重构代码以允许对 HttpClient 进行单元测试 [英] Refactoring code to allow for unit testing of HttpClient

问题描述

我正在处理一段如下所示的代码:

I am dealing with a piece of code that looks like this:

public class Uploader : IUploader
{
    public Uploader()
    {
        // assign member variables to dependency injected interface implementations
    }

    public async Task<string> Upload(string url, string data)
    {
        HttpResponseMessage result;
        try
        {
            var handler = new HttpClientHandler();
            var client = new HttpClient(handler);

            result = await client.PostAsync(url, new FormUrlEncodedContent(data));

            if (result.StatusCode != HttpStatusCode.OK)
            {
                return "Some Error Message";
            }
            else
            {
                return null; // Success!
            }
        }
        catch (Exception ex)
        {
            // do some fancy stuff here
        }
    }
}

我正在尝试对 Upload 功能进行单元测试.特别是，我需要模拟 HttpClient.阅读此处的其他答案和这些后href="https://gingter.org/2018/07/26/how-to-mock-httpclient-in-your-net-c-unit-tests/" rel="nofollow noreferrer">两个文章，我知道解决这个问题的更好方法之一是模拟 HttpMessageHandler 并将其传递给 HttpClient 并让它返回我想要的任何内容.

I am trying to unit test the Upload function. In particular, I need to mock the HttpClient. After reading the other answers on here and these two articles, I know that one of the better ways to solve this is to mock the HttpMessageHandler instead and pass that to HttpClient and have it return whatever I want.

所以，我首先在构造函数中传入 HttpClient 作为依赖项:

So, I started along that path by first passing in HttpClient in the constructor as a dependency:

public class Uploader : IUploader
{
    private readonly HttpClient m_httpClient; // made this a member variable

    public Uploader(HttpClient httpClient) // dependency inject this
    {
        m_httpClient = httpClient;
    }

    public async Task<string> Upload(string url, string data)
    {
        HttpResponseMessage result;
        try
        {
            var handler = new HttpClientHandler();

            result = await m_httpClient.PostAsync(url, new FormUrlEncodedContent(data));

            if (result.StatusCode != HttpStatusCode.OK)
            {
                return "Some Error Message";
            }
            else
            {
                return null; // Success!
            }
        }
        catch (Exception ex)
        {
            // do some fancy stuff here
        }
    }
}

并在Startup.cs的ConfigureServices方法中添加:services.AddSingleton();.

但现在我面临一个小问题，原始代码专门创建了一个 HttpClientHandler 来传入.那么我如何重构它以接收一个可模拟的处理程序?

But now I face a slight issue where the original code specifically creates a HttpClientHandler to pass in. How then do I refactor that to take in a mockable handler?

推荐答案

我觉得最简单的方法是继续使用HttpClient，但是传入一个mocking的HttpClientHandler，比如https://github.com/richardszalay/mockhttp

I find the simplest way is to continue using HttpClient, but pass in a mocking HttpClientHandler such as https://github.com/richardszalay/mockhttp

来自上面链接的代码示例:

Code sample from the link above:

var mockHttp = new MockHttpMessageHandler();

mockHttp.When("http://localhost/api/user/*")
        .Respond("application/json", "{'name' : 'Test McGee'}");

// Inject the handler or client into your application code
var client = mockHttp.ToHttpClient();

var response = await client.GetAsync("http://localhost/api/user/1234");

var json = await response.Content.ReadAsStringAsync();

Console.Write(json); // {'name' : 'Test McGee'}

.NET Core 中内置的依赖注入框架忽略了internal 构造函数，因此在这种情况下它将调用无参数构造函数.

The Dependency Injection framework built into .NET Core ignores internal constructors, so it will call the parameter-less constructor in this scenario.

public sealed class Uploader : IUploader
{
    private readonly HttpClient m_httpClient;

    public Uploader() : this(new HttpClientHandler())
    {
    }

    internal Uploader(HttpClientHandler handler)
    {
        m_httpClient = new HttpClient(handler);
    }

    // regular methods
}

在您的单元测试中，您可以使用接受 HttpClientHandler 的构造函数:

In your unit tests, you can use the constructor accepting the HttpClientHandler:

[Fact]
public async Task ShouldDoSomethingAsync()
{
    var mockHttp = new MockHttpMessageHandler();

    mockHttp.When("http://myserver.com/upload")
        .Respond("application/json", "{'status' : 'Success'}");

    var uploader = new Uploader(mockHttp);

    var result = await uploader.UploadAsync();

    Assert.Equal("Success", result.Status);
}

通常我不太喜欢使用内部构造函数来促进测试，但是，我发现这比注册共享的 HttpClient 更明显和独立.

Normally I'm not a big fan of having an internal constructor to facilitate testing, however, I find this more obvious and self-contained than registering a shared HttpClient.

HttpClientFactory 可能是另一个不错的选择，但我还没有玩过太多，所以我只会提供我自己发现有用的信息.

HttpClientFactory might be another good option, but I haven't played around with that too much, so I'll just give info on what I've found useful myself.

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