EF Core 8 RC1: Complex types as value objects

Arthur Vickers

The first release candidate of Entity Framework Core (EF Core) 8 is available on NuGet today!

Basic information

EF Core 8, or just EF8, is the successor to EF Core 7, and is scheduled for release in November 2023, at the same time as .NET 8.

EF8 requires .NET 8 and this RC1 release should be used with the .NET 8 RC1 SDK.

EF8 will align with .NET 8 as a long-term support (LTS) release. See the .NET support policy for more information.

New in EF8

EF8 RC1 contains all the major feature features we intend to ship in EF8, although further tweaks and bug fixes are coming for RC2. These features include:

In this post we’re going to take a more detailed look at using complex types to represent value objects.

Complex types as value objects

Objects saved to the database can be split into three broad categories:

  • Objects that are unstructured and hold a single value. For example, int, Guid, string, IPAddress. These are (somewhat loosely) called “primitive types”.
  • Objects that are structured to hold multiple values, and where the identity of the object is defined by a key value. For example, Blog, Post, Customer. These are called “entity types”.
  • Objects that are structured to hold multiple values, but the object has no key defining identity. For example, Address, Coordinate.

Prior to EF8, there was no good way to map the third type of object. Owned types can be used, but since owned types are actually entity types, they have a key and semantics based on the value of that key, even when the key value is hidden.

EF8 now supports “Complex Types” to cover this third type of object. Complex types in EF Core are very similar to complex types in EF6, but there are some differencee. Complex type objects:

  • Are not identified or tracked by key value.
  • Must be defined as part of an entity type. (In other words, you cannot have a DbSet of a complex type.)
  • Can be either .NET value types or reference types. (EF6 only supports reference types.)
  • Can share the same instance across multiple properties. (See below for more details. EF6 does not allow sharing.)

Simple example

For example, consider an Address type:

public class Address
{
    public required string Line1 { get; set; }
    public string? Line2 { get; set; }
    public required string City { get; set; }
    public required string Country { get; set; }
    public required string PostCode { get; set; }
}

Address is then used in three places in a simple customer/orders model:

public class Customer
{
    public int Id { get; set; }
    public required string Name { get; set; }
    public required Address Address { get; set; }
    public List<Order> Orders { get; } = new();
}

public class Order
{
    public int Id { get; set; }
    public required string Contents { get; set; }
    public required Address ShippingAddress { get; set; }
    public required Address BillingAddress { get; set; }
    public Customer Customer { get; set; } = null!;
}

Let’s create and save a customer with their address:

var customer = new Customer
{
    Name = "Willow",
    Address = new() { Line1 = "Barking Gate", City = "Walpole St Peter", Country = "UK", PostCode = "PE14 7AV" }
};

context.Add(customer);
await context.SaveChangesAsync();

This results in the following row being inserted into the database:

INSERT INTO [Customers] ([Name], [Address_City], [Address_Country], [Address_Line1], [Address_Line2], [Address_PostCode])
OUTPUT INSERTED.[Id]
VALUES (@p0, @p1, @p2, @p3, @p4, @p5);

Notice that the complex types do not get their own tables. Instead, they are saved inline to columns of the Customers table. This matches the table sharing behavior of owned types.

NOTE We don’t plan to allow complex types to be mapped to their own table. However, in a future release, we do plan to allow the complex type to be saved as a JSON document in a single column. Vote for Issue #31252 if this is important to you.

