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Fluent Assertions Documentation
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    JSON
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    For projects targeting .NET 6 or later, there's built-in support for assertions on the types JsonNode and JsonArray (including JsonNode derived classes, e.g. JsonValue and JsonObject) from the System.Text.Json namespace.

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    var jsonNode = JsonNode.Parse("{ \"name\": \"John\" }");
jsonNode.Should().HaveProperty("name").Which.ToString().Should().Be("John");
jsonNode.Should().NotHaveProperty("code");
jsonNode.BeAnArray().Which.Should().HaveCount(3);
jsonNode.NotBeAnArray();

var jsonNode = JsonNode.Parse("\"Hello World\"");
jsonNode.Should().BeString().Which.Should().Be("Hello World");
jsonNode.Should().NotBeString();

var jsonNode = JsonNode.Parse("42");
jsonNode.Should().BeNumeric().Which.Should().Be(42);
jsonNode.Should().NotBeNumeric();

var jsonNode = JsonNode.Parse("true");
jsonNode.Should().BeBool().Which.Should().BeTrue();
jsonNode.Should().NotBeBool();

    Although there's no official date time type in JSON like there is for numbers and arrays, there still is the ISO 8601 standard. So Fluent Assertions recognizes those dates like this:

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    var jsonNode = JsonNode.Parse("\"2025-09-11T21:17:00\"");
jsonNode.Should().BeLocalDate().Which.Should().Be(11.September(2025).At(21, 17).AsLocal());
jsonNode.Should().NotBeUtcDate();

var jsonNode = JsonNode.Parse("\"2025-09-11T21:17:00Z\"");
jsonNode.Should().BeUtcDate().Which.Should().Be(11.September(2025).At(21, 17).AsUtc());
jsonNode.Should().NotBeLocalDate();

    JSON arrays get special treatment as well as the same assertions that are available to collections are available here as well.

    C#
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    JsonArray array = JsonNode.Parse("[1, 2, 3]")!.AsArray();
array.Should().NotBeEmpty();
    See Also
    Available Assertions

    Reference

    JsonNodeAssertions<T> Class
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