Changed TypeLocator to use ConcurrentDictionary for its cache

Build.ps1

@@ -66,4 +66,4 @@ Else {
     Write-Output "RUNNING dotnet pack .\src\JsonApiDotNetCore -c Release -o .\artifacts --version-suffix=alpha1-$revision"
     dotnet pack .\src\JsonApiDotNetCore -c Release -o .\artifacts --version-suffix=alpha1-$revision --include-symbols
     CheckLastExitCode
-}
+}

src/JsonApiDotNetCore/Graph/IdentifiableTypeCache.cs

@@ -0,0 +1,35 @@
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using JsonApiDotNetCore.Models;
+
+namespace JsonApiDotNetCore.Graph
+{
+    /// <summary>
+    /// Used to cache and locate types, to facilitate auto-resource discovery
+    /// </summary>
+    internal sealed class IdentifiableTypeCache
+    {
+        private readonly ConcurrentDictionary<Assembly, List<ResourceDescriptor>> _typeCache = new ConcurrentDictionary<Assembly, List<ResourceDescriptor>>();
+
+        /// <summary>
+        /// Get all implementations of <see cref="IIdentifiable"/> in the assembly
+        /// </summary>
+        public IEnumerable<ResourceDescriptor> GetIdentifiableTypes(Assembly assembly)
+        {
+            return _typeCache.GetOrAdd(assembly, asm => FindIdentifiableTypes(asm).ToList());
+        }
+
+        private static IEnumerable<ResourceDescriptor> FindIdentifiableTypes(Assembly assembly)
+        {
+            foreach (var type in assembly.GetTypes())
+            {
+                if (TypeLocator.TryGetResourceDescriptor(type, out var descriptor))
+                {
+                    yield return descriptor;
+                }
+            }
+        }
+    }
+}

src/JsonApiDotNetCore/Graph/ResourceDescriptor.cs

@@ -13,6 +13,6 @@ public ResourceDescriptor(Type resourceType, Type idType)
         public Type ResourceType { get; }
         public Type IdType { get; }
 
-        internal static ResourceDescriptor Empty => new ResourceDescriptor(null, null);
+        internal static ResourceDescriptor Empty { get; } = new ResourceDescriptor(null, null);
     }
 }

src/JsonApiDotNetCore/Graph/ServiceDiscoveryFacade.cs

@@ -47,6 +47,7 @@ public class ServiceDiscoveryFacade : IServiceDiscoveryFacade
         };
         private readonly IServiceCollection _services;
         private readonly IResourceGraphBuilder _resourceGraphBuilder;
+        private readonly IdentifiableTypeCache _typeCache = new IdentifiableTypeCache();
 
         public ServiceDiscoveryFacade(IServiceCollection services, IResourceGraphBuilder resourceGraphBuilder)
         {
@@ -67,7 +68,7 @@ public ServiceDiscoveryFacade AddAssembly(Assembly assembly)
         {
             AddDbContextResolvers(assembly);
 
-            var resourceDescriptors = TypeLocator.GetIdentifiableTypes(assembly);
+            var resourceDescriptors = _typeCache.GetIdentifiableTypes(assembly);
             foreach (var resourceDescriptor in resourceDescriptors)
             {
                 AddResource(assembly, resourceDescriptor);
@@ -77,7 +78,6 @@ public ServiceDiscoveryFacade AddAssembly(Assembly assembly)
             return this;
         }
 
-
         public IEnumerable<Type> FindDerivedTypes(Type baseType)
         {
             return baseType.Assembly.GetTypes().Where(t =>
@@ -108,9 +108,9 @@ private void AddDbContextResolvers(Assembly assembly)
         /// <param name="assembly">The assembly to search for resources in.</param>
         public ServiceDiscoveryFacade AddResources(Assembly assembly)
         {
-            var identifiables = TypeLocator.GetIdentifiableTypes(assembly);
-            foreach (var identifiable in identifiables)
-                AddResource(assembly, identifiable);
+            var resourceDescriptors = _typeCache.GetIdentifiableTypes(assembly);
+            foreach (var resourceDescriptor in resourceDescriptors)
+                AddResource(assembly, resourceDescriptor);
 
             return this;
         }
@@ -148,7 +148,7 @@ private void AddResourceToGraph(ResourceDescriptor identifiable)
         /// <param name="assembly">The assembly to search for resources in.</param>
         public ServiceDiscoveryFacade AddServices(Assembly assembly)
         {
-            var resourceDescriptors = TypeLocator.GetIdentifiableTypes(assembly);
+            var resourceDescriptors = _typeCache.GetIdentifiableTypes(assembly);
             foreach (var resourceDescriptor in resourceDescriptors)
             {
                 AddServices(assembly, resourceDescriptor);
@@ -170,7 +170,7 @@ private void AddServices(Assembly assembly, ResourceDescriptor resourceDescripto
         /// <param name="assembly">The assembly to search for resources in.</param>
         public ServiceDiscoveryFacade AddRepositories(Assembly assembly)
         {
-            var resourceDescriptors = TypeLocator.GetIdentifiableTypes(assembly);
+            var resourceDescriptors = _typeCache.GetIdentifiableTypes(assembly);
             foreach (var resourceDescriptor in resourceDescriptors)
             {
                 AddRepositories(assembly, resourceDescriptor);
@@ -186,7 +186,7 @@ private void AddRepositories(Assembly assembly, ResourceDescriptor resourceDescr
                 RegisterServiceImplementations(assembly, serviceInterface, resourceDescriptor);
             }
         }
-        
+
         private void RegisterServiceImplementations(Assembly assembly, Type interfaceType, ResourceDescriptor resourceDescriptor)
         {
             if (resourceDescriptor.IdType == typeof(Guid) && interfaceType.GetTypeInfo().GenericTypeParameters.Length == 1)