Now let’s say we want to ship an order to a customer and use the customer’s address as both the default billing an shipping address. The natural way to do this is to copy the Address object from the Customer into the Order. For example:

customer.Orders.Add(
    new Order { Contents = "Tesco Tasty Treats", BillingAddress = customer.Address, ShippingAddress = customer.Address, });

await context.SaveChangesAsync();

With complex types, this works as expected, and the address is inserted into the Orders table:

INSERT INTO [Orders] ([Contents], [CustomerId],
    [BillingAddress_City], [BillingAddress_Country], [BillingAddress_Line1], [BillingAddress_Line2], [BillingAddress_PostCode],
    [ShippingAddress_City], [ShippingAddress_Country], [ShippingAddress_Line1], [ShippingAddress_Line2], [ShippingAddress_PostCode])
OUTPUT INSERTED.[Id]
VALUES (@p0, @p1, @p2, @p3, @p4, @p5, @p6, @p7, @p8, @p9, @p10, @p11);

So far you might be saying, “but I could do this with owned types!” However, the “entity type” semantics of owned types quickly get in th way. For example, running the code above with owned types results in a slew of warnings and then an error:

warn: 8/20/2023 12:48:01.678 CoreEventId.DuplicateDependentEntityTypeInstanceWarning[10001] (Microsoft.EntityFrameworkCore.Update) 
      The same entity is being tracked as different entity types 'Order.BillingAddress#Address' and 'Customer.Address#Address' with defining navigations. If a property value changes, it will result in two store changes, which might not be the desired outcome.
warn: 8/20/2023 12:48:01.687 CoreEventId.DuplicateDependentEntityTypeInstanceWarning[10001] (Microsoft.EntityFrameworkCore.Update) 
      The same entity is being tracked as different entity types 'Order.ShippingAddress#Address' and 'Customer.Address#Address' with defining navigations. If a property value changes, it will result in two store changes, which might not be the desired outcome.
warn: 8/20/2023 12:48:01.687 CoreEventId.DuplicateDependentEntityTypeInstanceWarning[10001] (Microsoft.EntityFrameworkCore.Update)
      The same entity is being tracked as different entity types 'Order.ShippingAddress#Address' and 'Order.BillingAddress#Address' with defining navigations. If a property value changes, it will result in two store changes, which might not be the desired outcome.
fail: 8/20/2023 12:48:01.709 CoreEventId.SaveChangesFailed[10000] (Microsoft.EntityFrameworkCore.Update) 
      An exception occurred in the database while saving changes for context type 'NewInEfCore8.ComplexTypesSample+CustomerContext'.
      System.InvalidOperationException: Cannot save instance of 'Order.ShippingAddress#Address' because it is an owned entity without any reference to its owner. Owned entities can only be saved as part of an aggregate also including the owner entity.
         at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.PrepareToSave()
         ...

This is because a single instance of the Address entity type (with the same hidden key value) is being used for three different entity instances. On the other hand, sharing the same instance between complex properties is allowed, and so the code works as expected when using complex types.

Configuration of complex types

Complex types must be configured in the model using either mapping attributes or by calling ComplexProperty API in OnModelCreating. Complex types are not discovered by convention.

For example, the Address type can be configured using the ComplexTypeAttribute:

[ComplexType]
public class Address
{
    public required string Line1 { get; set; }
    public string? Line2 { get; set; }
    public required string City { get; set; }
    public required string Country { get; set; }
    public required string PostCode { get; set; }
}

Or in OnModelCreating:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Customer>()
        .ComplexProperty(e => e.Address);

    modelBuilder.Entity<Order>(b =>
    {
        b.ComplexProperty(e => e.BillingAddress);
        b.ComplexProperty(e => e.ShippingAddress);
    });
}

Mutability

In the example above, we ended up with the same Address instance used in three places. This is allowed and doesn’t cause any issues for EF Core when using complex types. However, sharing instances of the same reference type means that if a property value on the instance is modified, then that change will be reflected in all three usages. For example, following on from above, let’s change Line1 of the customer address and save the changes:

customer.Address.Line1 = "Peacock Lodge";
await context.SaveChangesAsync();

This results in the following update to the database when using SQL Server:

UPDATE [Customers] SET [Address_Line1] = @p0
OUTPUT 1
WHERE [Id] = @p1;
UPDATE [Orders] SET [BillingAddress_Line1] = @p2, [ShippingAddress_Line1] = @p3
OUTPUT 1
WHERE [Id] = @p4;

Notice that all three Line1 columns have changed, since they are all sharing the same instance. This is usually not what we want.

