测试对象相等性的最佳方法是什么 - 不重写Equals＆GetHashCode，或实现IEquatable <T>？

3 回答

• 1

  如果不重写它，则无法更改 Equals 实例方法的结果 . 因此，您需要使用 IEqualityComparer<T> ，需要将其显式传递给需要进行相等性检查的代码 . 幸运的是，大多数内置集合都接受这样的相等比较器作为其构造函数的参数 .

  如果您不想以任何方式更改原始类，则使用反射实现 IEqualityComparer<T> 似乎是您唯一的选择 .

  您可以使用 GetField("_personName",BindingFlags.NonPublic) 获取 FieldInfo ，然后在其上调用 GetValue 以获取值 .

  void Main()
  {
          Guid id = Guid.NewGuid();
          string personName = "MyName";
          MyClass object1 = new MyClass(id, personName);
          MyClass object2 = new MyClass(id, personName);
          //This returns false, but I'd like it to return true:
          Console.WriteLine(object1.Equals(object2));
          //[edit]...by using, for example:
          Console.WriteLine(MyClassEqualityComparer.Instance.Equals(object1, object2));
  }
  
  public class MyClass
  {
      private Guid _id;
      private string _personName;
      public MyClass(Guid id, string personName)
      {
          _id = id;
          _personName = personName;
      }
  }
  
  public class MyClassEqualityComparer:IEqualityComparer<MyClass>
  {
    private static FieldInfo personNameField=typeof(MyClass).GetField("_personName",BindingFlags.Instance|BindingFlags.NonPublic);
    private static FieldInfo idField=typeof(MyClass).GetField("_id",BindingFlags.Instance|BindingFlags.NonPublic);
  
    public bool Equals(MyClass o1,MyClass o2)
    {
      if(o1==o2)
        return true;
      if(o1==null||o2==null)
        return false;
      string name1=(string)personNameField.GetValue(o1);
      string name2=(string)personNameField.GetValue(o2);
      if(name1!=name2)
        return false;
      Guid id1=(Guid)idField.GetValue(o1);
      Guid id2=(Guid)idField.GetValue(o2);
      return id1==id2;
    }
  
    public int GetHashCode(MyClass o)
    {
      if(o==null)
        return 0;
      string name=(string)personNameField.GetValue(o);
      Guid id=(Guid)idField.GetValue(o);
      return name.GetHashCode()^id.GetHashCode();
    }
  
    private MyClassEqualityComparer()
    {
    }
  
    public static readonly IEqualityComparer<MyClass> Instance=new MyClassEqualityComparer();
  }
  

  回复于 2024-04-27T04:24:04+08:00
• 2

  您可以创建使用反射的自定义IEqualityComparer<T>实现：

  var comparer = new MyClassEqualityComparer();
  Console.WriteLine(comparer.Equals(object1, object2));
  
  // ...
  
  public class MyClassEqualityComparer : EqualityComparer<MyClass>
  {
      private static readonly string[] _names = { "_id", "_personName" };
  
      private static readonly FieldInfo[] _infos =
          typeof(MyClass).GetFields(BindingFlags.Instance | BindingFlags.NonPublic)
                         .Where(fi => _names.Contains(fi.Name))
                         .ToArray();
  
      public override bool Equals(MyClass x, MyClass y)
      {
          return _infos.All(fi => object.Equals(fi.GetValue(x), fi.GetValue(y)));
      }
  
      public override int GetHashCode(MyClass obj)
      {
          unchecked
          {
              int hash = 31;
              foreach (FieldInfo fi in _infos)
              {
                  object val = fi.GetValue(obj);
                  hash = (hash * 17) + ((val == null) ? 0 : val.GetHashCode());
              }
              return hash;
          }
      }
  }
  

  回复于 2024-04-27T04:24:04+08:00
• 2

  您可以向MyClass添加成员函数并检查其中的私有字段：

  class MyClass
  {
      private Guid _id;
      private string _personName;
      public MyClass(Guid id, string personName)
      {
          _id = id;
          _personName = personName;
      }
  
      public bool IsEqual(MyClass otherInstance)
      {
          return (_id == otherInstance._id && _personName == otherInstance._personName);
      }
  }
  

  或者，如果无法更改MyClass实现，则添加扩展方法，并使用反射进行比较，如 CodeInChaos 所述：

  static class MyClassExtensions
  {
      public static bool IsEqual(this MyClass myInstance, MyClass otherInstance)
      {
         // Reflection goes here - read all private field from myInstance     
         // and otherInstance and compare them
      }
  }
  

  回复于 2024-04-27T04:24:04+08:00