title: Call custom function from excel in C# Program
body: <p>I have an excel addin (*.xla) that contains many complex functions that all take input parameters and return values.  I unfortunately cannot create a *.XLL as I don't have access to the *.xla file.</p>
  
  <p>I've managed to use C# to write the excel function into a cell (as a string), calculate the cell, and then retrieve the result back into C#.  This method works but it feels very ad hoc and inefficient.</p>
  
  <p>Given the situation described above, is there a better way for calling a custom excel function (that takes parameters) from C#? </p>