TIP If order addresses should change automatically when the customer address changes, then consider mapping the address as an entity type. Order and Customer can then safely reference the same address instance (which is now identified by a key) via a navigation property.

A good way to deal with issues like this it to make the type immutable. Indeed, this immutability often natural when a type is a good candidate for being a complex type. For example, it usually makes sense to supply a complex new Address object rather than to just mutate, say, the country while leaving the rest the same. C# records are a great fit for this!

Immutable record

C# 9 introduced record types, which makes creating and using immutable objects easier. For example, the Address object can be made a record type:

public record Address(string Line1, string? Line2, string City, string Country, string PostCode);

It now becomes very easy to update the immutable recored with just one property value changed. For example:

customer.Address = customer.Address with { Line1 = "Peacock Lodge" };

await context.SaveChangesAsync();

Immutable struct record

C# 10 introduced struct record types, which makes it easy to create and work with immutable struct records like it is with immutable class records. For example, we can define Address as an immutable struct record:

public readonly record struct Address(string Line1, string? Line2, string City, string Country, string PostCode);

The code for changing the address now looks the same as when using an immutable class record:

customer.Address = customer.Address with { Line1 = "Peacock Lodge" };

await context.SaveChangesAsync();

Nested complex types

A complex type can contain properties of other complex types. For example, let’s use our Address complex type from above together with a PhoneNumber complex type, and nest them both inside another complex type:

public record Address(string Line1, string? Line2, string City, string Country, string PostCode);

public record PhoneNumber(int CountryCode, long Number);

public record Contact
{
    public required Address Address { get; init; }
    public required PhoneNumber HomePhone { get; init; }
    public required PhoneNumber WorkPhone { get; init; }
    public required PhoneNumber MobilePhone { get; init; }
}

We’re using immutable records here, since these are a good match for the semantics of our complex types, but nesting of complex types can be done with any flavor of .NET type.

NOTE We’re not using a primary constructor for the Contact type because EF Core does not yet support constructor injection of complex type values. Vote for Issue #31621 if this is important to you.

We will add Contact as a property of the Customer:

public class Customer
{
    public int Id { get; set; }
    public required string Name { get; set; }
    public required Contact Contact { get; set; }
    public List<Order> Orders { get; } = new();
}

And PhoneNumber as properties of the Order:

public class Order
{
    public int Id { get; set; }
    public required string Contents { get; set; }
    public required PhoneNumber ContactPhone { get; set; }
    public required Address ShippingAddress { get; set; }
    public required Address BillingAddress { get; set; }
    public Customer Customer { get; set; } = null!;
}

Configuration of nested complex types can again be achieved using ComplexTypeAttribute:

[ComplexType]
public record Address(string Line1, string? Line2, string City, string Country, string PostCode);

[ComplexType]
public record PhoneNumber(int CountryCode, long Number);

[ComplexType]
public record Contact
{
    public required Address Address { get; init; }
    public required PhoneNumber HomePhone { get; init; }
    public required PhoneNumber WorkPhone { get; init; }
    public PhoneNumrequired ber MobilePhone { get; init; }
}

Or in OnModelCreating:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Customer>(
        b =>
        {
            b.ComplexProperty(
                e => e.Contact,
                b =>
                {
                    b.ComplexProperty(e => e.Address);
                    b.ComplexProperty(e => e.HomePhone);
                    b.ComplexProperty(e => e.WorkPhone);
                    b.ComplexProperty(e => e.MobilePhone);
                });
        });

    modelBuilder.Entity<Order>(
        b =>
        {
            b.ComplexProperty(e => e.ContactPhone);
            b.ComplexProperty(e => e.BillingAddress);
            b.ComplexProperty(e => e.ShippingAddress);
        });
}

Queries

Properties of complex types on entity types are treated like any other non-navigation property of the entity type. This means that they are always loaded when the entity type is loaded. This is also true of any nested complex type properties. For example, querying for a customer:

var customer = await context.Customers.FirstAsync(e => e.Id == customerId);

