pythonnet · lostmsu · Oct 12, 2020 · Oct 6, 2020 · Oct 9, 2020
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
@@ -26,7 +26,7 @@ public static MethodInfo Method
            }
        }

-        public static bool CanAssgin(Type clrType)
+        public static bool CanAssign(Type clrType)
        {
            if (typeof(ICollection).IsAssignableFrom(clrType))
            {
@@ -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/runtime/typemanager.cs b/src/runtime/typemanager.cs
@@ -225,7 +225,7 @@ internal static IntPtr CreateType(ManagedType impl, Type clrType)
            InitializeSlots(type, impl.GetType(), slotsHolder);

            if (Marshal.ReadIntPtr(type, TypeOffset.mp_length) == IntPtr.Zero
-                && mp_length_slot.CanAssgin(clrType))
+                && mp_length_slot.CanAssign(clrType))
            {
                InitializeSlot(type, TypeOffset.mp_length, mp_length_slot.Method, slotsHolder);
            }

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