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Codenizer.HttpClient.Testable

A NuGet package to help you test the usage of the .Net HttpClient in your applications.

It supports matching on HTTP verbs, URL, query parameters, content type and much more. It allows you to return specific responses for each matching request as well as asserting that requests were made with the expected parameters and HTTP headers.

Build status

Codenizer.HttpClient.Testable

Proudly built with NCrunch.

Have a look at the changelog for recent changes in the latest versions.

Introduction

Most people would call this a stub, a double or a mock and they are probably right. This was written to be an easy way to have your code interact with a HttpClient that just works instead of having to create wrappers around it.

It supports setting up various behaviours to responses it receives (actual: that are sent by the HttpClient) and allows you to assert that calls are made and to verify the properties of those requests.

Usage

To include this package in your applications, add the NuGet package:

dotnet CLI:

dotnet add package Codenizer.HttpClient.Testable

PowerShell:

Install-Package -Name Codenizer.HttpClient.Testable

Example use in a test

Let's say we have a class that calls an external API using HttpClient. It's a simple one:

public class InfoApiClient
{
    private readonly HttpClient _httpClient;
    
    public InfoApiClient(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }
    
    public async Task<string> GetLatestInfo()
    {
        var response = await _httpClient.GetAsync("/api/info/latest");
        
        return await response.Content.ReadAsStringAsync();
    }
}

Unfortunately it doesn't deal with errors very well, in fact, it doesn't handle any errors at all! Let's change that and return a string Sorry, no content if we get a 404 Not Found.

public class WhenRequestingLatestInfo
{
    [Fact]
    public async void GivenApiReturns404_SorryNoContentIsReturned()
    {
        var infoClient = new InfoApiClient(new HttpClient());
        
        var latestInfo = await infoClient.GetLatestInfo();

        latestInfo
            .Should()
            .Be("Sorry, no content");
    }
}

Unless we're suddenly very unlucky, this test is going to fail with:

One or more errors occurred. (An invalid request URI was provided. The request URI must either be an absolute URI or BaseAddress must be set.)

because we didn't configure any URL to talk to. So how can we make sure a 404 Not Found is seen by the HttpClient and for us to start changing the implementation to handle that?

We will need to intercept the calls made by the HttpClient:

var messageHandler = new Codenizer.HttpClient.Testable.TestableMessageHandler();
var httpClient = new HttpClient(messageHandler) { BaseAddress = new Uri("http://localhost:5000") };
var infoClient = new InfoApiClient(httpClient);

Let's run the test again:

Expected string to be
"Sorry, no content" with a length of 17, but
"No response configured for /api/info/latest" has a length of 43.

Right. At least it tells us what's going on. We need to set up something to make this work. So let's instruct the handler to return a 404 Not Found when we try to hit /api/info/latest:

messageHandler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatusCode.NotFound);

Our test now looks like:

[Fact]
public void GivenApiReturns404_SorryNoContentIsReturned_Step3()
{
    var messageHandler = new Codenizer.HttpClient.Testable.TestableMessageHandler();
    var httpClient = new System.Net.Http.HttpClient(messageHandler) { BaseAddress = new Uri("http://localhost:5000") };
    var infoClient = new InfoApiClient(httpClient);
    
    messageHandler
        .RespondTo()
        .Get()
        .ForUrl("/api/info/latest")
        .With(HttpStatusCode.NotFound);

    infoClient
        .GetLatestInfo()
        .Result
        .Should()
        .Be("Sorry, no content");
}

Should work right....? Wrong:

Object reference not set to an instance of an object

This exception actually occurs in the InfoApiClient because it tries to read content that isn't there. Our final step is to implement a status code check in the InfoApiClient and make it work:

public async Task<string> GetLatestInfo()
{
    var response = await _httpClient.GetAsync("/api/info/latest");

    if (response.StatusCode == HttpStatusCode.NotFound)
    {
        return "Sorry, no content";
    }
    
    return await response.Content.ReadAsStringAsync();
}

Run the test again:

Success

Yay!

Taking it further

The message handler has a number of additional methods to control the responses it will generate. They follow a fluent style so are meant to be used in a chained fashion.

