Assign correct slots during IEnumerable resolution

src/libraries/Microsoft.Extensions.DependencyInjection.Specification.Tests/src/DependencyInjectionSpecificationTests.cs

@@ -761,6 +761,28 @@ public void ClosedServicesPreferredOverOpenGenericServices()
             Assert.IsType<FakeService>(service);
         }
 
+        // Reproduces https://github.com/dotnet/runtime/issues/79938
+        [Fact]
+        public void ResolvingEnumerableContainingOpenGenericServiceUsesCorrectSlot()
+        {
+            // Arrange
+            TestServiceCollection collection = new();
+            collection.AddTransient<IFakeOpenGenericService<PocoClass>, FakeService>();
+            collection.AddTransient(typeof(IFakeOpenGenericService<>), typeof(FakeOpenGenericService<>));
+            collection.AddSingleton<PocoClass>(); // needed for FakeOpenGenericService<>
+            IServiceProvider provider = CreateServiceProvider(collection);
+
+            // Act
+            IFakeOpenGenericService<PocoClass> service = provider.GetService<IFakeOpenGenericService<PocoClass>>();
+            IFakeOpenGenericService<PocoClass>[] services = provider.GetServices<IFakeOpenGenericService<PocoClass>>().ToArray();
+
+            // Assert
+            Assert.IsType<FakeService>(service);
+            Assert.Equal(2, services.Length);
+            Assert.True(services.Any(s => s.GetType() == typeof(FakeService)));
+            Assert.True(services.Any(s => s.GetType() == typeof(FakeOpenGenericService<PocoClass>)));
+        }
+
         [Fact]
         public void AttemptingToResolveNonexistentServiceReturnsNull()
         {
@@ -905,6 +927,7 @@ public void ResolvesMixedOpenClosedGenericsAsEnumerable()
             Assert.NotNull(enumerable[2]);
 
             Assert.Equal(instance, enumerable[2]);
+            Assert.True(enumerable[0] is FakeService, string.Join(", ", enumerable.Select(e => e.GetType())));
             Assert.IsType<FakeService>(enumerable[0]);
         }
 

src/libraries/Microsoft.Extensions.DependencyInjection/src/ServiceLookup/CallSiteFactory.cs

@@ -6,6 +6,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
+using System.Linq;
 using System.Reflection;
 using Microsoft.Extensions.Internal;
 
@@ -240,65 +241,80 @@ private static bool AreCompatible(DynamicallyAccessedMemberTypes serviceDynamica
             {
                 callSiteChain.Add(serviceType);
 
-                if (serviceType.IsConstructedGenericType &&
-                    serviceType.GetGenericTypeDefinition() == typeof(IEnumerable<>))
+                if (!serviceType.IsConstructedGenericType ||
+                    serviceType.GetGenericTypeDefinition() != typeof(IEnumerable<>))
                 {
-                    Type itemType = serviceType.GenericTypeArguments[0];
-                    CallSiteResultCacheLocation cacheLocation = CallSiteResultCacheLocation.Root;
+                    return null;
+                }
 
-                    var callSites = new List<ServiceCallSite>();
+                Type itemType = serviceType.GenericTypeArguments[0];
+                CallSiteResultCacheLocation cacheLocation = CallSiteResultCacheLocation.Root;
+                ServiceCallSite[] callSites;
 
