Turning COM Errors and HRESULTs into .Net Exceptions

.NET will automatically convert an HRESULT into a specific type of Exception based on the HRESULT.  This is fine for many of the built-in HRESULTs closely related to Win32 API like function calls.  In my case though, I wanted to turn an application specific error that occurs in an ATL COM server (ATL3.0 no less!) into a pretty Exception so that managed code would get a useful error message.

To do this:

1. Create an empty error object via CreateErrorInfo( &pcerrInfo )
2. Set the description via pcerrInfo->SetDescription().  CreateErrorInfo allocates a pointer to an object that implements ICreateErrorInfo.  SetDescription() is a method defined on ICreateErrorInfo.  Whatever gets set here will be assigned to the .NET Exception.Message property.
3. call SetErrorInfo(0, perrInfo) where perrInfo is a pointer to IErrorInfo. 
1. To get perrInfo pointer, call pcerrInfo->QueryInterface(IID_IErrorInfo, (LPVOID*) &perrInfo);
   
2. SetErroInfo() sets the error object for the current thread (actually it clears the current error object then sets it to the new one).
4. after Release()-ing both pointers, return an error HRESULT (e.g., return E_FAIL).
1. Don't make any other COM calls before returning the HRESULT as these might overwrite the error object just set.
5. In .NET this will show up as a COMException where COMException.Message = IErrorInfo->GetDescription()

Since this will be used a lot, I put it into a single function that takes a string.  In practice, I immediately return an error HRESULT after calling the function.

void 
CSimpleTestObj::AppSetErrorInfo(LPWSTR pszErr) 

{ 

 ICreateErrorInfo *pcerrinfo; 

 IErrorInfo *perrinfo; 

 HRESULT hr; 

 hr = CreateErrorInfo(&pcerrinfo); // create generic error object 

 if 
(SUCCEEDED(hr)) 

 { 

 // set the 
text - in .NET this will map to Exception.Message 

 
pcerrinfo->SetDescription(pszErr); 

 // need to 
get IErrorInfo because this is what the .NET code will see 


 hr = 
pcerrinfo->QueryInterface(IID_IErrorInfo, (LPVOID FAR*) &perrinfo); 

 if 
(SUCCEEDED(hr)) 

 { 

 // SetErrorInfo sets the 
error object for the current thread 

 // .Net will take it, 
make an exception, 

 // then set 
ApplicationException.Message = IErrorInfo.GetDescription() 


 SetErrorInfo(0, perrinfo); 


 perrinfo->Release(); 

 } 

 pcerrinfo->Release(); 

 } 

}