Question

I am trying to return the object type of the class that implements the interface defined in the code below.

The linq statement only returns the interface itself, and so the console output is just:

AssignableExperiment.IRule

Why isn’t the concrete class being returned?

using System;
using System.Linq;

namespace AssignableExperiment
{
    public interface IRule
    {
        void Validate(string s);
    }

    public class ConcreteRule : IRule
    {
        public void Validate(string s)
        {
           // some logic
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var ruleType = typeof(IRule);

            var ruleTypes = from t in ruleType.Assembly.GetTypes()
                            where t.IsAssignableFrom(ruleType)
                            select t;

            foreach (var type in ruleTypes)
            {
                Console.WriteLine(type);
            }

            Console.ReadLine();
        }
    }
}

Answer 1

You should turn it around the IsAssignableFrom MSDN. Because IsAssignableFrom works another way around as expected: BaseType.IsAssignableFrom(DerviedType) returns true.

var ruleTypes = from t in ruleType.Assembly.GetTypes()
                            where ruleType.IsAssignableFrom(t)
                            select t;

If you don’t want to get back IRule:

var ruleTypes = from t in ruleType.Assembly.GetTypes()
                            where ruleType.IsAssignableFrom(t) && t != ruleType
                            select t;