MongoDB C#/.NET Driver：UUID与ISODate反序列化异常求助

MongoDB集合中的文档如下：

{
  _id: new UUID("31daac77-bcbc-4cd5-bb93-382440f46f16"),
  CompanyId: 'XYZ',
  RequestDate: ISODate("2023-01-10T07:52:32.840Z")
}

对应的C#实体类：

public class ReportRequest
{
    public ReportRequest(string companyId)
    {
        this.Id = Guid.NewGuid();
        this.CompanyId = companyId;
        this.RequestDate = DateTime.UtcNow;
    }

    [BsonId]
    [BsonElement("_id")]
    [BsonGuidRepresentation(GuidRepresentation.Standard)]
    public Guid Id { get; }

    [BsonElement]
    public string CompanyId { get; }

    [BsonElement]
    [BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
    public DateTime RequestDate { get; }
}

查询代码：

public async Task PrintAvailableReportDatesAsync(string companyId)
{
    var filter = Builders<ReportRequest>.Filter.Eq(r => r.CompanyId, companyId);
    var cursor = await _collection.FindAsync(filter);
    var reportRequests = await cursor.ToListAsync();

    foreach (var req in reportRequests)
        Console.WriteLine($"Id: {req.Id.ToString()}, Date: {req.RequestDate}");
}

期望输出：

Id: 31daac77-bcbc-4cd5-bb93-382440f46f16, Date: 2023-01-10T07:52:32.840Z

实际输出：

Id: 00000000-0000-0000-0000-000000000000, Date: 0001-01-01T00:00:00

你的ReportRequest类中所有属性都只有只读的getter（get;），没有setter。MongoDB .NET驱动在反序列化文档到实体类时，需要能够设置这些属性的值，但当前的只读属性无法被驱动赋值，所以只能使用类型的默认值（Guid默认是全0，DateTime默认是0001-01-01）。

有几种可行的修正方式：

方式1：添加私有setter #

给每个属性添加私有setter，这样驱动可以通过反射访问私有成员来赋值，同时保持外部代码无法修改属性：

public class ReportRequest
{
    public ReportRequest(string companyId)
    {
        this.Id = Guid.NewGuid();
        this.CompanyId = companyId;
        this.RequestDate = DateTime.UtcNow;
    }

    [BsonId]
    [BsonElement("_id")]
    [BsonGuidRepresentation(GuidRepresentation.Standard)]
    public Guid Id { get; private set; }

    [BsonElement]
    public string CompanyId { get; private set; }

    [BsonElement]
    [BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
    public DateTime RequestDate { get; private set; }
}

方式2：使用C# 9+的init关键字 #

如果使用C# 9及以上版本，可以用init代替私有setter，它允许在对象初始化时赋值，之后属性变为只读：

public class ReportRequest
{
    public ReportRequest(string companyId)
    {
        this.Id = Guid.NewGuid();
        this.CompanyId = companyId;
        this.RequestDate = DateTime.UtcNow;
    }

    [BsonId]
    [BsonElement("_id")]
    [BsonGuidRepresentation(GuidRepresentation.Standard)]
    public Guid Id { get; init; }

    [BsonElement]
    public string CompanyId { get; init; }

    [BsonElement]
    [BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
    public DateTime RequestDate { get; init; }
}

方式3：添加无参构造函数（配合私有setter） #

如果驱动需要无参构造函数（旧版本驱动可能有此要求），可以添加私有无参构造函数，同时保留私有setter：

public class ReportRequest
{
    private ReportRequest() { } // 供驱动反序列化使用

    public ReportRequest(string companyId)
    {
        this.Id = Guid.NewGuid();
        this.CompanyId = companyId;
        this.RequestDate = DateTime.UtcNow;
    }

    [BsonId]
    [BsonElement("_id")]
    [BsonGuidRepresentation(GuidRepresentation.Standard)]
    public Guid Id { get; private set; }

    [BsonElement]
    public string CompanyId { get; private set; }

    [BsonElement]
    [BsonDateTimeOptions(Kind = DateTimeKind.Utc)]
    public DateTime RequestDate { get; private set; }
}

内容的提问来源于stack exchange，提问作者Brian Reading