в Гугле Groups набираем в строке запроса

serial port C#

и читаем, читаем, читаем...

например

While MSComm is easy, it has some 'problems' (license)
http://www.devhood.com/tutorials/tutorial_details.aspx?tutorial_id=320

you could to use the Win32 API as described here:
http://msdn.microsoft.com/library/en-us/dnwbgen/html/msdn_serial.asp

and use PInvoke like in this C# sample:
http://www.gotdotnet.com/userarea/keywordsrch.aspx?keyword=SerialStream

or VB.NET serial RS232:
http://www.allapi.net/classlib/class.php?id=15

или вот целый FAQ

My Serial Port FAQ:

.NET 1.0/1.1 has no support for legacy ports (COM/LPT).

There are plans to add support for serial ports in a future version:
http://groups.google.com/groups?&selm=O%23vQLn9bCHA.392%40tkmsftngp09
"this sample is very similar to what we will be adding" :
http://www.gotdotnet.com/community/usersamples/Default.aspx?query=SerialPort
(note, project built with beta VS.NET, read gotdotnet comment)

With 1.0/1.1 you have to use PInvoke or Interop :

First understand the Win32 API as described here (C++):
http://msdn.microsoft.com/library/en-us/dnfiles/html/msdn_serial.asp

MSDN article for .NET (mostly C#):
http://msdn.microsoft.com/msdnmag/issues/02/10/NETSerialComm/

PInvoke samples for C#:
http://www.gotdotnet.com/community/usersamples/Default.aspx?query=SerialPort
http://msdn.microsoft.com/msdnmag/issues/02/10/NETSerialComm/
http://www.gotdotnet.com/community/usersamples/Default.aspx?query=SerialStream

or for VB.NET:
http://msdn.microsoft.com/library/en-us/dnvssamp/html/vbcs_usingthecomportinvbnet.asp
http://www.gotdotnet.com/community/usersamples/Default.aspx?query=rs232
http://www.allapi.net/classlib/class.php?id=15

or you can use the "Managed Extensions for C++" and write wrappers.
http://msdn.microsoft.com/vstudio/techinfo/articles/upgrade/managedext.asp
http://www.gotdotnet.com/team/cplusplus/
on your VS.NET path:
...\VC7\managedextensionsspec.doc
...\VC7\migration_guide.doc
MC++ Sample:
http://www.codeproject.com/managedcpp/howtocomport.asp

or reusing the VB6 MSComm ActiveX is easy, but it has some 'problems' (license)
http://support.microsoft.com/?kbid=318597
http://www.devhood.com/tutorials/tutorial_details.aspx?tutorial_id=320
http://ourworld.compuserve.com/homepages/richard_grier/NETCommOCX.htm

commercial:
http://www.sax.net/dotnet/communications/
http://www.winsoft.sk/ncomport.htm

for Compact Framework (Windows CE)
http://ourworld.compuserve.com/homepages/richard_grier/CFSerial.htm

for Interop, use newsgroup:
microsoft.public.dotnet.framework.interop
--
Thomas Scheidegger - MVP .NET - 'NETMaster'
http://www.cetus-links.org/oo_dotnet.html - http://dnetmaster.net/

авторИ неизвестно насколько они рабочие, сколько и какие в них подводные камни.
Ну так надо пробовать, никто за вас целиком вашу задачу не решит, тем более, что она очень нечётко сформулирована - поле для деятельности тут большое. Если нужна была конкретная помощь, то м.б. стоило указать прибор, с которым вы собирались "общаться" по последовательному порту, тогда и помощь была бы конкретнее.