@@ -12,6 +12,7 @@
 
 ## Contributors
 
+-   Alex Earl ([@slide](https://github.com/slide))
 -   Alex Helms ([@alexhelms](https://github.com/alexhelms))
 -   Alexandre Catarino([@AlexCatarino](https://github.com/AlexCatarino))
 -   Arvid JB ([@ArvidJB](https://github.com/ArvidJB))
 
@@ -12,6 +12,7 @@ This document follows the conventions laid out in [Keep a CHANGELOG][].
 -   Added automatic NuGet package generation in appveyor and local builds
 -   Added function that sets Py_NoSiteFlag to 1.
 -   Added support for Jetson Nano.
+-   Added support for __len__ for .NET classes that implement ICollection
 
 ### Changed
 
 
@@ -142,6 +142,7 @@
     <Compile Include="Util.cs" />
     <Compile Include="platform\Types.cs" />
     <Compile Include="platform\LibraryLoader.cs" />
+    <Compile Include="slots\mp_length.cs" />
   </ItemGroup>
   <ItemGroup Condition=" '$(PythonInteropFile)' != '' ">
     <Compile Include="$(PythonInteropFile)" />
 
@@ -244,16 +244,5 @@ public static int sq_contains(IntPtr ob, IntPtr v)
 
             return 0;
         }
-
-
-        /// <summary>
-        /// Implements __len__ for array types.
-        /// </summary>
-        public static int mp_length(IntPtr ob)
-        {
-            var self = (CLRObject)GetManagedObject(ob);
-            var items = self.inst as Array;
-            return items.Length;
-        }
     }
 }
@@ -0,0 +1,50 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+
+namespace Python.Runtime.Slots
+{
+    internal static class mp_length_slot
+    {
+        /// <summary>
+        /// Implements __len__ for classes that implement ICollection
+        /// (this includes any IList implementer or Array subclass)
+        /// </summary>
+        public static int mp_length(IntPtr ob)
+        {
+            var co = ManagedType.GetManagedObject(ob) as CLRObject;
+            if (co == null)
+            {
+                Exceptions.RaiseTypeError("invalid object");
+            }
+
+            // first look for ICollection implementation directly
+            if (co.inst is ICollection c)
+            {
+                return c.Count;
+            }
+
+            Type clrType = co.inst.GetType();
+
+            // now look for things that implement ICollection<T> directly (non-explicitly)
+            PropertyInfo p = clrType.GetProperty("Count");
+            if (p != null && clrType.GetInterfaces().Any(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(ICollection<>)))
+            {
+                return (int)p.GetValue(co.inst, null);
+            }
+
+            // finally look for things that implement the interface explicitly
+            var iface = clrType.GetInterfaces().FirstOrDefault(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(ICollection<>));
+            if (iface != null)
+            {
+                p = iface.GetProperty(nameof(ICollection<int>.Count));
+                return (int)p.GetValue(co.inst, null);
+            }
+
+            Exceptions.SetError(Exceptions.TypeError, $"object of type '{clrType.Name}' has no len()");
+            return -1;
+        }
+    }
+}
@@ -1,9 +1,11 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Linq;
 using System.Reflection;
 using System.Runtime.InteropServices;
 using Python.Runtime.Platform;
+using Python.Runtime.Slots;
 
 namespace Python.Runtime
 {
@@ -153,6 +155,13 @@ internal static IntPtr CreateType(ManagedType impl, Type clrType)
             Marshal.WriteIntPtr(type, TypeOffset.tp_itemsize, IntPtr.Zero);
             Marshal.WriteIntPtr(type, TypeOffset.tp_dictoffset, (IntPtr)tp_dictoffset);
 
+            // add a __len__ slot for inheritors of ICollection and ICollection<>
+            if (typeof(ICollection).IsAssignableFrom(clrType) || clrType.GetInterfaces().Any(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(ICollection<>)))
+            {
+                InitializeSlot(type, TypeOffset.mp_length, typeof(mp_length_slot).GetMethod(nameof(mp_length_slot.mp_length)));
+            }
+
+            // we want to do this after the slot stuff above in case the class itself implements a slot method
             InitializeSlots(type, impl.GetType());
 
             if (base_ != IntPtr.Zero)
@@ -193,6 +202,12 @@ internal static IntPtr CreateType(ManagedType impl, Type clrType)
             return type;
         }
 
+        static void InitializeSlot(IntPtr type, int slotOffset, MethodInfo method)
+        {
+            IntPtr thunk = Interop.GetThunk(method);
+            Marshal.WriteIntPtr(type, slotOffset, thunk);
+        }
+
         internal static IntPtr CreateSubType(IntPtr py_name, IntPtr py_base_type, IntPtr py_dict)
         {
             // Utility to create a subtype of a managed type with the ability for the
 
@@ -92,6 +92,7 @@
     <Compile Include="doctest.cs" />
     <Compile Include="subclasstest.cs" />
     <Compile Include="ReprTest.cs" />
+    <Compile Include="mp_lengthtest.cs" />
   </ItemGroup>
   <ItemGroup>
     <Reference Include="Microsoft.CSharp" />
 
