object/spring/21 matches the site

public static HumanMadeObject Assertion_that_Spring_Is_Canvas()
{
    var spring = new HumanMadeObject()
        .WithContext()
        .WithId("https://linked.art/example/object/spring/21")
        .WithLabel("Spring")
        .WithClassifiedAs(Aat.Painting, Aat.TypeOfWork);

    spring.AttributedBy = [
        new AttributeAssignment()
        {
            TimeSpan = LinkedArtTimeSpan.FromYear(2015),
            AssignedProperty = "made_of",
            Assigned = [
                new Material { Id = Aat.Uri("300014078") }
            ]
        }
    ];

    // remove label from year to match docs

    return spring;
}

• Metatype classification The second argument classifies the first, not the object: WithClassifiedAs(Aat.Description, Aat.Statement) says this is a Description, and Description is a kind of Statement. The Plain C# form shows the nesting.

var humanMadeObject = new HumanMadeObject
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/object/spring/21",
    Label = "Spring",
    ClassifiedAs =
    [
        new Type("http://vocab.getty.edu/aat/300033618", "Painting")
        {
            ClassifiedAs = [Aat.TypeOfWork],
        },
    ],
    AttributedBy =
    [
        new AttributeAssignment
        {
            TimeSpan = new LinkedArtTimeSpan
            {
                BeginOfTheBegin = new LinkedArtDate(2015, 1, 1),
                EndOfTheEnd = new LinkedArtDate(2015, 12, 31, 23, 59, 59),
            },
            AssignedProperty = "made_of",
            Assigned = [new Material("http://vocab.getty.edu/aat/300014078", null)],
        },
    ],
};

Generated from the object the example builds: every property that is set, as object initialisers, with the vocabulary constants where a term is in the registry. The fluent helpers are a convenience, not a requirement; this form and the one as written produce the same JSON.