在不覆盖 Equals 和 GetHashCode 或实现 IEquatable 的情况下测试对象的最佳方法是什么<T>？

Question

我想检查两个没有公共属性的对象是否相等。但是，我不想重写Equals &GetHashCode方法，或者实现等价。例如，考虑以下代码:

class Program
{
    static void Main(string[] args)
    {
        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(ObjectsAreEqual(object1, object2));
    }
}
class MyClass
{
    private Guid _id;
    private string _personName;
    public MyClass(Guid id, string personName)
    {
        _id = id;
        _personName = personName;
    }
}

我知道标准方法是重写Equals &GetHashCode，但由于各种原因，我不想更改类中的任何代码。另外，没有公共属性，所以我不能比较它们。还有其他的实现方法吗?

例如，通过反射?或者通过将对象序列化为JSON，并比较结果字符串?

谢谢。

Answer 1

你不能改变Equals实例方法的结果而不重写它。因此，您需要使用IEqualityComparer<T>，它需要显式地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构造函数的参数。

如果你不想以任何方式改变原始类，那么用反射实现IEqualityComparer<T>似乎是你唯一的选择。

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();
}

Answer 2

您可以创建一个使用反射的自定义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;
        }
    }
}

Answer 3

您可以向MyClass添加一个成员函数，并检查那里的私有字段:

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);
    }
}

或添加扩展方法，如果你不能改变MyClass的实现，并比较使用反射CodeInChaos描述:

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
    }
}