Windows 10 declare COM port "access denied"

Question

Two existing EXEs, one written in C++ and Windows SDK, one written in C# and .NET 4.5.1, suddenly cannot open COM ports, "access denied". HyperTrm.exe has no problem opening the same ports that both EXE cannot open. Some recent update for Windows 10 has caused this problem. What is necessary to regain access to the COM ports?

Answer 1

Make sure that the port is not already open by another program that uses TAPI, such as a Fax program. Hyperterminal is TAPI-compliant, so it can use grab the port away from the other TAPI program. This would explain why Hyperterminal has no problem opening the port while your own program fails with "access denied".

Answer 2

Every online discussion of COM port Access Denied errors says the same thing - make sure the COM port is not in use by another program. Does Windows have a BUILT-IN way to "make sure" or do I have to look for a 3rd party tool?

Answer 3

Well, you can use netstat -an to see if the port appears in the list of ports in use.
Netstat has additional options that you can see by means of "netstat -?". If I remember correctly, one of these options lists which is the program that is using the port.