毎回テストを書こうとするたびに忘れてしまうのでメモ。

テスト対象のコードはこんな感じ。

public class ProductClass
{
    public ProductClass(IHttpClientFactory httpClientFactory)
    {
        HttpClientFactory = httpClientFactory;
    }

    public IHttpClientFactory HttpClientFactory { get; }

    public async Task<ResponseObject> Get()
    {
        using var client = HttpClientFactory.CreateClient("ClientName");
        using var request = new HttpRequestMessage(HttpMethod.Get, "/request/path");

        var response = await client.SendAsync(request).ConfigureAwait(false);
        if (!response.IsSuccessStatusCode)
        {
            return null;
        }
        var json = await response.Content.ReadAsStringAsync().ConfigureAwait(false);

        return JsonConvert.DeserializeObject<ResponseObject>(json);
    }
}

テストコードはこうなる。
ポイントは Moq.Protected を使って HttpMessageHandler をモックすること。

[TestFixture]
[TestOf(typeof(ProductClass))]
public class ProductClassTests
{
    [Test]
    public async Task Get()
    {
        var responseObject = new ResponseObject
        {
            SomeProperty = "value"
        };
        var mockHttpMessageHandler = new Mock<HttpMessageHandler>();
        mockHttpMessageHandler.Protected()
            .Setup<Task<HttpResponseMessage>>(
                "SendAsync",
                ItExpr.IsAny<HttpRequestMessage>(),
                ItExpr.IsAny<CancellationToken>())
            .Returns(() =>
            {
                var response = new HttpResponseMessage
                {
                    StatusCode = HttpStatusCode.OK,
                    Content = new StringContent(JsonSerializer.Serialize(responseObject))
                };
                return Task.FromResult(response);
            });

        var baseUrl = "https://test.example.jp";
        var httpClient = new HttpClient(mockHttpMessageHandler.Object)
        {
            BaseAddress = new Uri(baseUrl)
        };

        var mockHttpClientFactory = new Mock<IHttpClientFactory>();
        mockHttpClientFactory.Setup(m => m.CreateClient(It.IsAny<string>())).Returns(httpClient);

        var targetClient = new MedicalInstituteClient(mockHttpClientFactory.Object);
        var result = await targetClient.Get();

        Assert.That(result, Is.Not.Null);
        Assert.That(result.SomeProperty, Is.EqualTo("value"));

        mockHttpMessageHandler.Protected()
            .Verify(
                "SendAsync",
                Times.Once(),
                ItExpr.Is<HttpRequestMessage>(
                    request => request.Method == HttpMethod.Get
                    && request.RequestUri == new Uri($"{baseUrl}/request/path")),
                ItExpr.IsAny<CancellationToken>());

        mockHttpClientFactory.Verify(m => m.CreateClient(It.Is<string>(s => s == "ClientName")), Times.Once());
    }
}

参考サイト
Mocking HttpClient SendAsync