# Previewing a report definition that uses an external assembly
## Environment
Product
Progress® Telerik® Reporting
## Description
Previewing a report definition containing expressions with user-defined functions or other custom CLR objects coming from an external assembly. The application has to be configured to load the external assembly.
## Solution with User-defined functions
Without extending the application to load the external assembly, you will get the following error *"The expression contains object* 'MyReportExtensionsLibrary' *that is not defined in the current context."*
To avoid the error, follow the next steps:
* Add the Telerik.Reporting configuration section to the `` element. Notice that the section type attribute may differ according to the version. For the exact details, check [Telerik Reporting Configuration Section](https://www.telerik.com/products/reporting/documentation/doc-output/configure-the-report-engine/overview).
* Add `` section element and `` element in it, like in the Report Designer configuration file.
* Place the assembly extending the default functionality `MyAssembly.dll` in the directory from where the custom application will be executed. In the older .NET Framework, you may also use a subfolder described via relative to the execution folder paths using the [.NET `` element](https://learn.microsoft.com/en-us/previous-versions/dotnet/netframework-2.0/823z9h8w\(v=vs.80\)).
> Adding a reference to the assembly in Visual Studio with `CopyLocal` settings set to **True** will copy the DLL automatically.
The entire configuration for extending an application with `MyAssembly.dll` containing user-defined functions should look as follows:
```XML
```
```JSON
"telerikReporting": {
"assemblyReferences": [
{
"name": "MyAssembly",
"version": "1.0.0.0",
"culture": "neutral",
"publicKeyToken": null
}
]
}
```
Where **x.x.x.x** stands for the used Telerik Reporting assembly version, e.g., 20.3.26.1110.
While the assembly "name" is mandatory, you may skip the "version", "culture", and "publicKeyToken" when they are with their default values.
* Show the report definition in a report viewer control. For this, you may find useful the [Using Telerik Reporting in Applications](https://www.telerik.com/products/reporting/documentation/embedding-reports/display-reports-in-applications/overview) help article.
> important If the external assembly depends on other assemblies, all related assemblies have to be placed into the folder from which the application is executed.
## Solution with custom CLR objects
Without extending the application to load the external assembly, CLR objects used as data sources will not be loaded. To avoid the issue, you can:
* Add a reference to the external DLL in the project. If the reporting data source components refer to the CLR object by its [assembly qualified name](https://learn.microsoft.com/en-us/dotnet/api/system.type.assemblyqualifiedname?view=net-7.0), the Reporting Engine will be able to use [Reflection](https://learn.microsoft.com/en-us/dotnet/framework/reflection-and-codedom/reflection) and use the types since the assembly is already referenced and loaded in the application.
* Extend the application to load the assembly via Telerik.Reporting section as illustrated above for [Reports using external assemblies with User-defined functions](https://www.telerik.com/products/reporting/documentation/knowledge-base/deploying-trdx--xml-report-definition--that-uses-external-assembly#reports-using-external-assemblies-with-user-defined-functions). With this approach, you will have to copy the DLL in the application's execution folder manually, and thus you can skip adding a reference in the project. The configuration will tell the reporting engine to load the assembly.
> important If the external assembly depends on other assemblies, all related assemblies have to be placed into the folder from where the application is executed.
> The same configurations are applicable if you need to use external assemblies with User-defined functions or with custom CLR objects in Visual Studio Report Designer, where the application is `devenv.exe`, Standalone Report Designer, where the application is `Telerik.ReportDesigner.exe`, or other custom application with a viewer control.