It seems to me that there are major breaking changes in TagBuilder as of beta7 with no mention about them in the announcements repo.
Specifically .ToString no longer ren
@Memet Olsen
All my custom TagHelpers broke with an update to 4.6.1 (1.0.0-rc2). InnerHtml.Append() will no longer accept a TagBuilder.
Instead, AppendHtml() method should be used:
var container = new TagBuilder("div");
var input = new TagBuilder("input");
container.InnerHtml.AppendHtml(input);
2nd bug-fix [here]