by Craig Thomas
Excel Services UDFs represent another tier in the Excel Services capability to support custom solutions. UDFs provide a means to call custom-managed code functions from within a workbook. Without UDFs, a workbook is restricted to using only the intrinsic Excel functions. With UDFs, custom functions can be called from the workbook as well.
UDFs are very instrumental to custom server solutions because Excel Services does not support loading workbooks that contain code behind (VBA). Nor does Excel Services support the Excel client add-ins used by workbooks to extend functionality. With the right UDF-managed wrapper solution, you could leverage existing custom client solutions on the server, but that topic isn't discussed here.
This article describes how to author manage UDFs and deploy them to the server to make them available to workbooks that are loaded from the trusted file locations. An example solution is provided to demonstrate the authoring and building of a UDF assembly. Additional material is provided to explain how the workbook interacts with UDF methods, and how to pass and return arguments to the workbook.
