What's New in the C API for Excel 2010
In conjunction with Microsoft Excel 2010, the Microsoft Excel 2010 XLL Software Development Kit (SDK) includes support for the following new features.
The Microsoft Excel 2010 XLL SDK supports calling back to all of Excel 2010's new worksheet functions. For more information about calling Excel 2010 functions, see Calling into Excel from the DLL or XLL.
Asynchronous User-defined Functions
Excel 2010 now supports calling user-defined functions (UDF) asynchronously, which can improve performance by enabling several calculations to run at the same time. For more information about asynchronous UDFs, see Asynchronous User-Defined Functions.
Cluster connectors enable UDFs to run on high-performance compute clusters. For more information about creating cluster connectors, see Developing Excel 2010 Cluster Connectors.
You can now compile and link both 32-bit and 64-bit XLLs. For more information, see Creating XLLs.