ddieppa · October 8, 2020 12:38 · Oct 8, 2020
diff --git a/UseResolverScopedMediatorAttribute.cs b/UseResolverScopedMediatorAttribute.cs
@@ -0,0 +1,54 @@
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using HotChocolate.Types;
+using HotChocolate.Types.Descriptors;
+using MediatR;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace MyCompany.GraphQL
+{
+    /// <summary>
+    /// This attribute allows us to tap into the field resolution middleware
+    /// so that we can dynamically create an <see cref="IServiceScope"/> just for
+    /// this field resolver method, and create an <see cref="IMediator"/> instance inside it.
+    ///
+    /// This is used to solve a problem with using EF Core's DbContextPool, which scopes
+    /// DbContext instances per http request and will cause HotChocolate resolvers running
+    /// in parallel to execute on the same DbContext, which isn't allowed.
+    ///
+    /// By resolving our <see cref="IMediator"/> instance inside an isolated scope,
+    /// we ensure that if it depends itself on a DbContext that it will be unique in that scope
+    /// and there won't be multiple threads using it simultaneously.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method)]
+    public class UseResolverScopedMediatorAttribute : ObjectFieldDescriptorAttribute
+    {
+        private static readonly string _injectedArgumentName = "mediator";
+
+        private static readonly HashSet<string> _localSchemaNames = new HashSet<string>
+        {
+            SchemaNames.Content,
+            SchemaNames.ShopTyre,
+            SchemaNames.Task
+        };
+
+        public override void OnConfigure(
+            IDescriptorContext descriptorContext,
+            IObjectFieldDescriptor descriptor,
+            MemberInfo member)
+        {
+            descriptor.Use(next => async context =>
+            {
+                // Temp workaround for: https://github.com/ChilliCream/hotchocolate/issues/2246
+                using var scope = _localSchemaNames.Contains(context.Schema.Name)
+                    ? context.Service<Microsoft.AspNetCore.Http.IHttpContextAccessor>().HttpContext.RequestServices.CreateScope()
+                    : context.Service<IServiceProvider>().CreateScope();
+                var mediator = scope.ServiceProvider.GetRequiredService<IMediator>();
+                context.ModifyScopedContext(c => c.SetItem(_injectedArgumentName, mediator));
+
+                await next(context);
+            });
+        }
+    }
+}
diff --git a/_Usage.cs b/_Usage.cs
@@ -0,0 +1,8 @@
+[UseResolverScopedMediator]
+public async Task<string> GetAccountsAsync(
+    IResolverContext context,
+    [ScopedState] IMediator mediator)
+    => await mediator.Send(new SomeRequest());
+
+// Note: Here mediator is scoped, and when the handler is instantiated, all its dependencies are generated in the same scope,
+// including DbContext instances, so no multi-thread usage :)
No results found