这里是群友提供听说是issue提的问题，高手回答的。
var coherentState = _cache.GetType().GetField("_coherentState", BindingFlags.NonPublic | BindingFlags.Instance);
var coherentStateValue = coherentState.GetValue(_cache);

var entriesCollection = coherentStateValue.GetType().GetProperty("EntriesCollection", BindingFlags.NonPublic | BindingFlags.Instance);

var entriesCollectionValue = entriesCollection.GetValue(coherentStateValue) as ICollection;

var keys = new List<string>();

if (entriesCollectionValue != null)
{
foreach (var item in entriesCollectionValue)
{
var methodInfo = item.GetType().GetProperty("Key");

var val = methodInfo.GetValue(item);

keys.Add(val.ToString());
}
}

return keys;

  　　　　　var ggt = _cache.GetType();
           var _coherentStates = ggt.GetRuntimeFields(); 
            var keys = new List<string>();
            foreach (var f in _coherentStates)
            {
                if(f.Name == "_coherentState")
                {
                   var ftype = f.FieldType;
                    var ssss= ftype.GetRuntimeFields();
                  　foreach(var ss in ssss)
                    {
                        if (ss.Name == "_entries")
                        {
                            var eee = ss.GetValue(f.GetValue(_cache));
                            if (eee != null)
                            { 
                                var cacheItems = eee as IDictionary;
                                if (cacheItems == null) return keys;
                                foreach (DictionaryEntry cacheItem in cacheItems)
                                {
                                    if (!string.IsNullOrEmpty(key))
                                    {
                                        if (cacheItem.Key.ToString().Contains(key))
                                        {
                                            keys.Add(cacheItem.Key.ToString());
                                        }
                                    }
                                    else
                                    {
                                        keys.Add(cacheItem.Key.ToString());
                                    }
                                }
                            }
                        }
                    }
                }
            }
         
            return keys;