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Object Mapping

LiteDB supports POCO classes to strongly type documents. When you get a LiteCollection instance from LiteDatabase.GetCollection<T>, T will be your document type. If T is not a BsonDocument, LiteDB internally maps your class to BsonDocument. To do this, LiteDB uses the BsonMapper class:

// Simple strongly-typed document
public class Customer
    public ObjectId CustomerId { get; set; }
    public string Name { get; set; }
    public DateTime CreateDate { get; set; }
    public List<Phone> Phones { get; set; }
    public bool IsActive { get; set; }

var typedCustomerCollection = db.GetCollection<Customer>("customer");

var schemelessCollection = db.GetCollection("customer"); // <T> is BsonDocument

Mapper conventions

BsonMapper.ToDocument() auto converts each property of a class to a document field following these conventions:

  • Properties can be read-only or read/write
  • The class should have an Id property, <ClassName>Id property, a property with [BsonId] attribute or mapped by the fluent API.
  • A property can be decorated with [BsonIgnore] in order not to be mapped to a document field
  • A property can be decorated with [BsonField("fieldName")] to customize the name of the document field
  • No circular references are allowed
  • Max depth of 20 inner classes
  • Class fields are not converted to document
  • You can use BsonMapper global instance (BsonMapper.Global) or a custom instance and pass to LiteDatabase in its constructor. Keep this instance in a single place to avoid re-creating the mappings each time you use a database.

In addition to basic BSON types, BsonMapper maps others .NET types to BSON data type:

.NET type BSON type
Int16, UInt16, Byte, SByte Int32
UInt32 , UInt64 Int64
Single Double
Char, Enum String
IList<T> Array
T[] Array
NameValueCollection Document
IDictionary<K,T> Document
Any other .NET type Document
  • Nullable<T> are accepted. If value is null the BSON type is Null, otherwise the mapper will use T?.
  • For IDictionary<K, T>, K key must be String or a simple type (convertible using Convert.ToString(..)).


Starting with version 5 of LiteDB you can use BsonCtorAttribute to indicate which constructor the mapper must use. Fields no longer need to have a public setter and can be initialized by the constructor.

public class Customer
    public ObjectId CustomerId { get; }
    public string Name { get; }
    public DateTime CreationDate { get; }
    public bool IsActive { get; }

    public Customer(string name, bool isActive)
        CustomerId = ObjectId.NewObjectId();
        Name = name;
        CreationDate = DateTime.Now;
        IsActive = true;

    public Customer(ObjectId _id, string name, DateTime creationDate, bool isActive)
        CustomerId = _id;
        Name = name;
        CreationDate = creationDate;
        IsActive = isActive;

var typedCustomerCollection = db.GetCollection<Customer>("customer");

When GetCollection<T> is called, it tries to create instances of T by searching for a constructor in the following order:

  • First, it searches for a constructor with BsonCtorAttribute
  • Then, it searches for a parameterless constructor
  • Finally, it searches for a constructor whose parameters’ names match with the names of the fields in the document

Register a custom type

You can register your own map function, using the RegisterType<T> instance method. To register, you need to provide both serialize and deserialize functions.

    serialize: (uri) => uri.AbsoluteUri,
    deserialize: (bson) => new Uri(bson.AsString)
  • serialize function receives an instance of T and returns an instance of BsonValue
  • deserialize function receives an instance of BsonValue and returns an instance of T
  • RegisterType supports complex objects via BsonDocument or BsonArray

Mapping options

BsonMapper class settings:

Name Default Description
SerializeNullValues false Serialize field if value is null
TrimWhitespace true Trim strings properties before mapping to document
EmptyStringToNull true Empty strings convert to null
ResolvePropertyName (s) => s A function to map property name to document field name

BsonMapper offers 2 predefined functions to resolve property names: UseCamelCase() and UseLowerCaseDelimiter('_').


public class Customer
    public int CustomerId { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

var doc = BsonMapper.Global.ToDocument(new Customer { FirstName = "John", LastName = "Doe" });

var id = doc["_id"].AsInt;
var john = doc["first_name"].AsString;
var doe = doc["customerLastName"].AsString;


In v4, AutoId is moved to engine level (BsonDocument). There are 4 built-in auto-id functions implemented:

  • ObjectId: ObjectId.NewObjectId()
  • Guid: Guid.NewGuid() method
  • Int32/Int64: New collection sequence
  • DateTime: DateTime.Now

AutoId is only used when there is no _id field in the document upon insertion. In strongly-typed documents, BsonMapper removes the _id field for empty values (like 0 for Int or Guid.Empty for Guid). Please note that AutoId requires the id field to have a public setter.

Fluent Mapping

LiteDB offers a complete fluent API to create custom mappings without using attributes, keeping you domain classes without external references.

Fluent API uses EntityBuilder to add custom mappings to your classes.

var mapper = BsonMapper.Global;

    .Id(x => x.MyCustomKey) // set your document ID
    .Ignore(x => x.DoNotSerializeThis) // ignore this property (do not store)
    .Field(x => x.CustomerName, "cust_name"); // rename document field