event/operation_nightwatch/1 matches the site

public static Activity Nightwatch_Project()
{
    var operationNW = new Activity()
        .WithContext()
        .WithId("https://linked.art/example/event/operation_nightwatch/1")
        .WithLabel("Conservation Project")
        .WithClassifiedAs(Aat.ConservationActivity);

    operationNW.IdentifiedBy = [new Name("Operation Night Watch").AsPrimaryName()];
    operationNW.ReferredToBy = [
        new LinguisticObject()
            .WithClassifiedAs(Aat.Description, Aat.Statement)
            .WithContent("Operation Night Watch is the biggest and most wide-ranging ever study of Rembrandt’s most famous painting.")
    ];
    operationNW.TimeSpan = new LinkedArtTimeSpan
    {
        BeginOfTheBegin = new LinkedArtDate(2019, 1, 1),
        EndOfTheEnd = new LinkedArtDate(2023, 12, 31).LastSecondOfDay()
    };
    operationNW.CarriedOutBy = [
        new Group()
            .WithId("https://linked.art/example/group/rijksmuseum")
            .WithLabel("Rijksmuseum"),
        new Group()
            .WithId("https://linked.art/example/group/akzonobel")
            .WithLabel("AkzoNobel")
    ];

    return operationNW;

}

• 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.
• Role shortcuts AsPrimaryName, AsAccessionNumber and AsSystemAssignedNumber each add a single classification, with the AAT terms Primary Name, Accession Number and System Number.

var activity = new Activity
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/event/operation_nightwatch/1",
    Label = "Conservation Project",
    ClassifiedAs = [Aat.ConservationActivity],
    CarriedOutBy =
    [
        new Group
        {
            Id = "https://linked.art/example/group/rijksmuseum",
            Label = "Rijksmuseum",
        },
        new Group
        {
            Id = "https://linked.art/example/group/akzonobel",
            Label = "AkzoNobel",
        },
    ],
    IdentifiedBy =
    [
        new Name("Operation Night Watch")
        {
            ClassifiedAs = [Aat.PrimaryName],
        },
    ],
    TimeSpan = new LinkedArtTimeSpan
    {
        BeginOfTheBegin = new LinkedArtDate(2019, 1, 1),
        EndOfTheEnd = new LinkedArtDate(2023, 12, 31, 23, 59, 59),
    },
    ReferredToBy =
    [
        new LinguisticObject
        {
            Content = "Operation Night Watch is the biggest and most wide-ranging ever study of Rembrandt’s most famous painting.",
            ClassifiedAs =
            [
                new Type("http://vocab.getty.edu/aat/300435416", "Description")
                {
                    ClassifiedAs = [Aat.Statement],
                },
            ],
        },
    ],
};

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.