Here is the reason of the behavior and how to correct it.

It is because the BaseComponents/ArraySelector.razor doesn’t have @using Microsoft.AspNetCore.Components.Web (that’s where events like @onclick are defined, specifically in class EventHandlers). That’s because the Components/_Imports.razor file doesn’t have an effect on the folder BaseComponents. One can either add the @using to the component, or to a BaseComponents/_Imports.razor file or (probably the best option) move the _Imports.razor file to the root of the project.