OnixS low-latency .NET FIX Engine for the Microsoft .NET Framework. This documentation includes extensive class library reference, conceptual overviews, step-by-step procedures, and information about reference implementation samples.
Other useful references:
.NET Framework FIX Engine Release Notes
OnixS Support JIRA - .NET Framework FIX Engine (JIRA User access permissions required)
.NET Framework FIX Engine downloads (access permissions required)