@@ -0,0 +1,171 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Python.Test
+{
+    public class MpLengthCollectionTest : ICollection
+    {
+        private readonly List<int> items;
+
+        public MpLengthCollectionTest()
+        {
+            SyncRoot = new object();
+            items = new List<int>
+            {
+                1,
+                2,
+                3
+            };
+        }
+
+        public int Count => items.Count;
+
+        public object SyncRoot { get; private set; }
+
+        public bool IsSynchronized => false;
+
+        public void CopyTo(Array array, int index)
+        {
+            throw new NotImplementedException();
+        }
+
+        public IEnumerator GetEnumerator()
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    public class MpLengthExplicitCollectionTest : ICollection
+    {
+        private readonly List<int> items;
+        private readonly object syncRoot;
+
+        public MpLengthExplicitCollectionTest()
+        {
+            syncRoot = new object();
+            items = new List<int>
+            {
+                9,
+                10
+            };
+        }
+        int ICollection.Count => items.Count;
+
+        object ICollection.SyncRoot => syncRoot;
+
+        bool ICollection.IsSynchronized => false;
+
+        void ICollection.CopyTo(Array array, int index)
+        {
+            throw new NotImplementedException();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            throw new NotImplementedException();
+        }
+    }
+
+    public class MpLengthGenericCollectionTest<T> : ICollection<T>
+    {
+        private readonly List<T> items;
+
+        public MpLengthGenericCollectionTest() {
+            SyncRoot = new object();
+            items = new List<T>();
+        }
+
+        public int Count => items.Count;
+
+        public object SyncRoot { get; private set; }
+
+        public bool IsSynchronized => false;
+
+        public bool IsReadOnly => false;
+
+        public void Add(T item)
+        {
+            items.Add(item);
+        }
+
+        public void Clear()
+        {
+            items.Clear();
+        }
+
+        public bool Contains(T item)
+        {
+            return items.Contains(item);
+        }
+
+        public void CopyTo(T[] array, int arrayIndex)
+        {
+            items.CopyTo(array, arrayIndex);
+        }
+
+        public IEnumerator GetEnumerator()
+        {
+            return ((IEnumerable)items).GetEnumerator();
+        }
+
+        public bool Remove(T item)
+        {
+            return items.Remove(item);
+        }
+
+        IEnumerator<T> IEnumerable<T>.GetEnumerator()
+        {
+            return items.GetEnumerator();
+        }
+    }
+
+    public class MpLengthExplicitGenericCollectionTest<T> : ICollection<T>
+    {
+        private readonly List<T> items;
+
+        public MpLengthExplicitGenericCollectionTest()
+        {
+            items = new List<T>();
+        }
+
+        int ICollection<T>.Count => items.Count;
+
+        bool ICollection<T>.IsReadOnly => false;
+
+        public void Add(T item)
+        {
+            items.Add(item);
+        }
+
+        void ICollection<T>.Clear()
+        {
+            items.Clear();
+        }
+
+        bool ICollection<T>.Contains(T item)
+        {
+            return items.Contains(item);
+        }
+
+        void ICollection<T>.CopyTo(T[] array, int arrayIndex)
+        {
+            items.CopyTo(array, arrayIndex);
+        }
+
+        IEnumerator<T> IEnumerable<T>.GetEnumerator()
+        {
+            return items.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((IEnumerable)items).GetEnumerator();
+        }
+
+        bool ICollection<T>.Remove(T item)
+        {
+            return items.Remove(item);
+        }
+    }
+}
@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+
+"""Test __len__ for .NET classes implementing ICollection/ICollection<T>."""
+
+import System
+import pytest
+from Python.Test import MpLengthCollectionTest, MpLengthExplicitCollectionTest, MpLengthGenericCollectionTest, MpLengthExplicitGenericCollectionTest
+
+def test_simple___len__():
+    """Test __len__ for simple ICollection implementers"""
+    import System
+    import System.Collections.Generic
+    l = System.Collections.Generic.List[int]()
+    assert len(l) == 0
+    l.Add(5)
+    l.Add(6)
+    assert len(l) == 2
+
+    d = System.Collections.Generic.Dictionary[int, int]()
+    assert len(d) == 0
+    d.Add(4, 5)
+    assert len(d) == 1
+
+    a = System.Array[int]([0,1,2,3])
+    assert len(a) == 4
+
+def test_custom_collection___len__():
+    """Test __len__ for custom collection class"""
+    s = MpLengthCollectionTest()
+    assert len(s) == 3
+
+def test_custom_collection_explicit___len__():
+    """Test __len__ for custom collection class that explicitly implements ICollection"""
+    s = MpLengthExplicitCollectionTest()
+    assert len(s) == 2
+
+def test_custom_generic_collection___len__():
+    """Test __len__ for custom generic collection class"""
+    s = MpLengthGenericCollectionTest[int]()
+    s.Add(1)
+    s.Add(2)
+    assert len(s) == 2
+
+def test_custom_generic_collection_explicit___len__():
+    """Test __len__ for custom generic collection that explicity implements ICollection<T>"""
+    s = MpLengthExplicitGenericCollectionTest[int]()
+    s.Add(1)
+    s.Add(10)
+    assert len(s) == 2
Original file line number	Diff line number	Diff line change
@@ -244,16 +244,5 @@ public static int sq_contains(IntPtr ob, IntPtr v)
244	244
245	245	return 0;
246	246	}
247		-
248		-
249		- /// <summary>
250		- /// Implements __len__ for array types.
251		- /// </summary>
252		- public static int mp_length(IntPtr ob)
253		- {
254		- var self = (CLRObject)GetManagedObject(ob);
255		- var items = self.inst as Array;
256		- return items.Length;
257		- }
258	247	}
259	248	}