What is the error you are facing, I have just tried inside the View for an Article Page on the Alloy Templates and all seems to work 

I realized the problem was not in the code with categoryRepository but the next line of code:

 var filterBlock = EPiServer.ServiceLocation.ServiceLocator.Current.GetInstance<FilterBlock>();

This was causing this error: 

Exception thrown: 'System.InvalidOperationException' in Microsoft.Extensions.DependencyInjection.Abstractions.dll

I have removed this in code, and it works fine, but what is the substitute for that in .NET Core

Thanks

Im not sure if you are in a view or the block itself 

If you are trying to get the Content Type of a particular Block and know its ContentId I would use IContentRepository 

e.g. 

this.contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
// Could cause null reference exception so might be worth going down the .TryGet route 
var filterBlock= contentRepository.Get<FilterBlock>(x.ContentLink);