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Object Models - .NET SDK

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  • Create an Object Model
  • Object Schema
  • Property Annotations
  • Primary Key
  • Indexes
  • Full-Text Search Indexes
  • Default Field Values
  • Ignore a Property
  • Rename a Property
  • Rename a Class
  • Custom Setters
  • Omit Classes from your Realm Schema
  • Required and Optional Properties
  • Ignoring Nullability

Create an Object Model

Realm classes are regular C# classes that define the Realm schema.

Important

Inheritance

All Realm objects inherit from the IRealmObject, IEmbeddedObject, or IAsymmetricObject interface and must be declared partial classes.

In versions of the .NET SDK older than 10.18.0, objects derive from RealmObject, EmbeddedObject, or AsymmetricObject base classes. This approach to Realm model definition is still supported, but does not include new features such as the nullability annotations. In a future SDK release, the base classes will become deprecated. You should use the interfaces for any new classes that you write and should consider migrating your existing classes.

Note

Class names are limited to a maximum of 57 UTF-8 characters.

Object Schema

An object schema is a configuration object that defines the properties and relationships of a Realm object. Realm client applications define object schemas with the native class implementation in their respective language using the Object Schema.

Object schemas specify constraints on object properties such as the data type of each property and whether or not a property is required. Schemas can also define relationships between object types in a realm.

Every App has a App Services Schema composed of a list of object schemas for each type of object that the realms in that application may contain. Realm guarantees that all objects in a realm conform to the schema for their object type and validates objects whenever they're created, modified, or deleted.

Property Annotations

Schema properties are standard C# properties on a RealmObject. There are several property annotations that you can use to more finely define how a Realm handles a specific property.

Primary Key

A primary key is a property that uniquely identifies an object. You can create a primary key with any of the following types (or their nullable counterparts):

  • ObjectId

  • UUID

  • string

  • char

  • byte

  • short

  • int

  • long

You may define a primary key on a single property for an object type as part of the object schema. Realm automatically indexes primary key properties, which allows you to efficiently read and modify objects based on their primary key.

If an object type has a primary key, then all objects of that type must include the primary key property with a value that is unique among objects of the same type in a realm.

Note

Once you assign a property as a primary key, you cannot change it.

The following example demonstrates how to designate a primary key in an object schema:

public partial class Dog : IRealmObject
{
    [PrimaryKey]
    public string Name { get; set; }
    public int Age { get; set; }
    public Person? Owner { get; set; }
}

Tip

See also:

Indexes

Indexes significantly improve query times in a Realm. Without indexes, Realm scans every document in a collection to select the documents that match the given query. However, if an applicable index exists for a query, Realm uses the index to limit the number of documents that it must inspect.

You can index properties with the following types:

  • bool

  • byte

  • short

  • int

  • long

  • DateTimeOffset

  • char

  • string

  • ObjectId

  • UUID

Note

Adding an index speeds up queries at the cost of slightly slower write times and additional storage and memory overhead. Indexes require space in your realm file, so adding an index to a property increases disk space consumed by your realm file. Each index entry is a minimum of 12 bytes.

To index a property, use the Indexed attribute. With the Indexed attribute, you can specify the type of index on the property by using the IndexType enum. In the following example, we have a default ("General") index on the Name property:

public partial class Person : IRealmObject
{
    [Indexed(IndexType.General)]
    public string Name { get; set; }
    [Indexed(IndexType.FullText)]
    public string Biography { get; set; }
}

Note

When you create an index, you are creating it on the local realm and not on an Atlas collection. If you need to query an Atlas collection directly and want to improve performance, refer to Create, View, Drop, and Hide Indexes.

Full-Text Search Indexes

In addition to standard indexes, Realm also supports Full-Text Search (FTS) indexes on string properties. While you can query a string field with or without a standard index, an FTS index enables searching for multiple words and phrases and excluding others.

For more information on querying full-text indexes, see Full Text Search (LINQ) and Full Text Search (RQL).

To index an FTS property, use the Indexed attribute with the IndexType.FullText enum. In the following example, we have a FullText index on the Biography property:

public partial class Person : IRealmObject
{
    [Indexed(IndexType.General)]
    public string Name { get; set; }
    [Indexed(IndexType.FullText)]
    public string Biography { get; set; }
}

Default Field Values

You can use the built-in language features to assign a default value to a property. In C#, you can assign a default value on primitives in the property declaration. You cannot set a default value on a collection, except to set it to null!. Even if you set a collection to null!, collections are always initialized on first access, so will never be null.

public partial class Person : IRealmObject
{
    public string Name { get; set; } = "foo";
    public IList<PhoneNumber> PhoneNumbers { get; } = null!;
}

Note

Default Values and Nullability

While default values ensure that a newly created object cannot contain a value of null (unless you specify a default value of null), they do not impact the nullability of a property. To make a property non-nullable, see Required Properties.

Ignore a Property

If you don't want to save a property in your model to a realm, you can ignore that property. A property is ignored by default if it is not autoimplemented or does not have a setter.

Ignore a property from a Realm object model with the Ignored attribute:

// Rather than store an Image in Realm,
// store the path to the Image...
public string ThumbnailPath { get; set; }
// ...and the Image itself can be
// in-memory when the app is running:
[Ignored]
public Image? Thumbnail { get; set; }

Rename a Property

By default, Realm uses the name defined in the model class to represent properties internally. In some cases you might want to change this behavior:

  • To make it easier to work across platforms, since naming conventions differ.

  • To change a property name in .NET without forcing a migration.

