我应该在单元测试时总是使用Mocks，以便最小化测试范围内的单元

Question

Consider the following test which confirms that when an object is added to the HttpContentConatiner class it's resulting MD5, which is stored on the content container matches the result of serializing and hashing the body content externally (which is how it would be used).

[Fact]
public void When_body_added_correctly_MD5_matches_an_external_hash_of_the_same_content()
{
    var contentStub = new object();
    var serializer = new NewtonSoftJsonSerializer();
    var hasher = new Hmac256Hasher();
    var contentContainer = new HttpContentContainer(serializer, hasher);

    contentContainer.AddBody(contentStub);

    Assert.Equal(hasher.Hash(serializer.Serialize(contentStub), "Key"),
                             contentContainer.ContentMD5.Value);

}

In this test I use an actual implementation for both the serializer and the hasher as opposed to mocking out these two objects. This now means that the test depends on both the hasher and the serializer working correctly as their implementations have now been pulled into the test scope which I worry may make my tests brittle.

My Question
Given the above example, should I mock the serializer and hasher for maintainability purposes or is it ok to depend on external services like this?

New Code based on Answer:

[Fact]
public void When_a_valid_body_is_added_ContentMD5_Value_is_populated()
{
    var serializerMock = new Mock<ISerializer>();
    serializerMock.Setup(serializer => serializer.Serialize(It.IsAny<object>()))
                  .Returns("serializedContent");

    var hasherMock = new Mock<IHasher>();
    hasherMock.Setup(hasher => hasher.Hash(It.IsAny<string>(), It.IsAny<string()))
              .Returns("MD5");

    var contentContainer = 
        new HttpContentContainer(serializerMock.Object, hasherMock.Object);

    contentContainer.AddBody(new object());

    Assert.NotEmpty(contentContainer.ContentMD5.Value);
}

Answer 1

I would prefer to Mock both Hasher as well as the Serializer.

Reason - Both Hash and Serialize method of these dependencies could be assumed to perform expected functions, and this can be the expectation of the Mock. A test failure would then indicate the failure of Object under test with certainty.