Is there an equivalent for MvcHtmlString in ASP.NET 5 ? Otherwise, how can we render HTML output from my custom HTMLHelper method properly ?
Asp .Net Core (v.5) replaced MvcHtmlString for a new HtmlString type. Don't confuse this with the HtmlString type that is a part of the System.Web namespace introduced in .NET 4.0 (MvcHtmlString inherits from this type).
NOTE: After April 28, 2016 the HtmlString type was moved to the Microsoft.AspNetCore.Html.Abstractions package. Remember to include this in your project.json in the dependencies node.
Additionally, you need to declare the using directive
using Microsoft.AspNetCore.Html;
or declare it by its fully qualified name:
Microsoft.AspNetCore.Html.HtmlString
If you want more information on the HtmlString type look at the HtmlAbstractions package source code located here