I started working on this last year, and frankly found the task to be beyond me. Given the constraints imposed by my employer, I ended up writing only what I needed in C++.

My regrets. If you could sketch what you need (besides the low-level "Add a field" and "Determine the message check-sum") I'd be happy to give it another try.

• Does one include tag-validation? (My employer required custom-tags for his messages.)
• Does the user provide his own transport-mechanism, or do you need the FIX library to send and recieve messages?
• Support for all versions?

In the meantime, unless you're writing both sides of the FIX connection, whoever you're worjking with should have a QA environment you can test with. (NASDAQ, for example, helps you test a new FIX connection.)