In particular, IComponent is missing the following two attributes, which were defined on .NET Framework:

[Designer("System.ComponentModel.Design.ComponentDesigner, " + AssemblyRef.SystemDesign, typeof(IDesigner))]
[Designer("System.Windows.Forms.Design.ComponentDocumentDesigner, " + AssemblyRef.SystemDesign, typeof(IRootDesigner))]

This affects both designer and runtime IDesignerHost scenarios. cc @ericstj.

While we've worked around this in our designer by hard-coding knowledge of IComponent's missing attributes in our own IDesignerHost implementation, third-parties would be hard-pressed to do the same for runtime scenarios where the designer is hosted in an application at runtime. An example of such a scenario is DevExpress's Reporting library where they allow a report designer based on CodeDom serialization to embedded directly into an end user application. This is a very meaningful for certain line-of-business apps.