Is translated to the following SQL when using SQL Server:

SELECT TOP(1) [c].[Id], [c].[Name], [c].[Contact_Address_City], [c].[Contact_Address_Country],
    [c].[Contact_Address_Line1], [c].[Contact_Address_Line2], [c].[Contact_Address_PostCode],
    [c].[Contact_HomePhone_CountryCode], [c].[Contact_HomePhone_Number], [c].[Contact_MobilePhone_CountryCode],
    [c].[Contact_MobilePhone_Number], [c].[Contact_WorkPhone_CountryCode], [c].[Contact_WorkPhone_Number]
FROM [Customers] AS [c]
WHERE [c].[Id] = @__customerId_0

Notice two things from this SQL:

  • Everything is returned to populate the customer and all the nested Contact, Address, and PhoneNumber complex types.
  • All the complex type values are stored as columns in the table for the entity type. Complex types are never mapped to separate tables.

Projections

Complex types can be projected from a query. For example, selecting just the shipping address from an order:

var shippingAddress = await context.Orders
    .Where(e => e.Id == orderId)
    .Select(e => e.ShippingAddress)
    .SingleAsync();

This translates to the following when using SQL Server:

SELECT TOP(2) [o].[ShippingAddress_City], [o].[ShippingAddress_Country], [o].[ShippingAddress_Line1],
    [o].[ShippingAddress_Line2], [o].[ShippingAddress_PostCode]
FROM [Orders] AS [o]
WHERE [o].[Id] = @__orderId_0

Note that projections of complex types cannot be tracked, since complex type objects have no identity to use for tracking.

Use in predicates

Members of complex types can be used in predicates. For example, finding all the orders going to a certain city:

var city = "Walpole St Peter";
var walpoleOrders = await context.Orders.Where(e => e.ShippingAddress.City == city).ToListAsync();

Which translates to the following SQL on SQL Server:

SELECT [o].[Id], [o].[Contents], [o].[CustomerId], [o].[BillingAddress_City], [o].[BillingAddress_Country],
    [o].[BillingAddress_Line1], [o].[BillingAddress_Line2], [o].[BillingAddress_PostCode],
    [o].[ContactPhone_CountryCode], [o].[ContactPhone_Number], [o].[ShippingAddress_City],
    [o].[ShippingAddress_Country], [o].[ShippingAddress_Line1], [o].[ShippingAddress_Line2],
    [o].[ShippingAddress_PostCode]
FROM [Orders] AS [o]
WHERE [o].[ShippingAddress_City] = @__city_0

A full complex type instance can also be used in predicates. For example, finding all customers with a given phone number:

var phoneNumber = new PhoneNumber(44, 7777555777);
var customersWithNumber = await context.Customers
    .Where(
        e => e.Contact.MobilePhone == phoneNumber
             || e.Contact.WorkPhone == phoneNumber
             || e.Contact.HomePhone == phoneNumber)
    .ToListAsync();

This translates to the following SQL when using SQL Server:

SELECT [c].[Id], [c].[Name], [c].[Contact_Address_City], [c].[Contact_Address_Country], [c].[Contact_Address_Line1],
     [c].[Contact_Address_Line2], [c].[Contact_Address_PostCode], [c].[Contact_HomePhone_CountryCode],
     [c].[Contact_HomePhone_Number], [c].[Contact_MobilePhone_CountryCode], [c].[Contact_MobilePhone_Number],
     [c].[Contact_WorkPhone_CountryCode], [c].[Contact_WorkPhone_Number]
FROM [Customers] AS [c]
WHERE ([c].[Contact_MobilePhone_CountryCode] = @__entity_equality_phoneNumber_0_CountryCode
    AND [c].[Contact_MobilePhone_Number] = @__entity_equality_phoneNumber_0_Number)
OR ([c].[Contact_WorkPhone_CountryCode] = @__entity_equality_phoneNumber_0_CountryCode
    AND [c].[Contact_WorkPhone_Number] = @__entity_equality_phoneNumber_0_Number)
OR ([c].[Contact_HomePhone_CountryCode] = @__entity_equality_phoneNumber_0_CountryCode
    AND [c].[Contact_HomePhone_Number] = @__entity_equality_phoneNumber_0_Number)

Notice that equality is performed by expanding out each member of the complex type. This aligns with complex types having no key for identity and hence a complex type instance is equal to another complex type instance if and only if all their members are equal. Notice that this also aligns with equality for C# records.

