# Extending Expressions Overview

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 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. This can be done through the [assemblyReferences element](https://www.telerik.com/products/reporting/documentation/doc-output/configure-the-report-engine/assemblyreferences-element) in the application's configuration file.

```XML
<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>
```

```JSON
"telerikReporting": {
    "assemblyReferences": [
        {
            "name": "MyCustomAssembly",
            "version": "1.0.0.0"
        }
    ]
}
```

## See Also

* [Create User Functions](https://www.telerik.com/products/reporting/documentation/designing-reports/connecting-to-data/expressions/extending-expressions/user-functions)
* [Create User Aggregate Functions](https://www.telerik.com/products/reporting/documentation/designing-reports/connecting-to-data/expressions/extending-expressions/user-aggregate-functions)