How to implement a Distinct Count Aggregation Function that supports the Server Mode data processing

This example demonstrates how to define and register the Distinct Count function that allows calculating the count of unique field values if the Server Mode is enabled. This concept is described in the Implementing Custom Functions help topic. A custom function class should implement the following interfaces:
ICustomFunctionOperator 
ICustomFunctionOperatorFormattable 
IQueryableConvertible

To register a custom function in your application, use the CriteriaOperator.RegisterCustomFunction method.