What

I'm using GraphQL package which is really good. While it does support schema first federation, it doesn't support code first federation.

How

Looking at the specs, all we need here is a few things that our graphql server does. We need to define some scalars and some queries. Schema first federation already includes a FederatedSchemaPrinter we can use that to print SDL

Why

I've multiple projects and I'm sure other people are also trying to do federation. Even though it's not a lot of code in this project, we'll always have to copy same code into all projects untill GraphQL project include federation.

Getting started:

First install Micro.GraphQL.Federation from nuget.

Create EntityType:

public class EntityType : Micro.GraphQL.Federation.Types.EntityType
{
    public EntityType()
    {
        // register all types which uses @key directive
        Type<ApplicationType>();
        Type<UserType>();
    }
}

Enable federation on Startup.ConfigureServices:

services.EnableFederation<EntityType>();

Extend your schema:

public class AppRegistrationSchema : Schema<EntityType>
{
    public AppRegistrationSchema(IServiceProvider services, Query query) : base(services)
    {
        Query = query;
    }
}

extend your query:

public class Query : Query<EntityType>
{
}

extend your types:

public sealed class UserType : ObjectGraphType<User>
{
  public UserType() {
      ExtendByKeys("id email username");
  }
}

will produce

extend type User @key(fields: "id email username") {

}

if it's core type and don't want to extend on gql schema

public sealed class UserType : ObjectGraphType<User>
{
  public UserType() {
      Key("id email username");
  }
}

will produce

type User @key(fields: "id email username") {}

for more info, open a issue