Excel 2010 SDK: Excel 2010 XLL Software Development Kit (SDK) exposes all relevant Excel 2010 functionality to XLL add-in developers, and introduces two new "power UDF" capabilities – Asynchronous user-defined functions (UDF), and HPC function remoting.
NOTE: Asynchronous UDFs are exactly what they sound: you can create a user-defined function (UDF) which starts some asynchronous process (such as sending out a request) and immediately returns. Excel tracks the pending result. When the result becomes available, the add-in sends it back to Excel through a call-back function. This lets you send out many external requests at the same time, to be concurrently run on external resources.
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