When you add the assembly, you receive the following warning message:
'Microsoft.ReportingServices.ProcessingCore.dll', or one of its dependencies, requires a later version of the .NET Framework than the one specified in the project.
You can change the .NET Framework target by clicking Properties on the Project menu and then selecting a new target in the '.NET Framework' dropdown box. (In Visual Basic, this is located on the Compile tab by clicking the "Advanced Compiler Options..." button.)
Do you still want to add a reference to 'Microsoft.ReportingServices.ProcessingCore.dll' in the project?
After you change the target .NET Framework to .NET Framework 4.0, the project can be compiled successfully. However, the SSRS service cannot load the custom report item. The custom report item does not display in any report in SQL Server 2012 Reporting Services.
To resolve this problem, you can safely ignore the warning message. The project will be compiled and can be deployed successfully to the report server.