Choosing an internal name that differs from the name used in model classes has the following implications:

  • Migrations must use the internal name when creating classes and properties.

  • Schema errors reported will use the internal name.

Use the [MapTo] attribute to rename a property:

public partial class Person : IRealmObject
{
    [MapTo("moniker")]
    public string Name { get; set; }
}

Rename a Class

By default, Realm uses the name defined in the model class to represent classes internally. In some cases you might want to change this behavior:

  • To support multiple model classes with the same simple name in different namespaces.

  • To make it easier to work across platforms, since naming conventions differ.

  • To use a class name that is longer than the 57 character limit enforced by Realm.

  • To change a class name in .NET without forcing a migration.

Use the [MapTo] attribute to rename a class:

[MapTo("Human")]
public partial class Person : IRealmObject
{
    public string Name { get; set; }
}

Custom Setters

Realm will not store a property with a custom setter. To use a custom setter, store the property value in a private property and then map that value to a public property with the custom setter. Realm will store the private property, while you modify its value via the public property. In the following code, the private email property is stored in the realm, but the public Email property, which provides validation, is not persisted:

// This property will be stored in the Realm
private string email { get; set; }
// Custom validation of the email property.
// This property is *not* stored in Realm.
public string Email
{
    get { return email; }
    set
    {
        if (!value.Contains("@")) throw new Exception("Invalid email address");
        email = value;
    }
}

Omit Classes from your Realm Schema

By default, your application's Realm Schema includes all classes that implement IRealmObject or IEmbeddedObject. If you only want to include a subset of these classes in your Realm Schema, you can update your configuration to include the specific classes you want:

// Declare your schema
partial class LoneClass : IRealmObject
{
    public string Name { get; set; }
}
class AnotherClass
{
    private void SetUpMyRealmConfig()
    {
        // Define your config with a single class
        var config = new RealmConfiguration("RealmWithOneClass.realm");
        config.Schema = new[] { typeof(LoneClass) };
        // Or, specify multiple classes to use in the Realm
        config.Schema = new[] { typeof(Dog), typeof(Cat) };
    }
}

Required and Optional Properties

In C#, value types, such as int and bool, are implicitly non-nullable. However, they can be made optional by using the question mark (?) notation.

Beginning with C# 8.0, nullable reference types were introduced. If your project is using C# 8.0 or later, you can also declare reference types, such as string and byte[], as nullable with ?.

Note

Beginning with .NET 6.0, the nullable context is enabled by default for new projects. For older projects, you can manually enable it. For more information, refer to https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-reference-types#setting-the-nullable-context.

The Realm .NET SDK fully supports the nullable-aware context and uses nullability to determine whether a property is required or optional. The SDK has the following rules:

  • Realm assumes that both value- and reference-type properties are required if you do not designate them as nullable. If you designate them as nullable by using ?, Realm considers them optional.

  • You must declare properties that are Realm object types as nullable.

  • You cannot declare collections (list, sets, backlinks, and dictionaries) as nullable, but their parameters may be nullable according to the following rules:

    • For all types of collections, if the parameters are primitives (value- or reference-types), they can be required or nullable.

    • For lists, sets, and backlinks, if the parameters are Realm objects, they cannot be nullable.

    • For dictionaries with a value type of Realm object, you must declare the value type parameter as nullable.

The following code snippet demonstrates these rules:

#nullable enable
public partial class Person : IRealmObject
{
    /* Reference Types */
    public string NonNullableName { get; set; }
    public string? NullableName { get; set; }
    public byte[] NonNullableArray { get; set; }
    public byte[]? NullableArray { get; set; }
    /* Value Types */
    public int NonNullableInt { get; set; }
    public int? NullableInt { get; set; }
    /* Realm Objects */
    public Dog? NullableDog { get; set; }
    // public Dog NonNullableDog { get; set; } // Compile-time error
    /* Collections of Primitives */
    public IList<int> IntListWithNonNullableValues { get; }
    public IList<int?> IntListWithNullableValues { get; }
    // public IList<int>? NullableListOfInts { get; } // Compile-time error
    /* Collections of Realm Objects */
    public IList<Dog> ListOfNonNullableObjects { get; }
    // public IList<Dog>? NullableListOfObjects { get; }  // Compile-time error
    // public IList<Dog?> ListOfNullableObjects { get; } // Compile-time error
    public ISet<Dog> SetOfNonNullableObjects { get; }
    // public ISet<Dog>? NullableSetOfObjects { get; }  // Compile-time error
    // public ISet<Dog?> SetOfNullableObjects { get; } // Compile-time error
    public IDictionary<string, Dog?> DictionaryOfNullableObjects { get; }
    // public IDictionary<string, Dog> DictionaryOfNonNullableObjects { get; } // Compile-time error
    // public IDictionary<string, Dog>? NullableDictionaryOfObjects { get; } // Compile-time error
    [Backlink(nameof(Dog.Person))]
    public IQueryable<Dog> MyDogs { get; }
    // [Backlink(nameof(Dog.People))]
    // public IQueryable<Dog?> MyDogs { get; } // Compile-time error
}

Note

If you are using the older schema type definition (your classes derive from the RealmObject base class), or you do not have nullability enabled, you will need to use the [Required] attribute for any required string and byte[] property.

Ignoring Nullability

You may prefer to have more flexibility in defining the nullability of properties in your Realm objects. You can do so by setting realm.ignore_objects_nullability = true in a global configuration file.

If you enable realm.ignore_objects_nullability, nullability annotations will be ignored on Realm object properties, including collections of Realm objects.

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