Creating MongoDB Unique Key with C#

The unique index only needs to be created once, after that any document inserts that contain a duplicate email address will fail. Here's an example:

var server = MongoServer.Create("mongodb://localhost");
var db = server.GetDatabase("myapp");

var users = db.GetCollection<User>("users");

users.EnsureIndex(new IndexKeysBuilder()
    .Ascending("EmailAddress"), IndexOptions.SetUnique(true));

var user1 = new User { EmailAddress = "[email protected]" };
var user2 = new User { EmailAddress = "[email protected]" };

try
{
    users.Save(user1, WriteConcern.Acknowledged);
    users.Save(user2, WriteConcern.Acknowledged);  // <-- throws MongoSafeModeException
}
catch (MongoSafeModeException ex)
{
    Console.WriteLine(ex.Message);
}

EnsureIndex() is deprecated/obsolete per the C# mongo drivers version 2.0 spec: http://api.mongodb.org/csharp/current/html/M_MongoDB_Driver_MongoCollection_EnsureIndex_2.htm

heres how to do it async and via 2.0 code:

var mongoClient = new MongoClient("connection");
var db = mongoClient.GetDatabase("database");

var options = new CreateIndexOptions() { Unique = true };
var field = new StringFieldDefinition<User>("EmailAddress");
var indexDefinition = new IndexKeysDefinitionBuilder<User>().Ascending(field);
await db.GetCollection<Users>("users").Indexes.CreateOneAsync(indexDefinition, options);

In case of a non-string index:

   var options = new CreateIndexOptions() { Unique = true };
   IndexKeysDefinition<Foo> keyCode = "{ Code: 1 }";
   var codeIndexModel = new CreateIndexModel<Foo>(keyCode, options);

As of 2.8 below is the way to create a index. Please note last two lines. CreateOneAsync(indexDefinition, options) is obsolete.

var mongoClient = new MongoClient("connection");
var db = mongoClient.GetDatabase("database");

var options = new CreateIndexOptions() { Unique = true };
var field = new StringFieldDefinition<User>("EmailAddress");
var indexDefinition = new IndexKeysDefinitionBuilder<User>().Ascending(field);

var indexModel = new CreateIndexModel<User>(indexDefinition,options);
await db.GetCollection<Users>("users").Indexes.CreateOneAsync(indexModel);

Your code looks right. Here's a full running program for you to compare against:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using MongoDB.Bson;
using MongoDB.Driver;
using MongoDB.Driver.Builders;

namespace TestEnsureIndexOnEmailAddress {
    public class User {
        public ObjectId Id;
        public string FirstName;
        public string LastName;
        public string EmailAddress;
    }

    public static class Program {
        public static void Main(string[] args) {
            var server = MongoServer.Create("mongodb://localhost/?safe=true");
            var database = server["test"];
            var users = database.GetCollection<User>("users");
            if (users.Exists()) { users.Drop(); }

            users.EnsureIndex(IndexKeys.Ascending("EmailAddress"), IndexOptions.SetUnique(true));
            var john = new User { FirstName = "John", LastName = "Smith", EmailAddress = "[email protected]" };
            users.Insert(john);
            var joe = new User { FirstName = "Joe", LastName = "Smith", EmailAddress = "[email protected]" };
            users.Insert(joe); // should throw exception
        }
    }
}

You can also use the mongo shell to confirm that the index got created:

> db.users.getIndexes()
[
        {
                "name" : "_id_",
                "ns" : "test.users",
                "key" : {
                        "_id" : 1
                },
                "v" : 0
        },
        {
                "_id" : ObjectId("4de8152ee447ad2550e3b5fd"),
                "name" : "EmailAddress_1",
                "ns" : "test.users",
                "key" : {
                        "EmailAddress" : 1
                },
                "unique" : true,
                "v" : 0
        }
]
>

In my opinion a better and more feasible way to do this is by creating a custom attribute, then with c# you could get the properties decorated with that attribute and add a unique index, this approach would be generic and very simple to use. Example below:

