Don't list extension methods multiple times (#1620)

* Don't return same method multiple times

* Deduplicate in PythonExtensionBinder instead

* Re-enable test

* Disable on .NET Core 2.1

Author: slozier

Src/IronPython/Runtime/Binding/PythonExtensionBinder.cs

@@ -23,7 +23,8 @@ public PythonExtensionBinder(PythonBinder binder, ExtensionMethodSet extensionMe
         public override MemberGroup GetMember(MemberRequestKind actionKind, Type type, string name) {
             var res = base.GetMember(actionKind, type, name);
             if (res.Count == 0) {
-                List<MemberTracker> trackers = new List<MemberTracker>();
+                // GetExtensionMethods may return duplicate MethodInfo so use a HashSet - https://github.com/IronLanguages/ironpython2/issues/810
+                HashSet<MemberTracker> trackers = null;
 
                 foreach (var method in _extMethodSet.GetExtensionMethods(name)) {
                     var parameters = method.GetParameters();
@@ -34,11 +35,11 @@ public override MemberGroup GetMember(MemberRequestKind actionKind, Type type, s
                     var paramType = parameters[0].ParameterType;
 
                     if (IsApplicableExtensionMethod(type, paramType)) {
-                        trackers.Add(MemberTracker.FromMemberInfo(method, paramType));
+                        (trackers ??= new HashSet<MemberTracker>()).Add(MemberTracker.FromMemberInfo(method, paramType));
                     }
                 }
 
-                if (trackers.Count > 0) {
+                if (trackers is not null) {
                     return new MemberGroup(trackers.ToArray());
                 }
             }

Tests/test_cliclass.py

@@ -1872,9 +1872,9 @@ class MyXamlRootObject(XamlTestObject):
         finally:
             os.unlink(fname)
 
-    @unittest.skipIf(is_netcoreapp, "https://github.com/IronLanguages/ironpython2/issues/810")
+    @unittest.skipIf(is_netcoreapp21, "TODO: figure out")
     def test_extension_methods(self):
-        import clr, imp, os
+        import clr, os
         if is_netcoreapp:
             clr.AddReference('System.Linq')
         else: