Generic.cs

 class Program
    {
        static void Main(string[] args)
        {
			IEnumerable<Xpto> enumerable = new List<Xpto>
			{
				new Xpto(15,"a"),
				new Xpto(8,"c"),
				new Xpto(3,"e"),
				new Xpto(5,"t"),
				new Xpto(4,"b"),
				new Xpto(2,"v"),
				new Xpto(1,"q")
			};
	        IQueryable<Xpto> list = enumerable.AsQueryable();
			Expression<Func<Xpto, string>> expression = CreateExpression<Xpto>("_text");
	        IEnumerable<Xpto> newEnumerable = list.OrderBy(expression);
	        foreach (Xpto xpto in newEnumerable)
		        Console.WriteLine($"Id = {xpto._id} and Text = {xpto._text}");
			Console.Read();
        }

	    private static Expression<Func<T, string>> CreateExpression<T>(string fieldName)
	    {
		    ParameterExpression parameter = Expression.Parameter(typeof(T), "x");
		    MemberExpression body = Expression.PropertyOrField(parameter, fieldName);
		    return Expression.Lambda<Func<T, string>>(body, parameter); //x => x.Id
	    }
	}

	class Xpto
	{
		public int _id;
		public string _text;

		/// <inheritdoc />
		public Xpto(int id, string text)
		{
			_id = id;
			_text = text;
		}
	}