Implementing Custom Functions

• May 03, 2018
• 2 minutes to read

This document explains how you can implement custom functions for CriteriaOperator-based criteria used in filter conditions and expressions supported in many DevExpress products (including grids, pivot grids, Reporting, etc.) in addition to built-in functions (see Expression Operators, Functions, and Constants and FunctionOperatorType).

Note

Custom functions are not available in the visual tree of Filter Controls (FilterControl and FilterEditorControl).

Declaring Custom Functions

A custom function is a class implementing the following interfaces.

• ICustomFunctionOperator - Declares the base functionality of a custom function.
• ICustomFunctionOperatorBrowsable - Provides additional information (a function category, function description, parameter count, etc.) on a custom function for Expression Editors. Implementing this interface exposes a custom function in Expression Editors, allowing end-users to use it in expressions along with built-in functions.
• ICustomFunctionOperatorFormattable - Defines server-side processing (implementation) of a custom function. Implementing this interface allows you to use a custom function to build query criteria in our ORM tool - XPO.

The ICustomFunctionOperatorFormattable and ICustomFunctionOperatorBrowsable interfaces inherit from the ICustomFunctionOperator interface, and extend it with a specific advanced functionality. Thus, you can implement ICustomFunctionOperatorFormattable or ICustomFunctionOperatorBrowsable (or both of them, depending on the capabilities you want to offer with your custom function) and as a result provide an ICustomFunctionOperator implementation.

Refer to the How to: Implement a Custom Criteria Language Operator for a sample ICustomFunctionOperatorFormattable interface implementation (a custom function that returns the month of a date).

Registering Custom Functions

Once a custom function is declared, you can register it in your application using any of the following methods.

• Pass a custom function to CriteriaOperator.RegisterCustomFunction or CriteriaOperator.RegisterCustomFunctions.

  These static methods significantly simplify the registration process. With a single method call, you make custom functions available wherever CriteriaOperator-based criteria are processed - whether it is an expression or a database query built with XPO.

• Pass a custom function to a data store provider’s RegisterCustomFunctionOperator method and add it to a dictionary’s XPDictionary.CustomFunctionOperators collection.

  This is an outdated registration technique, which exposes the custom function only to XPO queries based on dictionary entries.

To unregister a custom function, call the CriteriaOperator.UnregisterCustomFunction method.

To see a code example on implementing custom functions, refer to the How to: Implement a Custom Criteria Language Operator topic in the XPO documentation.

To learn how to implement custom functions and criteria, and use them in LINQ expressions built with XPO, see How to: Implement Custom Functions and Criteria in LINQ to XPO.