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带一个字段(即整数)的结构-是否仍需要重写GetHashCode、Equals等?

gethashcode hashset struct performance c#

Dirk Boer · 技术社区 · 6 年前

通常,当有人谈论结构时,建议您重写 Equals GetHashCode 等等。

如果你有一个 struct 只需要一个 (或任何其他 简单值类型 )?

public struct LolCatId
{
    public int Id { get; }

    public LolCatId(int id)
    {
        Id = id;
    }
}

HashSets

1 回复 | 直到 6 年前

Dmitrii Bychenko 6 年前

Equals GetHashCode 基于反射的 缓慢的 ).

还有一些违约 等于 实现非常简单 ,例如:

  // Wrong Equals optimization demo: 
  // .Net (4.7) doesn't use reflection here but compare bytes 
  // and this decision is incorrect in the context
  struct MyDemo {
    public double x;
  }

  byte[] bits = BitConverter.GetBytes(double.NaN);

  bits[1] = 42;

  // a holds "standard" NaN
  MyDemo a = new MyDemo() { x = double.NaN };
  // b holds "modified" NaN
  MyDemo b = new MyDemo() { x = BitConverter.ToDouble(bits, 0)};

  Console.Write(string.Join(Environment.NewLine, 
    $"Are structs equal? {(a.Equals(b) ? "Yes" : "No")}",
    $"Are fields equal?  {(a.x.Equals(b.x) ? "Yes" : "No")}"));

结果:

Are structs equal? No
Are fields equal?  Yes

有关详细信息,请参阅

https://blogs.msdn.microsoft.com/seteplia/2018/07/17/performance-implications-of-default-struct-equality-in-c/

安全的一面 尤其是当两种方法都可以 易于实施

public struct LolCatId {
  public int Id { get; }

  public LolCatId(int id) {
    Id = id;
  }

  public override int GetHashCode() => Id;

  public override bool Equals(object obj) => 
    obj is LolCatId other ? other.Id == Id : false;
}