如何使用C#官方驱动程序创建具有Null值的Bson文档? [英] How to create Bson Document with Null value using C# official driver?

问题描述

我有带有3个字符串字段(国家，省，城市)的对象.它们可以包含null或某些字符串名称.

I have objects with 3 string fields Country, Province, City. They can contain null or some string name.

我想用完全相同的值查询所有数据.

I wanna query all data with the exact same values.

例如，我需要所有数据，其中

For Example i need all data where

City = null,
Province = "WA",
Country = "USA"

我创建了BsonDocument:

I created BsonDocument:

var lookup = new QueryDocument
{
    {"GeoPosition.City", userLocation.City},
    {"GeoPosition.Province", userLocation.Province},
    {"GeoPosition.Country", userLocation.Country}
};

但是空字段被扔掉了，文档看起来像这样:

But null field was thrown away and document looks like:

{
    "GeoPosition.Province" : "WA",
    "GeoPosition.Country" : "USA"
}

如果我想使用

Query.EQ("GeoPosition.City", userLocation.City)

我有一个例外，说参数不能为空.

I have exception saying that parametr cant be null.

正如我在文档中所看到的，如果值等于null，则构建查询检查不会有任何问题.因此，这是C#驱动程序的问题.有什么想法可以解决这个问题吗?

As i see in documentation there is no problem in building query cheking if value equals null. So that is a problem with C# driver. Any ideas how to solve this problem?

推荐答案

取决于城市变量的数据类型.如果city变量的类型为BsonValue，则可以使用??.直接操作:

Depends on the data type of your city variable. If the city variable is of type BsonValue you can use the ?? operator directly:

BsonValue city = null;
var query = Query.EQ("city", city ?? BsonNull.Value);
Console.WriteLine(query.ToJson());

如果您的city变量为字符串类型，则需要进行额外的转换以使编译器满意:

If your city variable is of type string you need an extra conversion cast to make the compiler happy:

string city = null;
var query = Query.EQ("city", (BsonValue)city ?? BsonNull.Value);
Console.WriteLine(query.ToJson());

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