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The existing limitation in .NET Core 3.1 and above does not allow the ActiveReports Integrated Designer to be used for designing Code-based Section Reports in WinForms applications in Visual Studio.
As a workaround, you need to use the Visual Studio's option to link report files from the .NET Core project into the .NET Framework project and use the .NET Framework WinForms Designer.
The steps to enable design-time report creation in .NET Core project for Code-Based Section Reports are as follows:
Select the Target Framework as 'NET 8.0' and select Create.
Double-click the linked SectionReport1.cs (in the .Net Framework project) to open the ActiveReports Integrated Designer for the report.
Now design the report. You will observe that the modification of a report in the .NET Framework project leads to the modification of a report in the .NET Core project.