Adding GraphQL middleware when using ASP.NET Core

Enabling a GraphQL endpoint in your ASP.NET Core application is quite easy thanks to the GraphQL.Server.Transports.AspNetCore NuGet package.

	public void ConfigureServices(IServiceCollection services)
	{
	services
	.AddGraphQL(new GraphQLOptions() { ExposeExceptions = true, EnableMetrics=false })
	.AddDataLoader();
	// .AddGraphQLAuthorization();
	services.AddSingleton<GraphQL.IDependencyResolver>(
	c => new FuncDependencyResolver(type => c.GetRequiredService(type)));
	}

view raw Startup.cs hosted with ❤ by GitHub

One of the nice features of GraphQL is that you can extend your resolvers using custom middleware. In the GraphQL.NET documentation they refer to the FieldsMiddleware property to register this extra middleware:

	schema.Execute(_ =>
	{
	_.Query = "...";
	_.FieldMiddleware.Use<InstrumentFieldsMiddleware>();
	});

view raw FieldMiddleware.cs hosted with ❤ by GitHub

Unfortunately when using the GraphQL.Server package you only have an GraphQLOptions object but the only thing you can do is set a SetFieldMiddleware flag:

	public class GraphQLOptions
	{
	public GraphQLOptions();
	public ComplexityConfiguration ComplexityConfiguration { get; set; }
	public bool EnableMetrics { get; set; }
	public bool ExposeExceptions { get; set; }
	public bool SetFieldMiddleware { get; set; }
	}

view raw GraphQLOptions.cs hosted with ❤ by GitHub

To register your own middleware you have to jump through some extra hoops:

• Create a custom GraphQLExecutor and override the GetOptions method:

	public class ValidationGraphQLExecutor<TSchema> : DefaultGraphQLExecuter<TSchema>
	where TSchema : ISchema
	{
	private readonly GraphQLOptions options;
	public ValidationGraphQLExecutor(
	TSchema schema,
	IDocumentExecuter documentExecuter,
	IOptions<GraphQLOptions> options,
	IEnumerable<IDocumentExecutionListener> listeners,
	IEnumerable<IValidationRule> validationRules)
	: base(schema, documentExecuter, options, listeners, validationRules) =>
	this.options = options.Value;
	protected override ExecutionOptions GetOptions(
	string operationName,
	string query,
	Inputs variables,
	object context,
	CancellationToken cancellationToken)
	{
	var options = base.GetOptions(operationName, query, variables, context, cancellationToken);
	if (this.options.SetFieldMiddleware)
	{
	options.FieldMiddleware.Use<ValidationMiddleware>();
	}
	return options;
	}
	}

view raw ValidationGraphQLExecutor.cs hosted with ❤ by GitHub

• Register this Executor instance in your IoC container(here I'm using StructureMap):

	public class ServerRegistry : Registry
	{
	public ServerRegistry()
	{
	For(typeof(IGraphQLExecuter<>)).Use(typeof(ValidationGraphQLExecutor<>));
	}
	}

view raw ServerRegistry.cs hosted with ❤ by GitHub