using System;

using System.Linq;

using System.Collections.Generic;

using System.Reflection;

public class Program

{

    public static void Main()

    {

        var list = new List<dynamic>()

        {

            new { CreatedAt = DateTime.Now, User = "user", Id = 1 },

            new { CreatedAt = DateTime.Now, User = "user1", Id = 2 },

            new { CreatedAt = DateTime.Now, User = "user2", Id = 3 },

        };

        PropertyInfo propertyInfo = list.GetType().GetGenericArguments().Single().GetProperty("User");

        Console.WriteLine(propertyInfo == null ? "Null" : "NotNull");

    }

}