.Net Interfaces Are Not Classes

Fri, Oct 14 2011

If you have an attention span as short as mine, feel free to skip to the TL;DR.

Well, duh! Right?

To be honest, if you know that already, and who doesn’t, then this post does not have much to offer, except a tale of DataBinding exceptions, TypeConverters, and a few interesting tid-bits I picked up along the way, along with what I think it’s a bug in a core .Net class.

Throughout this post I will use two interface and two classes implementing them to make my point:

interface IBase {
  string BaseMethod();
interface IDerived : IBase {
  string DerivedMethod();

class Base: IBase {
  public string BaseMethod() { return "BaseMethod"; }
class Derived : Base, IDerived {
  public string DerivedMethod() { return "DerivedMethod"; }

Rocket surgery, innit?

data binding and interfaces

Good OOP code monkeys have designed and programmed against abstractions, e.g., interfaces, before Uncle Bob articulated the SOLID principles, and I like to believe that even before the advent of the Dependency Inversion principle.

It only makes sense that we apply the same principle, programming against interfaces, to MVVM design, as it shows in this overly simplistic example:

class MyForm : Form {
  public IBase Base { get; set; }
  public IDerived Derived { get ; set; }

  protected override void OnLoad(EventArgs e) {
    this.Text += ":" + (Base == null 
                          ? "Base is null" 
                          : Base.BaseMethod());
    this.Text += ":" + (Derived == null 
                          ? "Derived is null" 
                          : Derived.DerivedMethod());
class MyViewModel  {
  public IBase Base { get { return new Base(); } }
  public IDerived Derived { get { return new Derived(); } }

I’m using Windows.Forms because WPF would’ve been too verbose for such a tiny task, but the concept translates almost word for word.

MVVM was designed to make use of binding, DataBinding to be more specific:

void Main()
  MyForm viewBase = new MyForm { Text = "Base" }, 
         viewDerived = new MyForm { Text = "Derived" };
  var viewModel = new MyViewModel();

  viewBase.DataBindings.Add("Base", viewModel, "Base", 
  viewDerived.DataBindings.Add("Derived", viewModel, "Derived", 


Running that code (here is the script to use with LINQPad) we’d expect to get two windows with titles “Base:BaseMethod”, and respectively “Derived:DerivedMethod”, yet instead we get two big, fat, FormatExceptions that make as much sense as a fish with a bicycle (full stack trace here and also notice how early in the control’s lifecyle this error happens):

System.FormatException: Cannot format the value to the desired type.
   at System.Windows.Forms.Binding.FormatObject(Object value)
   at System.Windows.Forms.Control.UpdateBindings()
   at System.Windows.Forms.Control.CreateControl()
   at System.Windows.Forms.Control.WmShowWindow(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
   at System.Windows.Forms.Form.WmShowWindow(Message& m)
   at System.Windows.Forms.Form.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Cannot format ?! And format what – a Base into an IBase? How is that possible?

the devil is in the code

Very puzzled, I pulled out a decompiler, JetBrains’ dotPeek, took a look at Binding.FormatObject(Object), and found these interesting lines:

//-  propertyType is typeof(IBase)
//-  value is our new Base()
//-  so is obj1
if (propertyType == typeof (object))
  return value;
if (obj1 != null && 
    (obj1.GetType().IsSubclassOf(propertyType) || 
     obj1.GetType() == propertyType))
  return obj1;

I’m pretty sure this would’ve jumped out at you, dear reader, because you’re so much smarter, but in my head it didn’t make any sense; I thought “of course Base is a child of IBase – what’s your problem?”.

Yes, Base is a child of IBase, for very fuzzy values of child, but it is not a subclass of IBase: Base implements IBase but does not inherit from IBase.

Given the direction of the assignment, I believe this is a bug: IsSubclassOf is an unnecessary restriction, they should have used propertyType.IsAssignableFrom(obj1.GetType()) instead, which would’ve taken care of this mess and allow for binding onto an interface to happen as expected.

Well, let’s debate that later, how do we fix it?

Obviously we could change MyForm.Base to be of type Base and MyForm.Derived to be of either type Derived or Base. That would work, and it’s a reasonable fix if you happen to dislike abstractions.

You could also change both properties to be of type object, but that just means you have OOP in general.

The next few instructions in Binding.FormatObject contain another solution:

TypeConverter converter = 
    TypeDescriptor.GetConverter(value != null 
                                ? value.GetType() 
                                : typeof (object));
if (converter != null && converter.CanConvertTo(propertyType))
  return converter.ConvertTo(value, propertyType);

You can read more about the TypeConverter, while I create one for the Derived class and then decorate it using the TypeConverterAttribute:

class DerivedConverter : TypeConverter {
  private readonly Type typeD = typeof(Derived);
  public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType) {
    if (destinationType.IsAssignableFrom(typeD)) 
      return true;
    return base.CanConvertFrom(context, destinationType);
  public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, 
         object value, Type destinationType) {  
    if (destinationType.IsAssignableFrom(typeD)) 
      return value;
    return base.ConvertTo(context, culture, value, destinationType);

class Derived : Base, IDerived {
  public string DerivedMethod(){return "DerivedMethod"; }

If you rerun the script with the TypeConverter in place, you’ll only get one exception for IBase, while the IDerived example performs as expected.