Configuring the URLs it will respond to

RespondTo()
    .Get() // This can be any HTTP verb such as .Delete(), .Post(), .Put() etc
    .ForUrl("/your/url/here")
    .AndContentType("application/json");

This pattern allows you to specify the HTTP verb, the relative part of the URI and optionally the content type to respond to. The handler will match against the entire path and query string including the HTTP method. If multiple responses are configured against the same URL a MultipleResponsesConfigureException will be thrown.

Since version 0.5.0 you can configure an endpoint with route parameters.
Let's say you want to configure an endpoint that retrieves editions of a book (for example: /api/books/56789/editions) and it doesn't quite matter which id the book has you can now define the endpoint as /api/books/{id}/editions. The handler will match the request to that configured response without knowing the specific id in advance.

NOTE: There is one case that currently breaks: if you do /api/entity/{id}?foo=bar then the handler will not match the response correctly.

Setting the response code

With(HttpStatusCode httpStatusCode)

Setting content to return

AndContent(string mimeType, string data)

You will need to specify the media type of the content to return. Data is supplied as a string, if you need to return JSON serialized data you can use this method as:

var serializedJson = JsonConvert.SerializeObject(myObject);

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndContent("application/json", serializedJson);

Using AndContent the handler will not serialize the data itself because it does not know about the required serialization settings.

Returning JSON data

As most APIs tend to return JSON data a convenience method is introduced on the testable handler. You can configure a request using the AndJsonContent method:

var myObject = new {
    Foo = "bar",
    Baz = "quux"
};

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndJsonContent(myObject);

In this case the response received by a caller will be serialized using the default Json.Net serialization settings.

To control how objects are serialized you can either pass a JsonSerializerSettings object to AndJsonContent:

var myObject = new {
    Foo = "bar",
    Baz = "quux"
};

var serializerSettings = new JsonSerializerSettings
{
    /* settings */
};

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndJsonContent(myObject, serializerSettings);

or if you have a handler for a specific API you can set that at construction time too:

var serializerSettings = new JsonSerializerSettings
{
    /* settings */
};

var handler = new TestableMessageHandler(serializerSettings);

in this case the handler will use those settings when serializing the object to return.

Returning binary data

You can now also configure a response to return binary data by passing in a byte array to AndContent. This will result in a ByteArrayContent object being used on the response:

var myPayload = new byte[] { 0x1, 0x2, 0x3 };

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndContent("image/png", myPayload);

Response headers

AndHeaders(Dictionary<string, string> headers)

For example:

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndContent("application/json", serializedJson)
    .AndHeaders(new Dictionary<string, string>
                {
                    {"Test-Header", "SomeValue"}
                });

If you need specific HTTP headers on the response you can add them using this method. This method can be called multiple times but be aware that it will append values to existing header names.

Cookies

If a response should include cookies you can configure the request with the AndCookie method:

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndContent("application/json", serializedJson)
    .AndCookie("cookie-name", "cookie-value");

This will add the Set-Cookie HTTP header to the response. The AndCookie method supports the following parameters:

  • expiresAt, DateTme will only be added to the cookie if this value is not null
  • domain, string will only be added to the cookie if this value is not null
  • path, string will only be added to the cookie if this value is not null
  • secure, bool will only be added to the cookie if this value is true
  • sameSite (Lax, Strict, None), if left null the SameSite parameter will not be set on the cookie
  • maxAge, int will only be added to the cookie if this value is not null

See the MDN article on the Set-Cookie header for more details on valid values.

If AddCookie does not suit your specific case you can always use AddHeaders and format the Set-Cookie header yourself. This can be particularly useful if you want to test malformed cookie responses.

Delaying responses

Let's say you want to test timeouts in your code, it would be useful to have the handler simulate that a request takes some time to process. With Taking(TimeSpan time) you can do that:

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndContent("application/json", serializedJson)
    .Taking(TimeSpan.FromMilliseconds(100));

This will make the handler delay for 100ms before returning the response you've configured. You do not necissarily need to define content first, Taking() can be applied to With() directly.

Handle requests only if they match a content type

You may want to configure a response only if the request has a specific Content-Type, for example on PUT or POST endpoints that require application/json. As of version 0.4.0 you can now do that like so:

handler
    .RespondTo()
    .Post()
    .ForUrl("/api/infos/")
    .AndContentType("application/json")
    .With(HttpStatus.Created);

If you make a request with a content type of text/plain:

await httpClient.PostAsync("/api/infos", new StringContent("test", Encoding.ASCII, "text/plain"));

the response returned from the handler will be 415 Unsupported Media Type

Handle requests only if they match the request body

For some tests you might want to configure the handler to return certain responses based on the request content.

