Interfaces

An interface is an abstract type that defines a certain set of fields that an object type or another interface must include to implement the interface.

SDL

interface Message {
  author: User!
  createdAt: DateTime!
}

type TextMessage implements Message {
  author: User!
  createdAt: DateTime!
  content: String!
}

Clients can query fields returning an interface like the following.

GraphQL

{
  messages {
    author {
      name
    }
    ... on TextMessage {
      content
    }
  }
}

Learn more about interfaces here.

Usage

Interfaces can be defined like the following.

[InterfaceType("Message")]
public interface IMessage
{
    User Author { get; set; }

    DateTime CreatedAt { get; set; }
}

public class TextMessage : IMessage
{
    public User Author { get; set; }

    public DateTime CreatedAt { get; set; }

    public string Content { get; set; }
}

public class Query
{
    public IMessage[] GetMessages()
    {
        // Omitted code for brevity
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services
            .AddGraphQLServer()
            .AddQueryType<Query>()
            .AddType<TextMessage>();
    }
}

We can also use classes to define an interface.

[InterfaceType]
public abstract class Message
{
    public User SendBy { get; set; }

    public DateTime CreatedAt { get; set; }
}

public class TextMessage : Message
{
    public string Content { get; set; }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services
            .AddGraphQLServer()
            // ...
            .AddType<TextMessage>();
    }
}

Note: We have to explicitly register the interface implementations:
C#
services.AddGraphQLServer().AddType<TextMessageType>()

Interfaces implementing interfaces

Interfaces can also implement other interfaces.

SDL

interface Message {
  author: User
}

interface DatedMessage implements Message {
  createdAt: DateTime!
  author: User
}

type TextMessage implements DatedMessage & Message {
  author: User
  createdAt: DateTime!
  content: String
}

We can implement this like the following.

[InterfaceType("Message")]
public interface IMessage
{
    User Author { get; set; }
}

[InterfaceType("DatedMessage")]
public interface IDatedMessage : IMessage
{
    DateTime CreatedAt { get; set; }
}

public class TextMessage : IDatedMessage
{
    public User Author { get; set; }

    public DateTime CreatedAt { get; set; }

    public string Content { get; set; }
}

public class Query
{
    public IMessage[] GetMessages()
    {
        // Omitted code for brevity
    }
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services
            .AddGraphQLServer()
            .AddQueryType<Query>()
            .AddType<IDatedMessage>()
            .AddType<TextMessage>();
    }
}

Note: We also have to register the DatedMessage interface manually, if we do not expose it through a field directly:
C#
services.AddGraphQLServer().AddType<DatedMessageType>()

Dynamic fields

We can also declare additional dynamic fields (resolvers) on our interfaces.

[InterfaceType("Message")]
public interface IMessage
{
    User Author { get; set; }

    DateTime GetCreatedAt();
}

public class TextMessage : IMessage
{
    public User Author { get; set; }

    public DateTime GetCreatedAt()
    {
        // Omitted code for brevity
    }
}

Last updated on 2021-08-10 by Tobias Tengler