public interface ICacheHelper
  {   
    T Get<T>(string key);
        
    void Set<T>(string key, T value);   

    void Remove(string key);        
  }

  public User Get(int id)
    {
      if (id <= 0)
        throw new ArgumentNullException("id");

      var key = string.Format(USER_CACHE_KEY, id);
      var user = _cacheHelper.Get<User>(key);
      if (user != null)
        return user;

      return _repository.Get(id);
    }  

public User GetV2(int id)
{
  if (id <= 0)
    throw new ArgumentNullException("id");

  var key = string.Format(USER_CACHE_KEY, id);
  var user = _cacheHelper.Get<User>(key);
  if (user != null)
    return user;
    user = _repository.Get(id);
  if (user != null)
    _cacheHelper.Set(key, user);
    return user;
}

public User GetV3(int id)
{
   if (id <= 0)
     throw new ArgumentNullException("id");

   var key = string.Format(USER_CACHE_KEY, id);
    return _cacheHelperV2.Get<User>(key, () => _repository.Get(id));      
}

//ICache Get<T>实现
public T Get<T>(string key, Func<T> fetch = null)
{
  T result = default(T);
  var obj = Cache.Get(key);
  if (obj is T)
  {
    result = (T)obj;
  }

  if(result == null)
  {
    result = fetch();

    if (result != null)
      Set(key, result);
  }

  return result;
}      

public T Get<T>(object id, Func<T> fetch = null)
{
  var type = typeof(T);
  var key = string.Format("urn:{1}:{2}", type.Name, id.ToString());//这里是关键，直接用TypeName来充当Key

  return Get(key, fetch);
}

public T Get<T>(string key, Func<T> fetch = null)
{
  T result = default(T);

  var obj = Cache.Get(key);
  if (obj is T)
  {
    result = (T)obj;
  }

  if (result == null)
  {
    result = fetch();

    if (result != null)
      Set(key, result);
   }

   return result;
}

public User GetV4(int id)
{
   if (id <= 0)
    throw new ArgumentNullException("id");

   return _cacheHelperV3.Get<User>(id, () => _repository.Get(id));
}

public class User
{
    [PrimaryKey] //这个Attribute是最重要的东西
    public int UserId { get; set;}

    public string UserName { get; set; }

    public string Cellphone { get; set; }
}
public void Set<T>(T obj)
{
   //此处应该被缓存以提高反射的效率
   var type = typeof(T);
   var primaryKey = type.GetProperties()
        .FirstOrDefault(t => t.GetCustomAttributes(false)
          .Any(c => c is PrimaryKeyAttribute));//这里通过取PrimaryKeyAttribute来获取ID的value
    var keyValue = primaryKey.GetValue(obj, null);      
    var key = string.Format("urn:{0}:{1}", type.Name, keyValue);

    var dt = DateTime.UtcNow.AddDays(1);//假设默认缓存1天
    var offset = new DateTimeOffset(dt);
    Cache.Set(key, obj, offset);
}

public class UserCacheEntity
{
    [PrimaryKey]
    public int ID
    {
      get
      {
        return string.Format("{0}:{1}", UserId, UserName);
      }
    }

    public int UserId { get; set; }

    public string UserName { get; set; }

    public string Cellphone { get; set; }
}