using System;

using System.Collections.Generic;

using System.Linq;

public class Program

    {

        public static void Main()

        {

            Program p = new Program();

            p.AddOrUpdate<User>(new User(){ id = 1});

        }

    public class User{

        public int id{get;set;}

​

    }

​

            public void AddOrUpdate<TEntity>(TEntity entity)

    {

​

        var key = entity.GetType().GetProperties().FirstOrDefault(p => p.Name.ToLower().Contains("id")).GetValue(entity);

​

                Console.WriteLine(key);

        if (entity == null)

            throw new ArgumentNullException("entity");

​

    }

}