Make len work for ICollection<> interface objects

danabr · lostmsu · commit 21f11dbb40c0 · 2020-10-12T12:12:08.000-07:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -31,6 +31,7 @@ details about the cause of the failure
 -    Fix non-delegate types incorrectly appearing as callable.
 -    Indexers can now be used with interface objects
 -    Fixed a bug where indexers could not be used if they were inherited
+-    Made it possible to use `__len__` also on `ICollection<>` interface objects
 
 ## [2.5.0][] - 2020-06-14
 
diff --git a/src/runtime/slots/mp_length.cs b/src/runtime/slots/mp_length.cs
@@ -36,6 +36,10 @@ public static bool CanAssgin(Type clrType)
             {
                 return true;
             }
+            if (clrType.IsInterface && clrType.IsGenericType && clrType.GetGenericTypeDefinition() == typeof(ICollection<>))
+            {
+                return true;
+            }
             return false;
         }
 
diff --git a/src/tests/test_mp_length.py b/src/tests/test_mp_length.py
@@ -47,3 +47,17 @@ def test_custom_generic_collection_explicit___len__():
     s.Add(1)
     s.Add(10)
     assert len(s) == 2
+
+def test_len_through_interface_generic():
+    """Test __len__ for ICollection<T>"""
+    import System.Collections.Generic
+    l = System.Collections.Generic.List[int]()
+    coll = System.Collections.Generic.ICollection[int](l)
+    assert len(coll) == 0
+
+def test_len_through_interface():
+    """Test __len__ for ICollection"""
+    import System.Collections
+    l = System.Collections.ArrayList()
+    coll = System.Collections.ICollection(l)
+    assert len(coll) == 0

Original file line number	Diff line number	Diff line change
`@@ -36,6 +36,10 @@ public static bool CanAssgin(Type clrType)`
`36`	`36`	`{`
`37`	`37`	`return true;`
`38`	`38`	`}`
	`39`	`+ if (clrType.IsInterface && clrType.IsGenericType && clrType.GetGenericTypeDefinition() == typeof(ICollection<>))`
	`40`	`+ {`
	`41`	`+ return true;`
	`42`	`+ }`
`39`	`43`	`return false;`
`40`	`44`	`}`
`41`	`45`