My custom attribute: MongoUniqueStringIndex

 public class DynamicFieldConfig
{
    public string InputType { get; set; }
    [MongoUniqueStringIndex]
    public string Name { get; set; }
    public string Label { get; set; }
    public List<string> Options { get; set; } = new List<string>();
    public string LookupTypeName { get; set; }
    public int Type { get; set; } //int to enum
    public string Value { get; set; }
    public List<DynamicFieldValidation> Validations { get; set; } = new List<DynamicFieldValidation>();
}

then where you get the collection, for example in my Data Access class I do this:

        public MongoDataAccess(IDatabaseSettings settings)
    {
        // todo: create a connection service/factory to get connection
        var client = new MongoClient(settings.ConnectionString);
        var database = client.GetDatabase(settings.DatabaseName);
        
        var entityName = new Pluralizer().Pluralize(typeof(TMongoEntity).Name);
        _entityStore = database.GetCollection<TMongoEntity>(entityName);

        var uniqueStringIndexProperties = typeof(TMongoEntity).GetProperties().Where(
            prop => Attribute.IsDefined(prop, typeof(MongoUniqueStringIndex))).ToList();
        if (uniqueStringIndexProperties.Any())
            foreach (var propertyInfo in uniqueStringIndexProperties)
            {
                var options = new CreateIndexOptions { Unique = true };
                var propertyInfoName = propertyInfo.Name;
                var field = new StringFieldDefinition<TMongoEntity>(propertyInfoName);
                var indexDefinition = new IndexKeysDefinitionBuilder<TMongoEntity>().Ascending(field);
                var indexModel = new CreateIndexModel<TMongoEntity>(indexDefinition, options);
                _entityStore.Indexes.CreateOne(indexModel);
            }
    }

here's a working program using MongoDB.Entities (disclaimer: I'm the author)

using System;
using MongoDB.Driver;
using MongoDB.Entities;

namespace StackOverflow
{
    public class User : Entity
    {
        public string Name { get; set; }
        public string EmailAddress { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            new DB("test");

            DB.Index<User>()
              .Options(o => o.Unique = true, o => o.Background = false)
              .Key(u => u.EmailAddress, Type.Ascending)
              .Create();

            var user1 = new User { Name = "First User", EmailAddress = "[email protected]" };
            user1.Save();

            try
            {
                var user2 = new User { Name = "Second User", EmailAddress = "[email protected]" };
                user2.Save();
            }
            catch (MongoWriteException x)
            {
                Console.WriteLine(x.Message);
                Console.ReadKey();
            }
        }
    }
}

trying to create a second user with the same email address results in the following exception:

A write operation resulted in an error. E11000 duplicate key error collection: test.User index: EmailAddress(Asc) dup key: { : "[email protected]" }

You have to wrap your IndexKeysDefinition<TDocument> into a CreateIndexModel<TDocument> to be able to specify options, such as Unique = true. Example:

collection.Indexes.CreateOne(
  new CreateIndexModel<DbModel>(
    Builders<DbModel>.IndexKeys.Ascending(d => d.SomeField),
    new CreateIndexOptions { Unique = true }
));

See also https://stackoverflow.com/a/55210026/2279059.

All answers are missing on one crucial point is to never create the index if already there

var db = _mongoClient.GetDatabase("db");
var coll = db.GetCollection<Coll>("collName");
var indexes = (await (await coll.Indexes.ListAsync()).ToListAsync()).Select(_ => _.GetElement("name").Value.ToString()).ToList();
// If index not present create it else skip.
if (indexes.Where(_ => _.Equals("<Index Name>")).Count() == 0)
{
    // Create Index here
}

I'm using <PackageReference Include="MongoDB.Driver" Version="2.11.5" />

this is my code to create a unique index

var client = new MongoClient(mongourl);
var database = client.GetDatabase(dbName);

IMongoCollection<CollectionModel> myCollection = database.GetCollection<CollectionModel>("collection_name");

myCollection .Indexes.CreateOne(
      new CreateIndexModel<CollectionModel>(Builders<CollectionModel>.IndexKeys.Descending(model => model.YOUR_PROPERTY),
      new CreateIndexOptions { Unique = true }));