The design-time preview is executed from Visual Studio as a running application. thus all assemblies and custom settings must be applied based on the devenv.exe
execution folder and devenv.exe.config
file's settings. For more details, check Report Designer Preview is not working when using ObjectDataSource.
To preview the report, test creating a sample project with a viewer and references to all related projects (reports, data, third-party libraries). Depending on the report's settings, you may need to declare external assemblies in a Telerik Reporting section, in order the reporting engine to recognize the types used by the report. For more details, please check Previewing a report definition that uses an external assembly
Let us know if you have any further questions.
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