When you write an expression, you have access to many built-in fields, collections, and functions that you can use alone or combine with other terms. You can add function calls, operators, constants, fields, parameters, items from the built-in collections to an expression. Built-in function calls include aggregation, commonly used utility functions, and conversion methods. For more information, please refer to Expressions Reference chapter, where you can find all built-in functions, etc.
However, in some more specific scenarios like in your case, you can also add references to functions defined in your code. These functions are called user functions and allow you to extend the default behavior of the Telerik Reporting engine. Check the User Functions article to see how to utilize them.
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