What is the best way to test for object equality - without overriding Equals & GetHashCode, or implementing IEquatable<T>?

Question

I'd like to check for equality among two objects that have no Public Properties. However, I don't want to override the Equals & GetHashCode method, or implement IEquatable. For example, consider the following code:

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;
    }
}

I know the standard method is to override Equals & GetHashCode, but for various reasons, I don't want to change any code in the class. Plus, there are no public properties, so I can't compare these. Is there any other way of implementing this?

For example, via reflection? Or perhaps by serialising the Objects to JSON, and comparing the resulting strings?

Thanks.

Answer 1

You could create a custom IEqualityComparer<T> implementation that uses reflection:

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 2

You can't change the result of the Equals instance method without overriding it. So you'll need to use IEqualityComparer<T> which needs to be passed explicitly to the code that requires equality checks. Luckily most built in collections accept such an equality comparer as a parameter to their constructor.

Implementing IEqualityComparer<T> with reflection seems to be your only option if you don't want to change the original class in any way.

You can get the FieldInfo using GetField("_personName",BindingFlags.NonPublic) and then call GetValue on it to get the value.

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 3

You could add a member function to MyClass and check the private fields there:

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);
    }
}

or add extension method if you can't change the MyClass implementation, and compare using reflection as CodeInChaos described:

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
    }
}