看了很多资料终于搞明白cache中absoluteExpiration，slidingExpiration这两个参数的含义。
absoluteExpiration：用于设置绝对过期时间，它表示只要时间一到就过期，所以类型为System.DateTime，当给这个参数设置了一个时间时，slidingExpiration参数的值就只能为Cache.NoSlidingExpiration，否则出错；
slidingExpiration：用于设置可调过期时间，它表示当离最后访问超过某个时间段后就过期，所以类型为System.TimeSpan，当给这个参数设置了一个时间段时，absoluteExpiration的值就只能为Cache.NoAbsoluteExpiration，否则出错；
两个使用实例
Cache.Add("name",
content, null, System.Web.Caching.Cache.NoAbsoluteExpiration,
TimeSpan.FromMinutes(10), System.Web.Caching.CacheItemPriority.Normal,
null);
Cache.Add("name",
content, null, DateTime.Now.AddMinutes(10),
System.Web.Caching.Cache.NoSlidingExpiration, CacheItemPriority.Normal,
null);
//
// 摘要:
// Inserts an object into the System.Web.Caching.Cache object together with dependencies,
// expiration policies, and a delegate that you can use to notify the application
// before the item is removed from the cache.
//
// 参数:
// key:
// The cache key that is used to reference the object.
//
// value:
// The object to insert into the cache.
//
// dependencies:
// The file or cache key dependencies for the item. When any dependency changes,
// the object becomes invalid and is removed from the cache. If there are no dependencies,
// this parameter contains null.
//
// absoluteExpiration:
// The time at which the inserted object expires and is removed from the cache.
// To avoid possible issues with local time such as changes from standard time to
// daylight saving time, use System.DateTime.UtcNow instead of System.DateTime.Now
// for this parameter value. If you are using absolute expiration, the slidingExpiration
// parameter must be set to System.Web.Caching.Cache.NoSlidingExpiration.
//
// slidingExpiration:
// The interval between the time that the cached object was last accessed and the
// time at which that object expires. If this value is the equivalent of 20 minutes,
// the object will expire and be removed from the cache 20 minutes after it was
// last accessed. If you are using sliding expiration, the absoluteExpiration parameter
// must be set to System.Web.Caching.Cache.NoAbsoluteExpiration.
//
// onUpdateCallback:
// A delegate that will be called before the object is removed from the cache. You
// can use this to update the cached item and ensure that it is not removed from
// the cache.
//
// 异常:
// T:System.ArgumentNullException:
// The key, value, or onUpdateCallback parameter is null.
//
// T:System.ArgumentOutOfRangeException:
// You set the slidingExpiration parameter to less than TimeSpan.Zero or the equivalent
// of more than one year.
//
// T:System.ArgumentException:
// The absoluteExpiration and slidingExpiration parameters are both set for the
// item you are trying to add to the Cache.-or-The dependencies parameter is null,
// and the absoluteExpiration parameter is set to System.Web.Caching.Cache.NoAbsoluteExpiration,
// and the slidingExpiration parameter is set to System.Web.Caching.Cache.NoSlidingExpiration.
public void Insert(string key, object value, CacheDependency dependencies, DateTime absoluteExpiration, TimeSpan slidingExpiration, CacheItemUpdateCallback onUpdateCallback);