Use explicit comparer for extension identifiers

csharp/src/Google.Protobuf/ExtensionRegistry.cs

@@ -42,6 +42,19 @@ namespace Google.Protobuf
     /// </summary>
     public sealed class ExtensionRegistry : ICollection<Extension>, IDeepCloneable<ExtensionRegistry>
     {
+        internal sealed class ExtensionComparer : IEqualityComparer<Extension>
+        {
+            public bool Equals(Extension a, Extension b)
+            {
+                return new ObjectIntPair<Type>(a.TargetType, a.FieldNumber).Equals(new ObjectIntPair<Type>(b.TargetType, b.FieldNumber));
+            }
+            public int GetHashCode(Extension a)
+            {
+                return new ObjectIntPair<Type>(a.TargetType, a.FieldNumber).GetHashCode();
+            }
+
+            internal static ExtensionComparer Instance = new ExtensionComparer();
+        }
         private IDictionary<ObjectIntPair<Type>, Extension> extensions;
 
         /// <summary>

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

@@ -448,7 +448,7 @@ namespace Google.Protobuf.Reflection
 
         private static IEnumerable<Extension> GetAllExtensions(FileDescriptor[] dependencies, GeneratedClrTypeInfo generatedInfo)
         {
-            return dependencies.SelectMany(GetAllDependedExtensions).Distinct().Concat(GetAllGeneratedExtensions(generatedInfo));
+            return dependencies.SelectMany(GetAllDependedExtensions).Distinct(ExtensionRegistry.ExtensionComparer.Instance).Concat(GetAllGeneratedExtensions(generatedInfo));
         }
 
         private static IEnumerable<Extension> GetAllGeneratedExtensions(GeneratedClrTypeInfo generated)