public class StarWarsSchema : Schema
{
  public StarWarsSchema()
  {
    Query = new StarWarsQuery();
  }
}

public class StarWarsQuery : ObjectGraphType
{
  public StarWarsQuery()
  {
    var data = new StarWarsData();
    Name = "Query";
    Field<CharacterInterface>(
      "hero",
      resolve: context => data.GetDroidById("3")
    );
  }
}

public class CharacterInterface : InterfaceGraphType
{
  public CharacterInterface()
  {
    Name = "Character";
    Field<NonNullGraphType<StringGraphType>>("id", "The id of the character.");
    Field<NonNullGraphType<StringGraphType>>("name", "The name of the character.");
    Field<ListGraphType<CharacterInterface>>("friends");
  }
}

public class DroidType : ObjectGraphType
{
  public DroidType()
  {
    var data = new StarWarsData();
    Name = "Droid";
    Field<NonNullGraphType<StringGraphType>>("id", "The id of the droid.");
    Field<NonNullGraphType<StringGraphType>>("name", "The name of the droid.");
    Field<ListGraphType<CharacterInterface>>(
        "friends",
        resolve: context => data.GetFriends(context.Source as StarWarsCharacter)
    );
    Interface<CharacterInterface>();
    IsTypeOf = value => value is Droid;
  }
}

public async Task<string> Execute(
  Schema schema,
  object rootObject,
  string query,
  string operationName = null,
  Inputs inputs = null)
{
  var executer = new DocumentExecuter();
  var writer = new DocumentWriter();

  var result = await executer.ExecuteAsync(schema, rootObject, query, operationName, inputs);
  return writer.Write(result);
}

var schema = new StarWarsSchema();

var query = @"
  query HeroNameQuery {
    hero {
      name
    }
  }
";

var result = await Execute(schema, null, query);

Console.Writeline(result);

// prints
{
  "data": {
    "hero": {
      "name": "R2-D2"
    }
  }
}