object obj = HttpRuntime.Cache.Get(key);
if (obj == null )
{
    Debug.WriteLine(key + "被新增進Cache, 過期時間一分鐘");
    HttpRuntime.Cache.Add(key, "1", null, Cache.NoAbsoluteExpiration, new TimeSpan(TimeSpan.TicksPerMinute), CacheItemPriority.Normal, CacheItemRemovedCallback);
}
else
{
    Debug.WriteLine(key 
 
+ "已存在Cache中, 勿重複新增");
}

CacheItemRemovedCallback為快取過期時的回撥方法