dump exceptions. See that blog entry for an explanation of how the code below works.

Here's sample code for a harness to print exceptions.

// Simple harness to dump exceptions.
// Be sure to reference MdbgCore.dll (which ships in the Whidbey beta 2 SDK)
using System;
using Microsoft.Samples.Debugging.MdbgEngine;

class Program
    static void Main(string[] args)
        if (args == null || args.Length != 1)
            Console.WriteLine("Usage: PrintEx <filename>");
            Console.WriteLine("   Will run <filename> and print all exceptions.");
        Console.WriteLine("Run '{0}' and print all exceptions.", args[0]);

        MDbgEngine debugger = new MDbgEngine();
        debugger.Options.CreateProcessWithNewConsole = true;

        // Specify which debug events we want to receive.
        // The underlying ICorDebug API will stop on all debug events.
        // The MDbgProcess object implements all of these callbacks, but only stops on a set of them
        // based off the Options settings.
        // See CorProcess.DispatchEvent and MDbgProcess.InitDebuggerCallbacks for more details.
        debugger.Options.StopOnException = true;
        //Do 'debugger.Options.StopOnExceptionEnhanced = true;' to get additional exception notifications.
        // Launch the debuggee.
        MDbgProcess proc = debugger.CreateProcess(args[0], "", DebugModeFlag.Default, null);
            // Let the debuggee run and wait until it hits a debug event.
            object o = proc.StopReason;

            // Process is now stopped. proc.StopReason tells us why we stopped.
            // The process is also safe for inspection.           
            ExceptionThrownStopReason m = o as ExceptionThrownStopReason;
            if (m != null)
                    MDbgThread t = proc.Threads.Active;
                    MDbgValue ex = t.CurrentException;
                    MDbgFrame f = t.CurrentFrame;
                    Console.WriteLine("Exception is thrown:" + ex.TypeName + "(" + m.EventType +
                        ") at function " + f.Function.FullName + " in source file:"
                         + t.CurrentSourcePosition.Path + ":" + t.CurrentSourcePosition.Line);
                catch(Exception e   )
                    Console.WriteLine("Exception is thrown, but can't inspect it.");

} // end class

So compile the above (making sure you include the reference to mdbgcore.dll) and then run it on an app like:

using System;

class Foo
static void Main()
try {
throw new Exception("Bong!"); // line 11

throw new Exception("Bong#2"); // line 15


And it prints:
Run 'c:\temp\t.exe' and print all exceptions.
Exception is thrown:System.Exception(DEBUG_EXCEPTION_FIRST_CHANCE) at function Foo.Main in source file:c:\temp\t.cs:11
Exception is thrown:System.Exception(DEBUG_EXCEPTION_FIRST_CHANCE) at function Foo.Main in source file:c:\temp\t.cs:15
Exception is thrown:System.Exception(DEBUG_EXCEPTION_UNHANDLED) at function Foo.Main in source file:c:\temp\t.cs:15

You could certainly make additional tweaks like:
1) printing the whole callstack instead of just the leaf-most method.
2) printing more of the exception (such as the message string, nested exceptions, or other fields)
3) printing other exception notifications (such as catch-handler-found or unwind notifications)
4) or support for attaching to an existing app.

Comments (9)

  1. Sunish Abraham says:

    I saw that your examples using MDbg (which exposes the ICorDebug APIs)…but it needs framework 2.0. Is there anyway to get MDbg and hence your examples to work with framework 1.1?

  2. Unfortunately, MDbg only works on V2.0. See here for details:

    However, the source for Cordbg (a pure native app) appears in the v1.0, v1.1 SDKs. You could canabalize that to produce a similar sort of harness. In particular, the debug event callbacks are implemented in Dshell.cpp. Have the exception callback print out stuff (by calling functions in Commands.cpp) and have all other callbacks just continue.

  3. akraus1 says:

    How can you get the managed and unmanaged call stack at runtime from an application which throws an unmanaged exception? The StalkWalk64 method does not work anymore when you want to get the native call stack. This way you can retrieve the call stack of an unmanaged app at runtime without any pdb in C++. Is there a similar way to get at least the unmanaged call stack in C++ with the CLR running?

  4. I’ve given sample code for MDbg-based harnesses that launch an app and then print all loaded modules…

  5. akraus1 says:

    I know it can be done with the debugging API. But what about a "self" debugging application. E.g when I throw an unmanaged exception I want the full call stack for my process as fast as possible. For speedy stack walking you can use e.g But this approach does not work anymore when the CLR is hosted in this process. Is there a similar performant way to get the call stack of your own process in the managed world?

  6. Akraus1 – great questions. I’ll write up some blog entries to answer each. Stay tuned…

  7. A vectored exception handler (see kernel32!AddVectoredExceptionHandler) lets you add to a global list…

  8. I often publish little samples in this blog based off MDbg (which generally show off the debugging services…

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