1. Introduction
2. The Essential Concepts: Visual Studio, XAML, and C#
3. Creating the User Interface: The Controls 

public class AccountType : ObjectGraphType<Account>
{
    public AccountType()
    {
        Field(x => x.Id, type: typeof(IdGraphType)).Description("Id property from the account object.");
        Field(x => x.Description).Description("Description property from the account object.");
        Field(x => x.OwnerId, type: typeof(IdGraphType)).Description("OwnerId property from the account object.");
    }
}

public interface IAccountRepository
{
    IEnumerable<Account> GetAllAccountsPerOwner(Guid ownerId);
}

public class AccountRepository : IAccountRepository
{
    private readonly ApplicationContext _context;
 
    public AccountRepository(ApplicationContext context)
    {
       _context = context;
    }
 
    public IEnumerable<Account> GetAllAccountsPerOwner(Guid ownerId) => _context.Accounts
        .Where(a => a.OwnerId.Equals(ownerId))
        .ToList();
}

public class OwnerType : ObjectGraphType<Owner>
{
    public OwnerType(IAccountRepository repository)
    {
        Field(x => x.Id, type: typeof(IdGraphType)).Description("Id property from the owner object.");
        Field(x => x.Name).Description("Name property from the owner object.");
        Field(x => x.Address).Description("Address property from the owner object.");
        Field<ListGraphType<AccountType>>(
            "accounts",
            resolve: context => repository.GetAllAccountsPerOwner(context.Source.Id)
        );
    }
}

https://localhost:5001/ui/playground

{
  owners{
    id,
    name,
    address,
    accounts{
      id,
      description,
      ownerId
    }
  }
}

public class AccountTypeEnumType : EnumerationGraphType<TypeOfAccount>
{
    public AccountTypeEnumType()
    {
        Name = "Type";
        Description = "Enumeration for the account type object.";
    }
}

public class AccountType : ObjectGraphType<Account>
{
    public AccountType()
    {
        ...
        Field<AccountTypeEnumType>("Type", "Enumeration for the account type object.");
    }
}

{
  owners{
    id,
    name,
    address,
    accounts{
      id,
      description,
      type,
      ownerId
    }
  }
}

public interface IOwnerRepository
{
    ...
    Owner GetById(Guid id);
}

public class OwnerRepository : IOwnerRepository
{
    ...
    Owner GetById(Guid id) => _context.Owners.SingleOrDefault(o => o.Id.Equals(id));
}

public class AppQuery : ObjectGraphType
{
    public AppQuery(IOwnerRepository repository)
    {
        ...

        Field<OwnerType>(
            "owner",
            arguments: new QueryArguments(new QueryArgument<NonNullGraphType<IdGraphType>> { Name = "ownerId" }),
            resolve: context =>
            {
                var id = context.GetArgument<Guid>("ownerId");
                return repository.GetById(id);
            }
        );
    }
}

Field<OwnerType>(
    "owner",
    arguments: new QueryArguments(new QueryArgument<NonNullGraphType<IdGraphType>> { Name = "ownerId" }),
    resolve: context =>
    {
        Guid id;
        if (!Guid.TryParse(context.GetArgument<string>("ownerId"), out id))
        {
            context.Errors.Add(new ExecutionError("Wrong value for guid"));
            return null;
        }
 
         return repository.GetById(id);
     }
);

{
  first:owners{
    ownerId:id,
    ownerName:name,
    ownerAddress:address,
    ownerAccounts:accounts
    {
      accountId:id,
      accountDescription:description,
      accountType:type
    }
  },
  second:owners{
    ownerId:id,
    ownerName:name,
    ownerAddress:address,
    ownerAccounts:accounts
    {
      accountId:id,
      accountDescription:description,
      accountType:type
    }
  }
}