If I implement a COM interface in a C# class, which will be called from a C++ module, then in case of a failure the C# code should throw an exception; this will be marshalled to a HRESULT failure code by the interop layer.
But will any message text (or other information) passed to the Exception constructor also be available to the calling C++ code? If so, how can it be obtained i.e. which API methods?
e.g. if in my C# code I do throw new ArgumentException("The email address is invalid") I would except my COM HRESULT to be E_INVALIDARG but will my custom text be marshalled or can I only look up a text description of E_INVALIDARG?
It's little known that the IErrorInfo object (returned by GetErrorInfo) also implements the _Exception COM interface, which exposes the public members of the System.Exception class to unmanaged code, including StackTrace, etc.
To access _Exception from C++, you'd need to import a correct version of mscorlib.tlb, e.g.:
#import "C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscorlib.tlb" raw_interfaces_only