-                    // If item type is not generic we can safely use descriptor cache
-                    if (!itemType.IsConstructedGenericType &&
-                        _descriptorLookup.TryGetValue(itemType, out ServiceDescriptorCacheItem descriptors))
+                // If item type is not generic we can safely use descriptor cache
+                if (!itemType.IsConstructedGenericType &&
+                    _descriptorLookup.TryGetValue(itemType, out ServiceDescriptorCacheItem descriptors))
+                {
+                    callSites = new ServiceCallSite[descriptors.Count];
+                    for (int i = 0; i < descriptors.Count; i++)
                     {
-                        for (int i = 0; i < descriptors.Count; i++)
-                        {
-                            ServiceDescriptor descriptor = descriptors[i];
+                        ServiceDescriptor descriptor = descriptors[i];
+
+                        // Last service should get slot 0
+                        int slot = descriptors.Count - i - 1;
+                        // There may not be any open generics here
+                        ServiceCallSite? callSite = TryCreateExact(descriptor, itemType, callSiteChain, slot);
+                        Debug.Assert(callSite != null);
+
+                        cacheLocation = GetCommonCacheLocation(cacheLocation, callSite.Cache.Location);
+                        callSites[i] = callSite;
+                    }
+                }
+                else
+                {
+                    // We need to construct a list of matching call sites in declaration order, but to ensure
+                    // correct caching we must assign slots in reverse declaration order and with slots being
+                    // given out first to any exact matches before any open generic matches. Therefore, we
+                    // iterate over the descriptors twice in reverse, catching exact matches on the first pass
+                    // and open generic matches on the second pass.
 
-                            // Last service should get slot 0
-                            int slot = descriptors.Count - i - 1;
-                            // There may not be any open generics here
-                            ServiceCallSite? callSite = TryCreateExact(descriptor, itemType, callSiteChain, slot);
-                            Debug.Assert(callSite != null);
+                    List<KeyValuePair<int, ServiceCallSite>> callSitesByIndex = new();
 
-                            cacheLocation = GetCommonCacheLocation(cacheLocation, callSite.Cache.Location);
-                            callSites.Add(callSite);
+                    int slot = 0;
+                    for (int i = _descriptors.Length - 1; i >= 0; i--)
+                    {
+                        if (TryCreateExact(_descriptors[i], itemType, callSiteChain, slot) is { } callSite)
+                        {
+                            AddCallSite(callSite, i);
                         }
                     }
-                    else
+                    for (int i = _descriptors.Length - 1; i >= 0; i--)
                     {
-                        int slot = 0;
-                        // We are going in reverse so the last service in descriptor list gets slot 0
-                        for (int i = _descriptors.Length - 1; i >= 0; i--)
+                        if (TryCreateOpenGeneric(_descriptors[i], itemType, callSiteChain, slot, throwOnConstraintViolation: false) is { } callSite)
                         {
-                            ServiceDescriptor descriptor = _descriptors[i];
-                            ServiceCallSite? callSite = TryCreateExact(descriptor, itemType, callSiteChain, slot) ??
-                                           TryCreateOpenGeneric(descriptor, itemType, callSiteChain, slot, false);
-
-                            if (callSite != null)
-                            {
-                                slot++;
-
-                                cacheLocation = GetCommonCacheLocation(cacheLocation, callSite.Cache.Location);
-                                callSites.Add(callSite);
-                            }
+                            AddCallSite(callSite, i);
                         }
-
-                        callSites.Reverse();
                     }
 
+                    callSitesByIndex.Sort((a, b) => a.Key.CompareTo(b.Key));
+                    callSites = callSitesByIndex.Select(p => p.Value).ToArray();
 
-                    ResultCache resultCache = ResultCache.None;
-                    if (cacheLocation == CallSiteResultCacheLocation.Scope || cacheLocation == CallSiteResultCacheLocation.Root)
+                    void AddCallSite(ServiceCallSite callSite, int index)
                     {
-                        resultCache = new ResultCache(cacheLocation, callSiteKey);
+                        slot++;
+
+                        cacheLocation = GetCommonCacheLocation(cacheLocation, callSite.Cache.Location);
+                        callSitesByIndex.Add(new(index, callSite));
                     }
+                }
 
-                    return _callSiteCache[callSiteKey] = new IEnumerableCallSite(resultCache, itemType, callSites.ToArray());
+                ResultCache resultCache = ResultCache.None;
+                if (cacheLocation == CallSiteResultCacheLocation.Scope || cacheLocation == CallSiteResultCacheLocation.Root)
+                {
+                    resultCache = new ResultCache(cacheLocation, callSiteKey);
                 }
 
-                return null;
+                return _callSiteCache[callSiteKey] = new IEnumerableCallSite(resultCache, itemType, callSites);
             }
             finally
             {