Csharp 尽管在 LinqWhere() 调用中判断该值不为空，但仍取消引用可能为空的引用

我准备了一个简单的C# Fiddle，它调用an OSRM map matching service URL并解析JSON响应.

我的问题是，有一条生产线生产warning CS8602: Dereference of a possibly null reference:

if (osrmResponse?.code == "Ok" && osrmResponse.matchings != null)
{
    return osrmResponse.matchings
        .Where(matching => matching != null)
        .SelectMany(matching => matching.legs ?? Enumerable.Empty<Leg>())
        .Where(leg => leg != null && leg.annotation != null && leg.annotation.nodes != null)
        // How to fix dereference of a possibly null value in the next line?
        .SelectMany(leg => leg.annotation.nodes ?? Enumerable.Empty<long>())
        // eliminate duplicate node ids by converting to an ISet
        .ToHashSet()
        .ToList();
}

我不明白，如果我在有问题的行之前判断.Where中的调用，为什么编译器认为leg或leg.annotaion为空？

原因可能是.Where()呼叫的签名，然后如何解决呢？

完整的测试用例复制如下:

namespace OsrmMapMatch
{
    public class OsrmResponse
    {
        public string? code { get; set; }
        public Matching[]? matchings { get; set; }
    }

    public class Matching
    {
        public Leg[]? legs { get; set; }
    }
    public class Leg
    {
        public Annotation? annotation { get; set; }
    }

    public class Annotation
    {
        public long[]? nodes { get; set; }
    }

    internal class Program
    {
        const string OsrmUri = "?overview=simplified&generate_hints=false&skip_waypoints=true&gaps=ignore&annotations=nodes&geometries=geojson&radiuses=";

        readonly static (double lng, double lat)[] Locations = 
        {
            (10.757938, 52.437444),
            (10.764379, 52.437314),
            (10.770562, 52.439067),
            (10.773268, 52.436633),
        };

        static async Task Main(string[] args)
        {
            const string HttpClientMapMatch = "HttpClientMapMatch";

            ServiceProvider serviceProvider = new ServiceCollection()
                .AddHttpClient(HttpClientMapMatch, httpClient =>
                {
                    httpClient.BaseAddress = new Uri("https://router.project-osrm.org/match/v1/driving/");
                }).Services.BuildServiceProvider();

            IHttpClientFactory? httpClientFactory = serviceProvider.GetService<IHttpClientFactory>();
            HttpClient? httpClient = httpClientFactory?.CreateClient(HttpClientMapMatch);
            if (httpClient == null)
            {
                Console.WriteLine("Error httpClient is null");
                return;
            }

            IEnumerable<long> nodes = await GetOsmNodesAsync(httpClient, Locations);
            Console.WriteLine($"Map matched OSM node ids: {JsonSerializer.Serialize(nodes.OrderBy(node => node))}\n");
        }

        private static async Task<IEnumerable<long>> GetOsmNodesAsync(HttpClient httpClient, IEnumerable<(double lng, double lat)> locations)
        {
            IEnumerable<string> lngLats = locations
                .Select(location => $"{location.lng:F6},{location.lat:F6}")
                .ToList();

            IEnumerable<int> radiuses = locations
                .Select(location => 50)
                .ToList();

            string requestUri = string.Join(";", lngLats) + OsrmUri + string.Join(";", radiuses);
            OsrmResponse? osrmResponse = await httpClient.GetFromJsonAsync<OsrmResponse>(requestUri);
            if (osrmResponse?.code == "Ok" && osrmResponse.matchings != null)
            {
                return osrmResponse.matchings
                    .Where(matching => matching != null)
                    .SelectMany(matching => matching.legs ?? Enumerable.Empty<Leg>())
                    .Where(leg => leg != null && leg.annotation != null && leg.annotation.nodes != null)
                    // How to fix dereference of a possibly null value in the next line?
                    .SelectMany(leg => leg.annotation.nodes ?? Enumerable.Empty<long>())
                    // eliminate duplicate node ids by converting to an ISet
                    .ToHashSet()
                    .ToList();
            }

            return Enumerable.Empty<long>();
        }
    }
}