How to handle WinRT exceptions that result in Exception?

Question

If a Windows runtime type raises a COM error .NET seems to wrap this error often (or always?) just into an Exception instance. The error message includes the COM HR

Answer 1

It is possible to catch Exception, handle particular errors by switching on the HRESULT, and re-throwing the Exception if the error was "unexpected." For example,

try
{
    // ...
}
catch (Exception ex)
{
    switch (ex->HResult)
    {
    case E_INVALID_USER_BUFFER: // 0x800706f8
        // handle invalid buffer case...
        break;
    default:
        // Unexpected exception; re-throw:
        throw;
    }
}

(I would note that providing an invalid buffer sounds more like a logic error than a runtime error, so I wonder whether this particular exception should really be caught.)

Alternatively, a more general solution would be to write a function or set of functions that handle Exception for known HRESULTs and re-throws a more specific exception. For example,

static void HandleKnownExceptions(Action f)
{
    try
    {
        f();
    }
    catch (Exception ex)
    {
        // Detect expected HRESULTs and throw the more-specific exception
        // type for each.
    }
}

Both of these approaches work equally well in both C++ and C#.

Note that it isn't necessarily the case that Exception is thrown directly by the platform or other components. At the Windows Runtime ABI layer, there are no exceptions: all errors are reported across the ABI boundary by HRESULT. The CLR translates a handful of known HRESULTs to more specific exception types, but it cannot perform a general translation.