Expressions can easily be extended by providing user functions written in any .NET language. This is very useful in addition to the built-in functions, providing more complex domain-specific calculations or for uniform calculations because it speeds up the processing and boosts the performance. Furthermore, you can add custom aggregate functions to apply domain-specific logic while accumulating values.

In order to expose your custom functions, so they can be utilized by the Reporting engine, you have to add a reference to the assembly they are defined in. The reference is added under the AssemblyReferences configuration section in the application's configuration file.

<configuration>
<configSections>
  <section
      name="Telerik.Reporting"
      type="Telerik.Reporting.Configuration.ReportingConfigurationSection, Telerik.Reporting"
      allowLocation="true"
      allowDefinition="Everywhere"/>
  </configSections>
  …
   <Telerik.Reporting>
  <AssemblyReferences>
    <add name="MyCustomAssembly" version="1.0.0.0" />
  </AssemblyReferences>
 </Telerik.Reporting>
</configuration>