How do I determine the HResult for a System.IO.IOException?

The System.Exception.HResult property is protected. How can I peek inside an exception and get the HResult without resorting to reflection or other ugly hacks?

Here's the situation:
I want to write a backup tool, which opens and reads files on a system. I open the file with FileAccess.Read and FileShare.ReadWrite, according to this guidance, because I don't care if the file is open for writing at the time I read it.

In some cases, when a file I am reading is open by another app, the System.IO.FileStream.Read() method throws a System.IO.IOException, "The process cannot access the file because another process has locked a portion of the file". This is error 33, or I think HResult 0x80070021. [EDIT: I believe this can be returned when another process calls LockFileEx to lock a byte range within a file.]

I'd like to pause and retry when I get this error. I think this is the appropriate action to take here. If the locking process releases the byte-range lock quickly, then I can proceed reading the file.

How can I distinguish an IOException for this reason, from others? I can think of these ways:

• private reflection - don't wanna do that. Perf will stink.
• call Exception.ToString() and parse the string. Feels hacky. Won't work in i18n versions.

I don't like these options. Isn't there a better, cleaner way?

I just searched around and found System.Runtime.InteropServices.Marshal.GetHRForException. Will that return a uint like 0x80070021?

For .Net Framework 4.5 and above, you can use the Exception.HResult property:

int hr = ex.HResult;

For older versions, you can use Marshal.GetHRForException to get back the HResult, but this has significant side-effects and is not recommended:

int hr = Marshal.GetHRForException(ex);

For what it's worth, System.Exception.HResult is no longer protected in .NET 4.5 -- only the setter is protected. That doesn't help with code that might be compiled with more than one version of the framework.

You can also use the ISerializable interface:

static class IOExceptionExtensions
{
    public static int GetHResult(this IOException ex)
    {
        var info = new SerializationInfo(typeof (IOException), new FormatterConverter());
        ex.GetObjectData(info, new StreamingContext());
        return info.GetInt32("HResult");
    }
}