我正在关注 stackoverflow 上的 previous post 关于从 C# 中的 List<T> 中删除重复项。
如果<T> 是一些用户定义的类型，例如：
class Contact
{
public string firstname;
public string lastname;
public string phonenum;
}
建议的 (HashMap) 不会删除重复项。我想，我必须重新定义一些比较两个对象的方法，不是吗？
【问题讨论】：
HashSet<T>确实删除重复项，因为它是一个集合...但仅当您的类型适当地定义了相等性时。
我怀疑“重复”是指“与另一个对象具有相同字段值的对象”-您需要覆盖 Equals/GetHashCode 才能使其工作，和/或实现 IEquatable<Contact>... 或你可以为HashSet<T> 构造函数提供一个IEqualityComparer<Contact>。
您可以只调用Distinct LINQ 扩展方法，而不是使用HashSet<T>。例如：
list = list.Distinct().ToList();
但同样，您需要以某种方式提供适当的平等定义。
这是一个示例实现。注意我是如何使它不可变的（可变类型的相等是奇怪的，因为两个对象可以在一分钟相等而下一分钟不相等）和 制成 字段私有，具有公共属性。最后，我已经密封了这个类——不可变类型通常应该是密封的，它使平等更容易讨论。
using System;
using System.Collections.Generic;
public sealed class Contact : IEquatable<Contact>
{
private readonly string firstName;
public string FirstName { get { return firstName; } }
private readonly string lastName;
public string LastName { get { return lastName; } }
private readonly string phoneNumber;
public string PhoneNumber { get { return phoneNumber; } }
public Contact(string firstName, string lastName, string phoneNumber)
{
this.firstName = firstName;
this.lastName = lastName;
this.phoneNumber = phoneNumber;
}
public override bool Equals(object other)
{
return Equals(other as Contact);
}
public bool Equals(Contact other)
{
if (object.ReferenceEquals(other, null))
{
return false;
}
if (object.ReferenceEquals(other, this))
{
return true;
}
return FirstName == other.FirstName &&
LastName == other.LastName &&
PhoneNumber == other.PhoneNumber;
}
public override int GetHashCode()
{
// Note: *not* StringComparer; EqualityComparer<T>
// copes with null; StringComparer doesn't.
var comparer = EqualityComparer<string>.Default;
// Unchecked to allow overflow, which is fine
unchecked
{
int hash = 17;
hash = hash * 31 + comparer.GetHashCode(FirstName);
hash = hash * 31 + comparer.GetHashCode(LastName);
hash = hash * 31 + comparer.GetHashCode(PhoneNumber);
return hash;
}
}
}
编辑：好的，响应对GetHashCode() 实现的解释请求：
EqualityComparer<T>.Default 总是处理这个，这很好......所以我用它来获取每个字段的哈希码。顺便说一下，两种处理无效的方法：
public override int GetHashCode()
{
// Unchecked to allow overflow, which is fine
unchecked
{
int hash = 17;
hash = hash * 31 + (FirstName ?? "").GetHashCode();
hash = hash * 31 + (LastName ?? "").GetHashCode();
hash = hash * 31 + (PhoneNumber ?? "").GetHashCode();
return hash;
}
}
或
public override int GetHashCode()
{
// Unchecked to allow overflow, which is fine
unchecked
{
int hash = 17;
hash = hash * 31 + (FirstName == null ? 0 : FirstName.GetHashCode());
hash = hash * 31 + (LastName == null ? 0 : LastName.GetHashCode());
hash = hash * 31 + (PhoneNumber == null ? 0 : PhoneNumber.GetHashCode());
return hash;
}
}
【讨论】：
class Contact {
public int Id { get; set; }
public string Name { get; set; }
public override string ToString()
{
return string.Format("{0}:{1}", Id, Name);
}
static private IEqualityComparer<Contact> comparer;
static public IEqualityComparer<Contact> Comparer {
get { return comparer ?? (comparer = new EqualityComparer()); }
}
class EqualityComparer : IEqualityComparer<Contact> {
bool IEqualityComparer<Contact>.Equals(Contact x, Contact y)
{
if (x == y)
return true;
if (x == null || y == null)
return false;
return x.Name == y.Name; // let's compare by Name
}
int IEqualityComparer<Contact>.GetHashCode(Contact c)
{
return c.Name.GetHashCode(); // let's compare by Name
}
}
}
class Program {
public static void Main()
{
var list = new List<Contact> {
new Contact { Id = 1, Name = "John" },
new Contact { Id = 2, Name = "Sylvia" },
new Contact { Id = 3, Name = "John" }
};
var distinctNames = list.Distinct(Contact.Comparer).ToList();
foreach (var contact in distinctNames)
Console.WriteLine(contact);
}
}
给予
1:John
2:Sylvia
【讨论】：
对于这项任务，我不一定认为实施 IComparable 是显而易见的解决方案。您可能希望以多种不同方式对唯一性进行排序和测试。
我倾向于实现IEqualityComparer<Contact>：
sealed class ContactFirstNameLastNameComparer : IEqualityComparer<Contact>
{
public bool Equals (Contact x, Contact y)
{
return x.firstname == y.firstname && x.lastname == y.lastname;
}
public int GetHashCode (Contact obj)
{
return obj.firstname.GetHashCode () ^ obj.lastname.GetHashCode ();
}
}
然后使用System.Linq.Enumerable.Distinct（假设您至少使用.NET 3.5）
var unique = contacts.Distinct (new ContactFirstNameLastNameComparer ()).ToArray ();
附言。说到HashSet<> 请注意HashSet<> 将IEqualityComparer<> 作为构造函数参数。
【讨论】：