Manipulation of complex type values

EF8 provides access to tracking information such as the current and original values of complex types and whether or not a property value has been modified. The API complex types is an extension of the change tracking API already used for entity types.

The ComplexProperty methods of EntityEntry return a entry for an entire complex object. For example, to get the current value of the Order.BillingAddress:

var billingAddress = context.Entry(order)
    .ComplexProperty(e => e.BillingAddress)
    .CurrentValue;

A call to Property can be added to access a property of the complex type. For example to get the current value of just the billing post code:

var postCode = context.Entry(order)
    .ComplexProperty(e => e.BillingAddress)
    .Property(e => e.PostCode)
    .CurrentValue;

Nested complex types are accessed using nested calls to ComplexProperty. For example, to get the city from the nested Address of the Contact on a Customer:

var currentCity = context.Entry(customer)
    .ComplexProperty(e => e.Contact)
    .ComplexProperty(e => e.Address)
    .Property(e => e.City)
    .CurrentValue;

Other methods are available for reading and changing state. For example, PropertyEntry.IsModified can be used to set a property of a complex type as modified:

context.Entry(customer)
    .ComplexProperty(e => e.Contact)
    .ComplexProperty(e => e.Address)
    .Property(e => e.PostCode)
    .IsModified = true;

Current limitations

Complex types represent a significant investment across the EF stack. We were not able to make everything work in this release, but we plan to close some of the gaps in a future release. Make sure to vote (👍) on the appropriate GitHub issues if fixing any of these limitations is important to you.

Complex type limitations in EF8 include:

How to get EF8 RC 1

EF8 is distributed exclusively as a set of NuGet packages. For example, to add the SQL Server provider to your project, you can use the following command using the dotnet tool:

dotnet add package Microsoft.EntityFrameworkCore.SqlServer --version 8.0.0-rc.1.23419.6

Installing the EF8 Command Line Interface (CLI)

The dotnet-ef tool must be installed before executing EF8 Core migration or scaffolding commands.

To install the tool globally, use:

dotnet tool install --global dotnet-ef --version 8.0.0-rc.1.23419.6

If you already have the tool installed, you can upgrade it with the following command:

dotnet tool update --global dotnet-ef --version 8.0.0-rc.1.23419.6

The .NET Data Community Standup

The .NET data access team is now live streaming every other Wednesday at 10am Pacific Time, 1pm Eastern Time, or 18:00 UTC. Join the stream learn and ask questions about many .NET Data related topics.

Documentation and Feedback

The starting point for all EF Core documentation is docs.microsoft.com/ef/. Please file issues found and any other feedback on the dotnet/efcore GitHub repo.

The following links are provided for easy reference and access.

6 comments

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  • Pavel Voronin 0

    So, what is the recommendation when to use OwnedType or ComplexType?

    • Nachman Berkowitz 0

      If it is on the same table it might as well be a ComplexType.

  • Tavi Cacina 0

    Is it possible to add an index on a field of an owned ComplexType?

  • Jan Seris 0

    Please when will you fix a critical performance bug for large project – unwanted client-side projection when mapping using functions?
    Imagine having documents in the DB and wanting selecting their names. When you map using a function and not with inline code, you get all byte[] selected as well only to throw them away on the client and return the names only.
    https://github.com/dotnet/efcore/issues/24509

  • Paolo Fornari 0

    Owned types still exists on EF 8 ? or are replaced by Complex types ?

  • Ahsan Shafqat 0

    there is typo on the following line of code

    public PhoneNumrequired ber MobilePhone { get; init; }

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