allow casting objects to generic .NET interfaces without specifying generic arguments as long as there is no ambiguity

lostmsu · lostmsu · commit 537ddf4b7fc9 · 2022-05-07T10:15:16.000-07:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -21,6 +21,7 @@ This document follows the conventions laid out in [Keep a CHANGELOG][].
 -   `__name__` and `__signature__` to reflected .NET methods
 -   .NET collection types now implement standard Python collection interfaces from `collections.abc`.
 See [Mixins/collections.py](src/runtime/Mixins/collections.py).
+-   you can cast objects to generic .NET interfaces without specifying generic arguments as long as there is no ambiguity.
 -   .NET arrays implement Python buffer protocol
 -   Python integer interoperability with `System.Numerics.BigInteger`
 -   Python.NET will correctly resolve .NET methods, that accept `PyList`, `PyInt`,
diff --git a/src/runtime/Types/GenericType.cs b/src/runtime/Types/GenericType.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 
 namespace Python.Runtime
 {
@@ -20,10 +21,58 @@ internal GenericType(Type tp) : base(tp)
         /// </summary>
         public static NewReference tp_new(BorrowedReference tp, BorrowedReference args, BorrowedReference kw)
         {
+            var self = (GenericType)GetManagedObject(tp)!;
+            if (!self.type.Valid)
+            {
+                return Exceptions.RaiseTypeError(self.type.DeletedMessage);
+            }
+            var type = self.type.Value;
+
+            if (type.IsInterface && !type.IsConstructedGenericType)
+            {
+                var nargs = Runtime.PyTuple_Size(args);
+                if (nargs == 1)
+                {
+                    var instance = Runtime.PyTuple_GetItem(args, 0);
+                    return AsGenericInterface(instance, type);
+                }
+            }
+
             Exceptions.SetError(Exceptions.TypeError, "cannot instantiate an open generic type");
+
             return default;
         }
 
+        static NewReference AsGenericInterface(BorrowedReference instance, Type targetType)
+        {
+            if (GetManagedObject(instance) is not CLRObject obj)
+            {
+                return Exceptions.RaiseTypeError("only .NET objects can be cast to .NET interfaces");
+            }
+
+            Type[] supportedInterfaces = obj.inst.GetType().GetInterfaces();
+            Type[] constructedInterfaces = supportedInterfaces
+                .Where(i => i.IsConstructedGenericType && i.GetGenericTypeDefinition() == targetType)
+                .ToArray();
+
+            if (constructedInterfaces.Length == 1)
+            {
+                BorrowedReference pythonic = ClassManager.GetClass(constructedInterfaces[0]);
+                using var args = Runtime.PyTuple_New(1);
+                Runtime.PyTuple_SetItem(args.Borrow(), 0, instance);
+                return Runtime.PyObject_CallObject(pythonic, args.Borrow());
+            }
+
+            if (constructedInterfaces.Length > 1)
+            {
+                string interfaces = string.Join(", ", constructedInterfaces.Select(TypeManager.GetPythonTypeName));
+                return Exceptions.RaiseTypeError("Ambiguous cast to .NET interface. "
+                                               + $"Object implements: {interfaces}");
+            }
+
+            return Exceptions.RaiseTypeError("object does not implement "
+                                           + TypeManager.GetPythonTypeName(targetType));
+        }
 
         /// <summary>
         /// Implements __call__ for reflected generic types.
