From f6cf49d5c8c840cb3b1062ee0e9d1b0723db689b Mon Sep 17 00:00:00 2001
From: Jimmy Bogard <jimmy.bogard@gmail.com>
Date: Mon, 9 Apr 2018 16:30:40 -0500
Subject: [PATCH] Adding test and returning a null if the open generic service
 fails to close because of a generic constraint. See #471 for discussion.

---
 .../Fakes/ConstrainedFakeOpenGenericService.cs   | 16 ++++++++++++++++
 src/DI/ServiceLookup/CallSiteFactory.cs          | 11 ++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 src/DI.Specification.Tests/Fakes/ConstrainedFakeOpenGenericService.cs

diff --git a/src/DI.Specification.Tests/Fakes/ConstrainedFakeOpenGenericService.cs b/src/DI.Specification.Tests/Fakes/ConstrainedFakeOpenGenericService.cs
new file mode 100644
index 00000000..811d9a33
--- /dev/null
+++ b/src/DI.Specification.Tests/Fakes/ConstrainedFakeOpenGenericService.cs
@@ -0,0 +1,16 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.Extensions.DependencyInjection.Specification.Fakes
+{
+    public class ConstrainedFakeOpenGenericService<TVal> : IFakeOpenGenericService<TVal>
+        where TVal : PocoClass
+    {
+        public ConstrainedFakeOpenGenericService(TVal value)
+        {
+            Value = value;
+        }
+
+        public TVal Value { get; }
+    }
+}
\ No newline at end of file
diff --git a/src/DI/ServiceLookup/CallSiteFactory.cs b/src/DI/ServiceLookup/CallSiteFactory.cs
index 878b64b5..95ccebba 100644
--- a/src/DI/ServiceLookup/CallSiteFactory.cs
+++ b/src/DI/ServiceLookup/CallSiteFactory.cs
@@ -197,7 +197,16 @@ private IServiceCallSite TryCreateOpenGeneric(ServiceDescriptor descriptor, Type
             {
                 Debug.Assert(descriptor.ImplementationType != null, "descriptor.ImplementationType != null");
 
-                var closedType = descriptor.ImplementationType.MakeGenericType(serviceType.GenericTypeArguments);
+                Type closedType;
+                try
+                {
+                    closedType = descriptor.ImplementationType.MakeGenericType(serviceType.GenericTypeArguments);
+                }
+                catch
+                {
+                    // This is the only way to reliably test generic constraints. See https://stackoverflow.com/questions/4864496/checking-if-an-object-meets-a-generic-parameter-constraint/4864565#4864565
+                    return null;
+                }
                 var constructorCallSite = CreateConstructorCallSite(serviceType, closedType, callSiteChain);
 
                 return ApplyLifetime(constructorCallSite, Tuple.Create(descriptor, serviceType), descriptor.Lifetime);