Using InteropServices in a WebService

[Russ]

I'm trying to use InteropServices in a web service, by following the
examples and info in the article "Consuming Unmanaged DLL Functions"
in the .NET Framework Developer's Guide.

I am able to add prototypes for unmanaged code and compile and link
the C++ managed code web service (based on the TempConvert3
walkthrough). I added web methods to try several calls to unmanaged
DLL's, but none of them work. Here is what I did:

in the web service header file, added the following, right after the
other using statements:

using namespace System::Runtime::InteropServices;
[DllImport ("DLib.dll")]
extern "C" bool IsValidCheckDigit (String* pDfi);
[DllImport ("DLib.dll")]
extern "C" String* GetComputerName ();

Then in the Service class (TempConvert3Class), added the following
member function declarations:

[System::Web::Services::WebMethod(
Description="Check a check digit.")]
bool CheckDigit (String* dfi);

[System::Web::Services::WebMethod(
Description="Get Computer Name.")]
String* GetName ();
Finally, in the cpp file, added the member functions:

bool TempConvert3Class::CheckDigit (String* dfi) {
bool bOk = IsValidCheckDigit (dfi);
return bOk;
}

String* TempConvert3Class::GetName () {
return GetComputerName ();
}
When I run the project, it still works properly when I invoke the
orignial temperature conversion method. The new methods are shown in
the web page, but when I invoke either of them I get a page that says:

The page cannot be displayed

The address of this page is:
http://localhost/TempConvert3/TempConvert3.asmx/GetName

There is no specific error or reason for the error shown.

The prototype for the two functions (in DLib.dll) are:

extern DllExport bool IsValidCheckDigit (const char* pDfi);
extern DllExport CString GetComputerName ();

Anyone know what is wrong?

Thanks, Russ