Can the new csproj file structure be used with a ASP.NET Framework project?

The new .csproj format includes some significant improvements over the classic files, including tight integration with NuGet package management and significantly less-verbose structure. I want to gain these benefits whilst still using the .NET Framework 4.6 and ASP.NET (because my project depends on Umbraco which has yet to produce a .NET Core version).

Httpclient This instance has already started one or more requests. Properties can only be modified before sending the first request

I am creating an application in .Net Core 2.1 and I am using http client for web requests. The issue is I have to send parallel calls to save time and for that I am using Task.WhenAll() method but when I hit this method I get the error “This instance has already started one or more requests. Properties can only be modified before sending the first request” Previously I was using RestSharp and everything was fine but I want to use httpclient. Here is the code: