ReadAsAsync is a .NET Standard extension that’s actually shared between ASP.NET Core and ASP.NET Web Api (via a NuGet library). However, it uses JSON.NET to do the deserialization, and as of .NET Core 3.0, ASP.NET Core now uses System.Text.Json instead. As such, this library (and the extension it contains) is not included in the .NET … Read more
I also came across this issue while following the documentation https://learn.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-3.0&tabs=visual-studio For your case I have checked and the code below will work, with the call to webBuilder.UseKestrel() removed. public static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args) .ConfigureWebHostDefaults(webBuilder => { webBuilder.UseContentRoot(Directory.GetCurrentDirectory()); webBuilder.UseIISIntegration(); webBuilder.UseStartup<Startup>(); });