https://github.com/dotnet/corefx/issues/9996#issuecomment-307870746
dotnet/aspnetcore#1805 (comment)

ASP.NET Core is getting many requests for users that want to impersonate the authenticated user of the request and perform actions on their behalf. The main problem is that many of these actions are asynchronous, such as database access, but RunImpersonated is synchronous. Users have repeatedly made the mistake of starting an asynchronous operation within RunImpersonated and then returning the Task. Exiting RunImpersonated revokes the impersonation and can cause errors or reverting to the base identity in the Task.

Proposal:
public static Task RunImpersonatedAsync(SafeAccessTokenHandle safeAccessTokenHandle, Func<Task> func)
public static Task<T> RunImpersonatedAsync<T>(SafeAccessTokenHandle safeAccessTokenHandle, Func<Task<T>> func)

@brentschmaltz