https://docs.mongodb.com/getting-started/csharp/query/

You can use the Find and FindAsync methods to issue a query to retrieve data from a collection in MongoDB.

All queries in MongoDB have the scope of a single collection.

Queries can return all documents in a collection or only the documents that match a specified filter or criteria.

You can specify the filter or criteria in a BsonDocument and pass as a parameter to the Find andFindAsync methods.

The FindAsync method returns query results in a IAsyncCursor, which is an iterable object that yields documents.

The Find method returns a IFindFluent object.

You can use the ToListAsync method to return the results as a list that contains all the documents returned by the cursor. 

ToListAsync requires holding the entire result set in memory.

Prerequisites

The examples in this section use the restaurants collection in the test database.

For instructions on populating the collection with the sample dataset, see Import Example Dataset.

Follow the Connect to MongoDB step to connect to a running MongoDB instance and declare and define the variable _database to access the test database.

Include the following using statements.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using FluentAssertions;
using MongoDB.Bson;

The examples use FluentAssertions to test the results.

If you are not using FluentAssertions, omit the using FluentAssertions; statement and the FluentAssertions test code below.

Query for All Documents in a Collection

To return all documents in a collection, call the FindAsync method with an empty filter document.

For example, the following operation queries for all documents in the restaurants collection.

 private async void Query()
        {
            var count = 0;
            var collection = MongoDatabase.GetCollection<BsonDocument>(collectionName);
            var filter = new BsonDocument();

            var cursor = await collection.FindAsync(filter);
            while (await cursor.MoveNextAsync())
            {
                var bacth = cursor.Current;
                foreach (var document in bacth)
                {
                    count++;
                    //process document
                }
            }
            Console.WriteLine($@"{nameof(count)}={count}");
        }

Optional.

The following code uses FluentAssertions to test the results.

If you are not usingFluentAssertions, omit.

count.Should().Be(25359);

If you have inserted, modified, or removed documents, such as specified in the other sections of the Getting Started guide, your count may differ.

The result set contains all documents in the restaurants collection.

Specify Equality Conditions

With the C# MongoDB driver, you can use the FilterDefinitionBuilder to implement the filter document.

For example, FilterDefinitionBuilder provides an Eq method to implement a filter document that specifies an equality condition:

var filter = Builders<BsonDocument>.Filter.Eq(<field>, <value>);

 If the <field> is in an embedded document or an array, use dot notation to access the field.

Query by a Top Level Field

The following operation finds documents whose borough field equals "Manhattan".

 private async void QueryTopLevelField()
        {
            var filter = Builders<BsonDocument>.Filter.Eq("borough", "Manhattan");
            var result = await collection.Find(filter).ToListAsync();
        }

Optional.

The following code uses FluentAssertions to test the results.

If you are not usingFluentAssertions, omit.

result.Count().Should().Be(10259);

If you have inserted, modified, or removed documents, such as specified in the other sections of the Getting Started guide, your count may differ.

Specify Conditions with Operators

Greater Than Operator ($gt)

Less Than Operator ($lt)

Combine Conditions

 You can combine multiple query conditions in logical conjunction (AND) and logical disjunctions (OR).

Logical AND

You can specify a logical conjunction (AND) for a list of query conditions by joining the conditions with an ampersand (e.g. &).

var collection = _database.GetCollection<BsonDocument>("restaurants");
var builder = Builders<BsonDocument>.Filter;
var filter = builder.Eq("cuisine", "Italian") & builder.Eq("address.zipcode", "10075");
var result = await collection.Find(filter).ToListAsync();

Optional. The following code uses FluentAssertions to test the results. If you are not usingFluentAssertions, omit.

result.Count().Should().Be(15);

If you have inserted, modified, or removed documents, such as specified in the other sections of the Getting Started guide, your count may differ.

The result set includes only the documents that matched all specified criteria.

Logical OR

Sort Query Results

Additional Information