public virtual abstract void Evaluate( 
   IArguments arguments,
   IValue result
)

Parameters

arguments

Provides access to the arguments to the custom function.

result

Assign the result of the custom function to result.

The implementation of this method must be thread safe, and must not use any API in the workbook set which is being calculated. All access to cells must be through arguments. No references to objects passed to, or acquired during the execution of this method should be used after this method completes execution.

If an exception is thrown during custom function evaluation, the custom function will return ValueError.Name.

Reference