Question

I have a dictionary of objects, all of different types but derived from the same base class, and wanted to find a specific one. I thought I could filter the dictionary for objects of a given type using “.OfType()”, but didn’t find anything.

What did I not understand ?

class Program
{
  static Dictionary<String, Animal> critters;

  static void Main(string[] args)
  {
     critters = new Dictionary<String, Animal>();
     critters.Add("Tiddles", new Cat());
     critters.Add("Rover", new Dog());
     critters.Add("Bob", new Fish());

     Animal rover = GetAnimal<Dog>();
  }

  // Finds first animal of the specified type.
  static T GetAnimal<T>() where T : Animal
  {
     var qry = from animal in critters.OfType<T>()
               select animal;
     return (qry.Count() > 0) ? qry.First() : null;
  }
}

class Animal { }
class Dog : Animal { }
class Cat : Animal { }
class Fish : Animal { }

Thanks,

Ross

Answer 1

It is because dictionary IEnumarable is returning a <KeyValuePair<TKey, TValue>. Use the Values from it.

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I would also suggest that you change the last line in your GetAnimal from

return (qry.Count() > 0) ? qry.First() : null;

There is no need to waste a .Count() operation on qry, just use FirstOrDefault, it does the same thing.

return qry.FirstOrDefault();

You can reduce it, and not lose any readability, to:

return critters.Values.OfType<T>().FirstOrDefault();