[Source: http://geekswithblogs.net/EltonStoneman]
David Meyer has a neat duck typing library for .NET which I’ve been looking at as I had a requirement to treat objects of various types in the same way. They all expose a “Value” property, but don’t implement an interface.
The library does more than I needed so I put together a quick duck typing implementation which just lets me treat an instance of an object which tacitly implements an interface, as a true implementation of the interface. In this example I can access BespokeDateTime and BespokeInt as IHasValue:
Using Castle’s Dynamic Proxy, this is simple to do – just requiring you to generate an interface proxy with a custom interceptor. The interceptor invokes the interface method on the real object, so you effectively wrap an instance in an interface it doesn’t explicitly implement:
BespokeDateTime bespoke = new BespokeDateTime(new DateTime(2000, 1, 1));
IHasValue duck = DuckType.As<IHasValue>(bespoke);
Console.WriteLine(duck.Value);
- or using an object extension to hide the implementation:
bespoke.As<IHasValue>().Value;
Iterating over collections is much cleaner with duck typing – if I have a collection that contains both BespokeDateTime and BespokeInt objects, to get all their values I can use:
foreach (object obj in GetObjects())
{
Assert.IsNotNull(DuckType.As<IHasValue>(obj).Value);
}
- compared to casting each object:
foreach (object obj in GetObjects())
{
BespokeDateTime bespokeDateTime = obj as BespokeDateTime;
if (bespokeDateTime != null)
{
Assert.IsNotNull(bespokeDateTime.Value);
}
else
{
BespokeInt bespokeInt = (BespokeInt) obj;
Assert.IsNotNull(bespokeInt.Value);
}
}
Use with caution over large collections, as there is a noticeable performance hit in using the duck type. Over 500,000 items the duck-typed code takes 40-50 times as long as the casting code, averaging 8 seconds to 0.2 seconds, with duck-typing by extension taking marginally longer again:
The full code (all 34 lines – including validation that the type to duck does tacitly implement the interface) is on MSDN Code Gallery here: Duck Typing with Castle.