Use HttpClientFactory from .NET 4.6.2

As @Panagiotis Kanavos said, you need to add Microsoft.Extensions.Http and Microsoft.Extensions.DependencyInjection. And this is my ConsoleApp code, you can refer to it.

class Program
{
     static void Main(string[] args)
     {
         Test();
         Console.Read();
     }

     static async void Test()
     {
        var serviceProvider = new ServiceCollection().AddHttpClient().BuildServiceProvider();
        var httpClientFactory = serviceProvider.GetService<IHttpClientFactory>();
        var client = httpClientFactory.CreateClient();
        var response = await client.SendAsync(new HttpRequestMessage(HttpMethod.Get, "http://www.baidu.com"));
        var content = await response.Content.ReadAsStringAsync();
        Console.WriteLine(content);
     }
}

All of the Microsoft.Extensions.* packages target .NET Standard 2.0. That means that you can use Dependency Injection, Configuration, Logging and HttpClientFactory if you add the appropriate package to your application.

You need to add Microsoft.Extensions.Http to use HttpClientFactory and Microsoft.Extensions.Http.Polly if you want to use it with Polly

To configure HttpClientFactory you'll have to add Microsoft.Extensions.DependencyInjection to your "main" project. Microsoft.Extensions.Http package only depends on Microsoft.Extensions.DependencyInjection.Abstractions which contains the interfaces, not the DI provider itself.