To do this you can configure the handler like so:

handler
    .RespondTo()
    .Post()
    .ForUrl("/search")
    .ForContent(@"{""params"":{""match"":""bar""}}")
    .With(HttpStatusCode.BadRequest);

and a second one like so:

handler
    .RespondTo()
    .Post()
    .ForUrl("/search")
    .ForContent(@"{""params"":{""match"":false}}")
    .With(HttpStatusCode.OK);

Here you can see that the same endpoint returns two different responses based on the request content.

A sequence of responses

In some cases you might want to have multiple responses configured for the same endpoint, for example when you call a status endpoint of a job. Using WithSequence you can configure an ordered set of responses for a single endpoint:

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/status")
    .WithSequence(builder => builder.With(HttpStatusCode.OK).AndContent("text/plain", "PENDING"))
    .WithSequence(builder => builder.With(HttpStatusCode.OK).AndContent("text/plain", "PENDING"))
    .WithSequence(builder => builder.With(HttpStatusCode.OK).AndContent("text/plain", "PENDING"))
    .WithSequence(builder => builder.With(HttpStatusCode.OK).AndContent("text/plain", "OK"));

here we have an endpoint /api/status that will respond with the content OK on the 4th call.

The builder argument is a new IRequestBuilder instance that you can configure the specific response with for the step in the sequence. It allows you to use all the options of configuring a response like you would have when you use With().

Verifying requests have been made

The handler exposes a Requests property that contains all requests made to the handler. You can use FluentAssertions to assert the properties of the requests.

For example:

handler
    .Requests
    .Should()
    .Contain(req => req.RequestUri.PathAndQuery == "/api/info");

Additionally you can verify the content of the request using the GetData() extension method like so:

// Configure the response
handler
    .RespondTo()
    .Post()
    .ForUrl("/api/info")
    .With(HttpStatusCode.Created);

// Call the endpoint
await httpClient.PostAsync("/api/info", new StringContent("{\"foo\":\"bar\"}"));

// Check the request content
var serializedContent = handler
    .Requests
    .Single(req => req.RequestUri.PathAndQuery == "/api/info")
    .GetData()
    .Should()
    .Be("{\"foo\":\"bar\"}");

Callback when request is made

For some cases it may be useful to have a callback when the request is handled by the testable handler. You can use WhenCalled to register one:

var wasCalled = false;

handler
    .RespondTo()
    .Get()
    .ForUrl("/api/info/latest")
    .With(HttpStatus.OK)
    .AndContent("application/json", serializedJson)
    .WhenCalled(request => wasCalled = true);

When you make a request to /api/info/latest the wasCalled variable will now be set to true.

Reset the handler

In situations where you need to reset the configured responses and you can't create a new instance easily you can now use the ClearConfiguredResponses() method. This will remove any configured response from the handler which allows you to configure new ones.

Working with dependency injection and IHttpClientFactory

When your application uses the IHttpClientFactory it can be more difficult to get the TestableMessageHandler to be used by the HttpClient used by your code. To help with this, you can use the TestableHttpClientFactory which allows you to configure the HttpClient name and the TestableMessageHandler it should use:

The code under test:

public class TheRealComponent
{
    private readonly IHttpClientFactory _httpClientFactory;

    public TheRealComponent(IHttpClientFactory httpClientFactory)
    {
        _httpClientFactory = httpClientFactory;
    }

    public async Task<string> ExecuteAsync()
    {
        var httpClient = _httpClientFactory.CreateClient("TheNameOfTheClient");
        return await httpClient.GetStringAsync("https://example.com");
    }
}

The test:

var httpClientFactory = new TestableHttpClientFactory();
var handler = httpClientFactory.ConfigureClient("TheNameOfTheClient");

handler
    .RespondTo()
    .Get()
    .ForUrl("https://example.com")
    .With(HttpStatus.OK)
    .AndContent("text/plain", "Hello, world!");

var sut = new TheRealComponent(httpClientFactory);

var result = await sut.ExecuteAsync();

result
    .Should()
    .Be("Hello, world!");

The TestableHttpClientFactory will return a new HttpClient instance which is backed by the TestableMessageHandler you've configured in the test.