为了获得最佳性能,我会尽量避免每次GetHashCode
调用时都迭代整个集合。的目的GetHashCode
是将性能提高到比评估每个元素更好的程度。因此,当列表中的元素像这样更改时,我可能会尝试维护哈希码。
class Program
{
static void Main(string[] args)
{
MyClassList l = new MyClassList() { new MyClass() {Type="Bob", Id=1}, new MyClass() {Type="Jones", Id=2}};
MyClassList l2 = new MyClassList() { new MyClass() { Type = "Jones", Id = 2 }, new MyClass() { Type = "Bob", Id = 1 } };
MyClassList l3 = new MyClassList() { new MyClass() { Type = "Jones", Id = 2 }};
Console.WriteLine("{0} {1} {2}", l.GetHashCode(), l2.GetHashCode(), l3.GetHashCode());
l3.Add(new MyClass() { Type = "Bob", Id = 1 });
Console.WriteLine("{0}", l3.GetHashCode());
}
}
public class MyClass
{
public string Type { get; set; }
public int Id { get; set; }
public override int GetHashCode()
{
return (Type.GetHashCode() % 0x8000) | (int)((uint)Id.GetHashCode() & 0xFFFF0000);
}
}
public class MyClassList : IList<MyClass>
{
List<MyClass> internalList;
int hashCode = 0;
public MyClassList()
{
internalList = new List<MyClass>();
}
private void IncludeInHash(MyClass item)
{
hashCode ^= item.GetHashCode();
}
private void ExcludeFromHash(MyClass item)
{
IncludeInHash(item);
}
public override int GetHashCode()
{
return hashCode;
}
public int IndexOf(MyClass item)
{
return internalList.IndexOf(item);
}
public void Insert(int index, MyClass item)
{
internalList.Insert(index, item);
// Make sure Insert is successful (doesn't throw an exception) before affecting the hash
IncludeInHash(item);
}
public void RemoveAt(int index)
{
MyClass reduce = internalList[index];
internalList.RemoveAt(index);
// Make sure RemoveAt is successful before affecting the hash
ExcludeFromHash(reduce);
}
public MyClass this[int index]
{
get
{
return internalList[index];
}
set
{
MyClass reduce = internalList[index];
internalList[index] = value;
// Make sure these happen atomically; don't allow exceptions to prevent these from being accurate.
ExcludeFromHash(reduce);
IncludeInHash(value);
}
}
public void Add(MyClass item)
{
internalList.Add(item);
IncludeInHash(item);
}
public void Clear()
{
internalList.Clear();
hashCode = 0;
}
public bool Contains(MyClass item)
{
return internalList.Contains(item);
}
public void CopyTo(MyClass[] array, int arrayIndex)
{
internalList.CopyTo(array, arrayIndex);
}
public int Count
{
get { return internalList.Count; }
}
public bool IsReadOnly
{
get { return false; }
}
public bool Remove(MyClass item)
{
if (internalList.Remove(item))
{
ExcludeFromHash(item);
return true;
}
else
return false;
}
public IEnumerator<MyClass> GetEnumerator()
{
return internalList.AsReadOnly().GetEnumerator();
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}