Check if Field Equals Null in MongoDb C# Driver 2.0

Question

I have a very simple query for mongo:

db.items.find( { MyFieldName: { $exists: true, $eq: null } } );

Not that it needs to be explained, but it finds documents which have a MyFieldName and where the value of that field is null. It seems like this would be really simple to do with the C# driver:

var fieldExistsFilter= Builders<BsonDocument>.Filter.Exists("MyFieldName", true);
var fieldValueIsNullFilter = Builders<BsonDocument>.Filter.Eq("MyFieldName", null);

However the second filter, fieldValueIsNullFilter, does not build if I try to check for null. It works fine if I write "testString" or anything like that, but not with null.

tl:dr; version: How do I create a filter to check if field is null in MongoDb C# driver?

Note, I checked other answers and they recommend $exists does what I want - it does not, as per mongo docs:

When is true, $exists matches the documents that contain the field, including documents where the field value is null. If is false, the query returns only the documents that do not contain the field.

Community · Accepted Answer · 2017-05-23 12:32:11Z

47

This actually works as expected with a little modification, which depends on:

BsonNull.Value

See this question for details: How to create Bson Document with Null value using C# official driver?

So the query is:

var fieldValueIsNullFilter = Builders<BsonDocument>.Filter.Eq("MyFieldName", BsonNull.Value);

Initially, I didn't realize BsonNull has a Value property.

edited May 23, 2017 at 12:32

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answered Jun 14, 2016 at 14:59

VSO

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2 Comments

Hari Prasad Over a year ago

It is the answer, you should accept it as an answer. Nothing wrong in it.

VSO Over a year ago

Alright, will do. Looks like there is a two day count-down before I can accept it.

Mike Polo · Accepted Answer · 2019-12-06 12:19:29Z

5

In addition to the comment above you can write like this if you have some entity (depends on the property type):

public class CustomEntity
{
    public string Id { get; set; }

    public string StringProperty { get; set; }

    public DateTime? DateTimeProperty { get; set; }
}


var filterIfStringPropertyNull = Builders<CustomEntity>.Filter.Eq(o => o.StringProperty, null); // if property is string

var filterIfDatePropertyNull = Builders<CustomEntity>.Filter.Eq(o => o.DateTimeProperty, BsonNull.Value.ToNullableUniversalTime());  // if property is DateTime?

And so on. It could be easier

answered Dec 6, 2019 at 12:19

Mike Polo

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1 Comment

ViV Over a year ago

Thanks for mentioning BsonNull.Value.ToNullableUniversalTime()

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