Kind of silly, isn’t it, to use a separate class to convert an instance into the interface it implements anyway.

It was at this point that I did what any sane developer would do when faced with this snafu: bitch on Twitter. Rory Primrose (@roryprimrose – you should follow him) replied with an example of his own, this one around interface inheritance.

interface inheritance

His assumption was that calling Type.GetMethods on a derived interface would also return the methods of its parent(s).

I had to try and cook some of my own code and here are the surprising results:

  inst_pub = BindingFlags.Instance | BindingFlags.Public,
  flat_inst_pub = BindingFlags.FlattenHierarchy | inst_pub,
  decl_inst_pub = BindingFlags.DeclaredOnly | inst_pub;
Action<string, Array> logm = (s, a) => a.Length.Dump(s);

Derived d = new Derived();
Type iB = typeof(IBase),
iD = typeof(IDerived),
cB = typeof(Base),
cD = d.GetType();

logm("Instance | Public", cD.GetMethods(inst_pub));                   //=> 6
logm("Flatten | Instance | Public", cD.GetMethods(flat_inst_pub));    //=> 6
logm("Declared | Instance | Public", cD.GetMethods(decl_inst_pub));   //=> 1

logm("Instance | Public", iD.GetMethods(inst_pub));                   //=> 1
logm("Flatten | Instance | Public", iD.GetMethods(flat_inst_pub));    //=> 1?!

cD.IsSubclassOf(cB).Dump("Derived subclass of Base?");                //=> True
(d is Base).Dump("Derived is Base");                                  //=> True
cD.IsSubclassOf(iD).Dump("Derived subclass of IDerived?");            //=> False
(d is IDerived).Dump("Derived is IDerived");                          //=> True

iD.IsAssignableFrom(cD).Dump("Derived assignable to IDerived?");       //=> True
iD.IsSubclassOf(iB).Dump("IDerived subclass of IBase?");               //=> False?!
iB.IsAssignableFrom(iD).Dump("IDerived assignable IBase?");            //=> True

The classes behave as expected (2 methods + 4 from Object), but I expected FlattenHierarchy with IDerived to return two methods: its own DerivedMethod and its parent BaseMethod.

Probing further, shows not only that classes are not subclasses of the interfaces they implement, but neither are interfaces subclasses of their parents.

Much sooner that I, Rory asked the question at the core of his post:

Does interface inheritance exist?

Much to my amazement, the answer is no, at least according to reflection. This is completely not what I had assumed.

when in doubt

… look up the specs.

In highsight, it should all have been obvious. My mistake lies in thinking of interfaces as purely virtual/abstract classes (there’s some C++ roots showing – only pointing that out to get some street cred).

However, that shows fault immediately because, even if it was so – even if interfaces were purely virtual classes, the CLI does not support multiple class inheritance (but you knew that already, didn’t you), yet it supports multiple interface inheritance using the exact same syntax.

The standard, ECMA-335, explicitly states in 8.9.9 Object type inheritance that:

[…] all object types shall either explicitly or implicitly declare support for (i.e., inherit from) exactly one other object type.

If I were to nitpick though, I would point out that the C# standard, ECMA-334, is a bit inconsistent in usage: within 8.9 Interfaces you will read “Interfaces can employ multiple inheritance”, which is contradicted by ECMA-335 8.10 Member inheritance:

While interface types can be derived from other interface types, they only “inherit” the requirement to implement method contracts, never fields or method implementations.

This is a CLI restriction: while ordinary C++ does support multiple inheritance, Managed C++ doesn’t. This blog post shares Managed C++ team’s decision and includes this grain of insight about Eiffel’s support of multiple inheritance on the CLI:

[…] the CLI does not, for example, support private inheritance, value inheritance (that is, the inheritance of implementation but not of type), or multiple inheritance (MI). While a language can choose to support these aspects of inheritance, that support requires a mapping onto the existing CLI object model because there is no direct support.

The Eiffel language under CLI, for example, choose to provide an MI mapping […]


What did we learn?

In truth, there’s no true inheritance in the CLI when it comes to interfaces – that is reserved for classes; in practice, we can consider interface inheritance exists as long as we use those relationships for typing rather than reflection.

If you perform runtime type inspection with a goal to see if casting is possible, use Type.IsAssignableFrom, rather than Type.IsSubclassOf. I cannot think of a good use case where you’d explicitly want IsSubclassOf and that would not betray your abstractions, but it’s late and I cannot think well during normal hours, let alone late into the night.

When you need to list the members an interface exposes, including the members of its parents, you have to perform recursive inspection using GetInterfaces() and then query those results in turn for members.

If DataBinding throws wrenches in your spokes because the target type is an interface, either replace it with an abstract class or use a TypeConverter in combination with a TypeConverterAttribute.