src/JsonApiDotNetCore/Graph/TypeLocator.cs

@@ -1,4 +1,4 @@
-using JsonApiDotNetCore.Extensions;
+using JsonApiDotNetCore.Extensions;
 using JsonApiDotNetCore.Models;
 using System;
 using System.Collections.Generic;
@@ -12,8 +12,6 @@ namespace JsonApiDotNetCore.Graph
     /// </summary>
     internal static class TypeLocator
     {
-        private static readonly Dictionary<Assembly, List<ResourceDescriptor>> _identifiableTypeCache = new Dictionary<Assembly, List<ResourceDescriptor>>();
-
         /// <summary>
         /// Determine whether or not this is a json:api resource by checking if it implements <see cref="IIdentifiable"/>.
         /// Returns the status and the resultant id type, either `(true, Type)` OR `(false, null)`
@@ -24,29 +22,6 @@ public static Type GetIdType(Type resourceType)
             return identifiableInterface?.GetGenericArguments()[0];
         }
 
-        /// <summary>
-        /// Get all implementations of <see cref="IIdentifiable"/> in the assembly
-        /// </summary>
-        public static IEnumerable<ResourceDescriptor> GetIdentifiableTypes(Assembly assembly)
-            => (_identifiableTypeCache.TryGetValue(assembly, out _) == false)
-                    ? FindIdentifiableTypes(assembly)
-                    : _identifiableTypeCache[assembly];
-
-        private static IEnumerable<ResourceDescriptor> FindIdentifiableTypes(Assembly assembly)
-        {
-            var descriptors = new List<ResourceDescriptor>();
-            _identifiableTypeCache[assembly] = descriptors;
-
-            foreach (var type in assembly.GetTypes())
-            {
-                if (TryGetResourceDescriptor(type, out var descriptor))
-                {
-                    descriptors.Add(descriptor);
-                    yield return descriptor;
-                }
-            }
-        }
-
         /// <summary>
         /// Attempts to get a descriptor of the resource type.
         /// </summary>

test/UnitTests/Graph/IdentifiableTypeCacheTests.cs

@@ -0,0 +1,39 @@
+using JsonApiDotNetCore.Graph;
+using JsonApiDotNetCore.Models;
+using UnitTests.Internal;
+using Xunit;
+
+namespace UnitTests.Graph
+{
+    public class IdentifiableTypeCacheTests
+    {
+        [Fact]
+        public void GetIdentifiableTypes_Locates_Identifiable_Resource()
+        {
+            // Arrange
+            var resourceType = typeof(Model);
+            var typeCache = new IdentifiableTypeCache();
+
+            // Act
+            var results = typeCache.GetIdentifiableTypes(resourceType.Assembly);
+
+            // Assert
+            Assert.Contains(results, r => r.ResourceType == resourceType);
+        }
+
+        [Fact]
+        public void GetIdentifiableTypes_Only_Contains_IIdentifiable_Types()
+        {
+            // Arrange
+            var resourceType = typeof(Model);
+            var typeCache = new IdentifiableTypeCache();
+
+            // Act
+            var resourceDescriptors = typeCache.GetIdentifiableTypes(resourceType.Assembly);
+
+            // Assert
+            foreach(var resourceDescriptor in resourceDescriptors)
+                Assert.True(typeof(IIdentifiable).IsAssignableFrom(resourceDescriptor.ResourceType));
+        }
+    }
+}

test/UnitTests/Graph/TypeLocator_Tests.cs

@@ -81,33 +81,6 @@ public void GetIdType_Correctly_Identifies_NonJsonApiResource()
             Assert.Equal(expectedIdType, idType);
         }
 
-        [Fact]
-        public void GetIdentifiableTypes_Locates_Identifiable_Resource()
-        {
-            // Arrange
-            var resourceType = typeof(Model);
-
-            // Act
-            var results = TypeLocator.GetIdentifiableTypes(resourceType.Assembly);
-
-            // Assert
-            Assert.Contains(results, r => r.ResourceType == resourceType);
-        }
-
-        [Fact]
-        public void GetIdentifiableTypes_Only_Contains_IIdentifiable_Types()
-        {
-            // Arrange
-            var resourceType = typeof(Model);
-
-            // Act
-            var resourceDescriptors = TypeLocator.GetIdentifiableTypes(resourceType.Assembly);
-
-            // Assert
-            foreach(var resourceDescriptor in resourceDescriptors)
-                Assert.True(typeof(IIdentifiable).IsAssignableFrom(resourceDescriptor.ResourceType));
-        }
-
         [Fact]
         public void TryGetResourceDescriptor_Returns_True_If_Type_Is_IIdentifiable()
         {