I am adding a "plug-in" mechanism to an existing C# application.  The
plug-ins are to be customer-written COM objects, dynamically loaded by my C#
host code.  I am able to load and talk to a few different sample plugins
knowing only the ProgID.  I made a sample C# plugin and a sample VB6 plugin
and a sample C++/ATL (C7) plug in so far and they all work fine except for
one glitch.  It would be nice for the methods in the plug-in to be able to
throw exceptions (including nice descriptive text) back to the C# caller when
warranted.  I can throw exceptions from the C# and VB6 plugins just fine, and
the exception message text is accessible in my C# catch block.  However, when
I throw a char* exception from the C++/ATL plugin, I can't seem to access the
exception text.   As an experiment I enabled managed extensions in the the
C++/ATL COM project and threw a System::Exception.  I can catch that in the
C# code and see the exception message text OK.  I am not sure if I can get
away with telling my customers that they must use managed C++ if they want to
use C++ to make a plugin for our program.  It would be nice if I could get
exception message text to work from unmanaged C++ back to C#.  Should that be
possible?  When caught, it looks like a SEHException when I use unmanaged
C++.  When I change to managed C++, it looks like a System.Exception wrapped
in a TargetInvocationException.

The C# catch block:

catch(Exception x)
{
    string z = "Error calling CapExpSendDocumentBuffer";
    z += "\r\n\r\n";
    z = z + x.InnerException.Message;
    z += "\r\n\r\n";
    z += x.InnerException.StackTrace;
    this.label1.Text = z;
    err = true;
}

The unmanaged C++ exception throw:
throw "this is a test of the emergency broadcast system";