Microsoft SQL Server Reporting Services (SSRS) supports extensions by including custom extensions and custom code. However, SSRS does not support Microsoft .NET Framework 4.x-based assemblies. Therefore, you cannot load a .NET Framework 4.x assembly.
You can load assemblies when Microsoft Reporting Server 2012 Service Pack 1 (SP1) in Microsoft SharePoint integration mode runs as a SharePoint shared service. This is because the service runs as a Microsoft .NET Framework 4.5 process and the report server can load custom assemblies and extensions that are built for the .NET Framework 4.x.
SSRS is built against the Microsoft .NET Framework 2.0 and the Microsoft .NET Framework 3.5. We recommend that you use .NET Framework 2.0 or .NET Framework 3.5 only in SSRS.
SQL Server Data Tools (SSDT) is a Microsoft Visual Studio environment that is included in SQL Server 2012. Report Designer in SSDT uses PreviewProcessingService.exe to render reports. Because PreviewProcessingService.exe uses the .NET Framework 4.0, you may encounter differences in behavior between design time and deployment to a Report Server.
For more information about the extending reporting services, see the following MSDN website:
Developer's guide for SQL Server Reporting Services
For more information about how to use custom assemblies or code in reporting services, see the following Microsoft Knowledge Base article:
920769 How to use custom assemblies or embedded code in Reporting Services