1\. 需先在 Startup.cs 檔案裡的 `ConfigureServices` 方法中加入 services.AddHttpClient();
``` C#
public class Startup
{
public IConfiguration _configuration { get; }
public Startup(IConfiguration configuration)
{
_configuration = configuration;
}
public void ConfigureServices(IServiceCollection services)
{
services.AddHttpClient();
// Remaining code deleted for brevity.
}
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
// Remaining code deleted for brevity.
}
}
```
2\. 然後在 Controller 中透過 DI 注入 IHttpClientFactory，並透過它取得圖檔。詳細部分如下：
``` C#
public class CrawlerController : Controller
{
private readonly IHttpClientFactory _httpClientFactory;
public CrawlerController(IHttpClientFactory httpClientFactory)
{
_httpClientFactory = httpClientFactory;
}
[HttpGet]
public async Task<ActionResult> Get()
{
// 1. 建立 HttpClient 物件
var client = _httpClientFactory.CreateClient();
var imageUrl = "https://raw.githubusercontent.com/linmasaki/SlimeRacing/master/demo_1.jpg";
var filename = new Uri(imageUrl).Segments.Last();
// 2. 不用 GetStreamAsync()，而是使用 GetByteArrayAsync() 方式取得圖片二進位檔，然後才再轉成 Stream 格式
using (var memoryStream = new MemoryStream(await client.GetByteArrayAsync(imageUrl)))
{
// 3. 使用 memoryStream 實做你的商業邏輯，e.g. 例如存到本地 C 磁碟的 temp 路徑下
using (var output = new FileStream($"C:/temp/{filename}", FileMode.Create))
{
await memoryStream.CopyToAsync(output).ConfigureAwait(false);
}
}
}
}
```
##參考資料 [\[Microsoft\] Make HTTP requests using IHttpClientFactory in ASP.NET Core](https://docs.microsoft.com/en-gb/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.1) [\[精緻碼農\] NET Core 中正确使用 HttpClient 的姿势](https://www.cnblogs.com/willick/p/net-core-httpclient.html)
##參考資料 [\[Microsoft\] Make HTTP requests using IHttpClientFactory in ASP.NET Core](https://docs.microsoft.com/en-gb/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.1) [\[精緻碼農\] NET Core 中正确使用 HttpClient 的姿势](https://www.cnblogs.com/willick/p/net-core-httpclient.html)
