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

public class MyClass<T>
{
    public T MainModel { get; set; }

    public Type GetTheType()
    {
return typeof(T).GetInterfaces().Where(i => i.IsGenericType)
    .Single(i => i.GetGenericTypeDefinition() == typeof(IEnumerable<>))
    .GetGenericArguments().First();
    }
}

public class Test
{
	public static void Main()
	{
var mainInstance = new MyClass<List<string>>();
Console.Out.WriteLine(mainInstance.GetTheType().FullName);		
	}
}