Mocking IHttpContextAccessor

The IHttpContextAccessor Interface Provides access to the current HttpContext, if one is available.

Example code to test

A common usage is a correlation id which is a unique identifier thats attached to requests/messages across distributed systems to reference to a particular transaction or event chain.

Note that IHttpContextAccessor httpContextAccessor would have been inject by the framework.

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public Guid GetCorrelationId()
{
    var correlationId = _httpContextAccessor // defensive coding could also be added to check for the existance of _httpContextAccessor
        .HttpContext
        .Request
        .Headers.FirstOrDefault(header =>
            header.Key.Equals(
                "My-Cool-Correlation-Id", 
                StringComparison.InvariantCultureIgnoreCase))
        .Value
        .ToString();

    if (Guid.TryParse(correlationId, out var parsedValue))
    {
        return parsedValue;
    }

    return Guid.NewGuid(); // Create new if one was not present
}

Example unit test

The key is to simply mock out the chain one node at a time so the test is easy to read - you can do this in one .Setup but it will be harder to read/maintain.

The ticky bit is to use new HeaderDictionary(new Dictionary<string, StringValues> for the headers, its a dictionary of dictionaries \ :D /

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[Fact]
public void GetCorrelationId_WhenHeaderContainsCorrelationId_ReturnIt()
{
    // Arrange
    var expected = Guid.NewGuid();
    var headers = new HeaderDictionary(new Dictionary<string, StringValues>
    {
        {
            "My-Cool-Correlation-Id",
            expected.ToString()
        }
    });
    
    var httpRequestMock = new Mock<HttpRequest>();
    var httpContextAccessorMock = new Mock<IHttpContextAccessor>(); 

    httpRequestMock
        .Setup(x => x.Headers)
        .Returns(headers);

    httpContextAccessorMock
        .Setup(a => a.HttpContext.Request)
        .Returns(httpRequestMock.Object);

    var classUnderTest = new MyRequestContext(httpContextAccessorMock.Object);

    // Act
    var actual = classUnderTest.GetCorrelationId();

    // Assert
    actual.Should().Be(expected);
}

References