I'd recommend posting on the nHapi mailing list rather than here.

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/- Sam Judson : Wrox Technical Editor -/

Think before you post: What have you tried?

I accept: if you mean HL7 v3 - this version is just XML, so obviously you can use .Net to create and edit HL7.
If you are using web endpoints then you can obviously use the HttpWebRequest classes to send and receive the XML.
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As other posters have said, NHAPI works great for parsing HL7 messages. For the communication part, sending HL7 messages is pretty easy. Most HL7 systems that send and receive HL7 messages by TCP/IP use the LLP protocol. This protocal is pretty easy, basically you just need to add a vertical tab character at the beginning of the message, and a field separator and carriage return at the end of the message. You can find a example of sending a HL7 message in C# at http://codingheadache.com/?p=10