Work around an "old runtime" issue with reflection

For oneofs, to get the case, we need to call the property that
returns the enum value. We really want it as an int, and modern
runtimes allow us to create a delegate which returns an int from the
method. (I suspect that the MS runtime has always allowed that.)
Old versions of Mono (e.g. used by Unity3d) don't allow that, so we
have to convert the enum value to an int via boxing. It's ugly, but
it should work.

csharp/src/Google.Protobuf/Reflection/ReflectionUtil.cs

@@ -30,6 +30,7 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #endregion
 
+using Google.Protobuf.Compatibility;
 using System;
 using System.Reflection;
 
@@ -63,10 +64,12 @@ namespace Google.Protobuf.Reflection
 
         /// <summary>
         /// Creates a delegate which will cast the argument to the appropriate method target type,
-        /// call the method on it, then convert the result to the specified type.
+        /// call the method on it, then convert the result to the specified type. The method is expected
+        /// to actually return an enum (because of where we're calling it - for oneof cases). Sometimes that
+        /// means we need some extra work to perform conversions.
         /// </summary>
         internal static Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method) =>
-            GetReflectionHelper(method.DeclaringType, typeof(object)).CreateFuncIMessageInt32(method);
+            GetReflectionHelper(method.DeclaringType, method.ReturnType).CreateFuncIMessageInt32(method);
 
         /// <summary>
         /// Creates a delegate which will execute the given method after casting the first argument to
@@ -103,11 +106,25 @@ namespace Google.Protobuf.Reflection
 
         private class ReflectionHelper<T1, T2> : IReflectionHelper
         {
+
             public Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method)
+            {
+                // On pleasant runtimes, we can create a Func<int> from a method returning
+                // an enum based on an int. That's the fast path.
+                if (CanConvertEnumFuncToInt32Func)
                 {
                     var del = (Func<T1, int>) method.CreateDelegate(typeof(Func<T1, int>));
                     return message => del((T1) message);
                 }
+                else
+                {
+                    // On some runtimes (e.g. old Mono) the return type has to be exactly correct,
+                    // so we go via boxing. Reflection is already fairly inefficient, and this is
+                    // only used for one-of case checking, fortunately.
+                    var del = (Func<T1, T2>) method.CreateDelegate(typeof(Func<T1, T2>));
+                    return message => (int) (object) del((T1) message);
+                }
+            }
 
             public Action<IMessage> CreateActionIMessage(MethodInfo method)
             {
@@ -127,5 +144,34 @@ namespace Google.Protobuf.Reflection
                 return (message, arg) => del((T1) message, (T2) arg);
             }
         }
+
+        // Runtime compatibility checking code - see ReflectionHelper<T1, T2>.CreateFuncIMessageInt32 for
+        // details about why we're doing this.
+
+        // Deliberately not inside the generic type. We only want to check this once.
+        private static bool CanConvertEnumFuncToInt32Func { get; } = CheckCanConvertEnumFuncToInt32Func();
+
+        private static bool CheckCanConvertEnumFuncToInt32Func()
+        {
+            try
+            {
+                MethodInfo method = typeof(ReflectionUtil).GetMethod(nameof(SampleEnumMethod));
+                // If this passes, we're in a reasonable runtime.
+                method.CreateDelegate(typeof(Func<int>));
+                return true;
+            }
+            catch (ArgumentException)
+            {
+                return false;
+            }
+        }
+
+        public enum SampleEnum
+        {
+            X
+        }
+
+        // Public to make the reflection simpler.
+        public static SampleEnum SampleEnumMethod() => SampleEnum.X;
     }
 }