internal ValueCollection(_Dictionary <TKey, TValue> dictionary)
 {
     this.dictionary = dictionary;
 }
 internal Enumerator(_Dictionary <TKey, TValue> dictionary)
 {
     this.en = dictionary.GetEnumerator();
 }
 internal Enumerator(_Dictionary <TKey, TValue> dictionary)
 {
     this.dictionary = dictionary;
     this.slot       = 0;
     this.current    = default(KeyValuePair <TKey, TValue>);
 }