SerialPortStream: Don't let IsOpen throw a NullReferenceException

Even though this commit protects the property IsOpen from throwing a NullReferenceException, other method calls are not protected for thread safety. Do not use any API except IsOpen on the class after it is Closed.

Issue: DOTNET-188
Issue: #90

CHANGES.md

@@ -7,6 +7,8 @@ Bugfixes
 * DOTNET-180: Allow compilation of libnserial on Ubuntu 16.04.5.
 * [Issue #104](https://github.com/jcurl/SerialPortStream/issues/104): Fix buffer
   handling. `Write()` would sometimes corrupt data.
+* [Issue #90](https://github.com/jcurl/SerialPortStream/issues/90): `IsOpen()`
+  might return `NullReferenceException` as it's not threadsafe with `Close()`.
 
 Features:
 

code/Native/WinNativeSerial.cs

@@ -639,7 +639,8 @@ public bool IsOpen
             get
             {
                 if (m_IsDisposed) return false;
-                return m_ComPortHandle != null && !m_ComPortHandle.IsClosed && !m_ComPortHandle.IsInvalid;
+                SafeFileHandle handle = m_ComPortHandle;
+                return handle != null && !handle.IsClosed && !handle.IsInvalid;
             }
         }
 
@@ -887,12 +888,13 @@ public void Close()
         {
             if (m_IsDisposed) throw new ObjectDisposedException("WinNativeSerial");
             if (IsOpen) {
+                SafeFileHandle handle = m_ComPortHandle;
+                m_ComPortHandle = null;
                 m_CommOverlappedIo.Dispose();
                 m_CommOverlappedIo = null;
                 m_CommState = null;
                 m_CommModemStatus = null;
-                m_ComPortHandle.Dispose();
-                m_ComPortHandle = null;
+                handle.Dispose();
             }
     }
 

code/SerialPortStream.cs

@@ -1,4 +1,4 @@
-// Copyright © Jason Curl 2012-2016
+// Copyright © Jason Curl 2012-2020
 // Sources at https://github.com/jcurl/SerialPortStream
 // Licensed under the Microsoft Public License (Ms-PL)