public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpClient("github", c =>
    {
        c.BaseAddress = new Uri("https://api.github.com/");
        c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); // Github requires a user-agent
    });
    services.AddHttpClient();
}

IHttpClientFactory _httpClientFactory;
public MyController(IHttpClientFactory httpClientFactory)
{
    _httpClientFactory = httpClientFactory;
}
public IActionResult Index()
{
    //This client doesn’t have any special configuration applied
    var defaultClient = _httpClientFactory.CreateClient();
    //This client has the header and base address configured for the “github” client above.
    var gitHubClient = _httpClientFactory.CreateClient("github");
    return View();
}

To get started with ASP.NET Core in .NET Core 3.0 Preview 9 install the .NET Core 3.0 Preview 9 SDK.

If you’re on Windows using Visual Studio, install the latest preview of Visual Studio 2019.

.NET Core 3.0 Preview 9 requires Visual Studio 2019 16.3 Preview 3 or later. 

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<LangVersion>9.0</LangVersion>

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.ComponentModel;

namespace System.Runtime.CompilerServices
{
   /// <summary>
   /// Reserved to be used by the compiler for tracking metadata.
   /// This class should not be used by developers in source code.
   /// </summary>
   [EditorBrowsable(EditorBrowsableState.Never)]
   internal static class IsExternalInit
   {
   }
}