Why can't I use LINQ to Objects when my class implements IEnumerable<T> multiple times?

Question

I have an interesting issue where a class inherits from a class that implements IEnumerable, but I also want the class to implement IEnumerable for a different type. Everything works except for IEnumerable extension methods, which means I can't do any LINQ to objects by default without always having to cast first. Does anyone have any ideas besides constantly casting?

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

namespace LinqTesting
{
    public class Trucks<T> : Vehicles, IEnumerable<Truck>
    {    
        public Trucks()
        {    
            // Does Compile
            var a = ((IEnumerable<Truck>)this).FirstOrDefault();
            // Doesn't Compile, Linq.FirstOrDefault not found
            var b = this.FirstOrDefault();
        }    

        public new IEnumerator<Truck> GetEnumerator() { throw new NotImplementedException(); }
    }    

    public class Vehicles : IEnumerable<Vehicle>
    {    
        public IEnumerator<Vehicle> GetEnumerator() { throw new NotImplementedException(); }
        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { throw new NotImplementedException(); }
    }    

    public class Vehicle { }

    public class Truck : Vehicle { }
}     

Answer 1

Actually you can, but you can't take the advantage of generic types inference, because your class implements two IEnumerable<T> of two different types and the compiler can't know which type you want to use.

You can specify it direclty, like:

var b = this.FirstOrDefault<Truck>();