using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

class Foo {
	public static Type GetEnumerableType(Type type)
	{
	    if (type == null)
	        throw new ArgumentNullException("type");
	
	    if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IEnumerable<>))
	        return type.GetGenericArguments()[0];
	
	    var iface = (from i in type.GetInterfaces()
	                 where i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IEnumerable<>)
	                 select i).FirstOrDefault();
	
	    if (iface == null)
	        throw new ArgumentException("Does not represent an enumerable type.", "type");
	
	    return GetEnumerableType(iface);
	}
	
	static void Main() {
	    foreach (var type in new[] {
	                 typeof(IEnumerable<string>),
	                 typeof(List<int>),
	                 typeof(EnumerableTest)
	             }) {
	    	Console.WriteLine("{0}: {1}",
	    		type.FullName,
	    		GetEnumerableType(type).FullName);
	    }
	}
}

class EnumerableTest : IEnumerable<long>, IEnumerable<double> {
    IEnumerator<long> IEnumerable<long>.GetEnumerator() {
        throw new NotImplementedException();
    }
    IEnumerator<double> IEnumerable<double>.GetEnumerator() {
        throw new NotImplementedException();
    }
    public IEnumerator GetEnumerator() {
        throw new NotImplementedException();
    }
}