动态构建Lambda 的问题,请指教,急!

Question

 public class Person
        {
            public string Name { get; set; }
            public int Age { get; set; }
            public int Salary { get; set; }
        }
 List<Person> Persons=new List<Person> ()
            Type TType=typeof(Person);
           
            ParameterExpression  pp = Expression.Parameter(TType, "p");

 Expression temp = Expression.GreaterThan(Expression.Call(Expression.Property(pp, TType.GetProperty("Age")), ??????),
                 Expression.Constant(20));
var lambda = Expression.Lambda<Func<Person, bool>>(temp, pp);

我想动态构建出Persons.Where(p=>p.Age>20).ToList(); 表达式请问怎么写啊!请帮帮忙!一个新手的求救!