MVC1006: Methods containing TagHelpers must be async and return Task

Cause

A tag helper was defined inside a Razor function that executes synchronously

Rule description

Tag Helper execution is asynchronous. When used inside a method or a lambda within a Razor Page, the containing function must also be declared to be async.

Consider the following cshtml file:

void Helper(string controller)
{
    <a asp-controller="@controller">Home</a>
}

asp-controller is a tag helper and will trigger this rule.

How to fix violations

Declare the function to be async and Task returning:

async Task Helper(string controller)
{
    <a asp-controller="@controller">Home</a>
}

When to suppress warnings

Do not suppress a warning from this rule.