ASP.NET Core 單元測試之如何 Mock HttpClient.GetStringAsync()

作者：汪宇杰 2021-05-11 15:50:28

在 ASP.NET Core 單元測試中模擬HttpClient.GetStringAsync() 的技巧。如果按正常思路像這樣去 Mock HttpClient.GetStringAsync()。

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在 ASP.NET Core 單元測試中模擬HttpClient.GetStringAsync() 的技巧。

問題

下面這個代碼

var html = await _httpClient.GetStringAsync(sourceUrl);

如果按正常思路像這樣去 Mock HttpClient.GetStringAsync()

var httpClientMock = new Mock<HttpClient>(); 
httpClientMock 
    .Setup(p => p.GetStringAsync(It.IsAny<string>())) 
    .Returns(Task.FromResult("..."));

Moq 框架就會爆

Exception

System.NotSupportedException : Unsupported expression: p => p.GetStringAsync(It.IsAny())Non-overridable members (here: HttpClient.GetStringAsync) may not be used in setup / verification expressions.

解決方法

我們需要 Mock HttpClient 底層使用的 HttpMessageHandler 而不是 HttpClient

var handlerMock = new Mock<HttpMessageHandler>(); 
var magicHttpClient = new HttpClient(handlerMock.Object);

然后我花了 9.96 分鐘研究了 HttpClient.GetStringAsync() 的源代碼，發現它最終調用的是 SendAsync() 方法

private async Task<string> GetStringAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken) 
{ 
    // ... 
    response = await base.SendAsync(request, cts.Token).ConfigureAwait(false); 
    // ... 
}

源代碼位置：https://source.dot.net/#System.Net.Http/System/Net/Http/HttpClient.cs,170

因此，我們的 Mock Setup 如下：

handlerMock 
    .Protected() 
    .Setup<Task<HttpResponseMessage>>( 
        "SendAsync", 
        ItExpr.IsAny<HttpRequestMessage>(), 
        ItExpr.IsAny<CancellationToken>() 
    ) 
    .ReturnsAsync(new HttpResponseMessage 
    { 
        StatusCode = HttpStatusCode.OK, 
        Content = new StringContent("the string you want to return") 
    }) 
    .Verifiable();

現在 Mock 就能運行成功了!

最后附上完整的 UT 代碼供參考：

using System.Net; 
using System.Net.Http; 
using System.Threading; 
using System.Threading.Tasks; 
using Microsoft.Extensions.Logging; 
using Moq; 
using Moq.Protected; 
using NUnit.Framework; 
 
namespace Moonglade.Pingback.Tests 
{ 
    [TestFixture] 
    public class PingSourceInspectorTests 
    { 
        private MockRepository _mockRepository; 
 
        private Mock<ILogger<PingSourceInspector>> _mockLogger; 
        private Mock<HttpMessageHandler> _handlerMock; 
        private HttpClient _magicHttpClient; 
 
        [SetUp] 
        public void SetUp() 
        { 
            _mockRepository = new(MockBehavior.Default); 
            _mockLogger = _mockRepository.Create<ILogger<PingSourceInspector>>(); 
            _handlerMock = _mockRepository.Create<HttpMessageHandler>(); 
        } 
 
        private PingSourceInspector CreatePingSourceInspector() 
        { 
            _magicHttpClient = new(_handlerMock.Object); 
            return new(_mockLogger.Object, _magicHttpClient); 
        } 
 
        [Test] 
        public async Task ExamineSourceAsync_StateUnderTest_ExpectedBehavior() 
        { 
            string sourceUrl = "https://996.icu/work-996-sick-icu"; 
            string targetUrl = "https://greenhat.today/programmers-special-gift"; 
 
            _handlerMock 
                .Protected() 
                .Setup<Task<HttpResponseMessage>>( 
                    "SendAsync", 
                    ItExpr.IsAny<HttpRequestMessage>(), 
                    ItExpr.IsAny<CancellationToken>() 
                ) 
                .ReturnsAsync(new HttpResponseMessage 
                { 
                    StatusCode = HttpStatusCode.OK, 
                    Content = new StringContent($"<html>" + 
                                                $"<head>" + 
                                                $"<title>Programmer's Gift</title>" + 
                                                $"</head>" + 
                                                $"<body>Work 996 and have a <a href=\"{targetUrl}\">green hat</a>!</body>" + 
                                                $"</html>") 
                }) 
                .Verifiable(); 
            var pingSourceInspector = CreatePingSourceInspector(); 
 
            var result = await pingSourceInspector.ExamineSourceAsync(sourceUrl, targetUrl); 
            Assert.IsFalse(result.ContainsHtml); 
            Assert.IsTrue(result.SourceHasLink); 
            Assert.AreEqual("Programmer's Gift", result.Title); 
            Assert.AreEqual(targetUrl, result.TargetUrl); 
            Assert.AreEqual(sourceUrl, result.SourceUrl); 
        } 
    } 
}

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ASP.NET 單元測試