public IEnumerable<K> GetEnumerator<K>() where K : class, T
......
public ICollection<K> GetItems<K>() where K : class, T
......
public K First<K>() where K : class, T
......


改为:

public IEnumerable<K> GetEnumerator<K>() where K : class
......
public ICollection<K> GetItems<K>() where K : class
......
public K First<K>() where K : class
......
拒绝显摆。