In order to enable the full design-time support including the Data Explorer, Expression builder and all editors you need to have the Report.DataSource initialized. The tricky part is that in order to load the component's designer Visual Studio do parse
the code inside the InitializeComponent method and create the objects it finds there in a specific manner. The parser (CodeDomSerializer) understands limited syntax and in general you should avoid writing code there - usually the designer generates the code inside for you. Please note that only the InitializeComponent() method matters. The Report's constructor is never invoked while loading the report into the designer (this is not a limitation of the Telerik Report's designer but for all .NET components).
We highly recommend that you review the Connecting Data to a Report
help section and specifically Adding a Data Source Programmatically
Generally when you want to achieve a certain task programmatically, it is a good idea to create the desired layout through the report designer and after that inspect the code generated by the designer (i.e. InitializeComponent). This would decrease the learning curve significantly as Telerik Reporting has a feasible and easy to understand API
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