Let me explain the necessary steps briefly:
1. Create a new Class Library and implement your custom function.
2. Build the project.
3. Navigate to the bin
folder of the project and copy the dll file.
4. Go to the installation folder of Telerik Reporting Designer C:\Program Files (x86)\Progress\Telerik Reporting [Version]\Report Designer
and paste the dll file.
(for example: C:\Program Files (x86)\Progress\Telerik Reporting R2 2019\Report Designer
5. Open Telerik.ReportDesigner.exe.config file through a text editor to register the copied assemblies. For example:
For additional information, refer to Extending Report Designer
article and How to use external assemblies with custom user functions in the Report Designer
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