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Maybe Assertions

maybe.ShouldBe() returns a MaybeAssertion<T>, the entry point for asserting optional values. From there, choose the branch you expect:

  • .Some(string? because = null) returns a SomeAssertion<T> carrying the value.
  • .None(string? because = null) returns a NoneAssertion.

Each method accepts an optional because reason that is included in the failure message when the assertion fails. The None failure message also reports the unexpected value when the Maybe was actually Some.

Asserting Some​

.Some() exposes the value through Value/Subject, or you can chain assertions with .And(...).

[Fact]
public void FindUser_ExistingEmail_ReturnsSome()
{
    // Arrange (Given)
    var email = "john@example.com";

    // Act (When)
    var maybe = FindUser(email);

    // Assert (Then)
    maybe.ShouldBe()
        .Some()
        .And(user => Assert.Equal(email, user.Email));
}

Chaining on the value: .And and .Where​

.And(Action<T>) runs xUnit assertions and returns the same SomeAssertion<T> so several can be chained. .Where(Func<T, bool>, because?) fails the test if the predicate is not satisfied.

[Fact]
public void FindActiveUser_ExistingEmail_ReturnsActiveUser()
{
    // Arrange (Given)
    var email = "john@example.com";

    // Act (When)
    var maybe = FindUser(email);

    // Assert (Then)
    maybe.ShouldBe()
        .Some()
        .Where(user => user.IsActive)
        .And(user => Assert.Equal(email, user.Email));
}

SomeAssertion<T> also offers BeEquivalentTo(expected), NotBeNull(), BeOfType<T, TExpected>(), SatisfyAll(params Action<T>[]), Map(...), and Inspect(...) for non-breaking debugging.

[Fact]
public void ParseConfig_ValidJson_ReturnsExpectedPort()
{
    // Arrange (Given)
    var json = "{ \"port\": 8080 }";

    // Act (When)
    var maybe = ParseConfig(json);

    // Assert (Then)
    maybe.ShouldBe()
        .Some()
        .Map(config => config.Port)
        .And(port => Assert.Equal(8080, port));
}

Asserting None​

.None() asserts the empty branch. There is nothing to chain onto, so the call typically ends the assertion.

[Fact]
public void FindUser_UnknownEmail_ReturnsNone()
{
    // Arrange (Given)
    var email = "missing@example.com";

    // Act (When)
    var maybe = FindUser(email);

    // Assert (Then)
    maybe.ShouldBe().None("no user is registered with that email");
}

See also​