如何仅在不破坏现有 Object.Equals() 的情况下检查两个对象的属性是否相等？

Question

Basically, GethashCode is different even though they contain the SAME values for the properties... so why is the default to return diff hashcodes?

public class User
{
    public Int32 Id { get; set; }
    public String Username { get; set; }
}

User a = new User();
a.Id = 1;
a.Username = "Hello";

User b = new User();
b.Id = 1;
b.Username = "Hello";

Console.WriteLine("Hash A: {0} | Hash B: {1}", a.GetHashCode(), b.GetHashCode());
//Hash A: 37121646 | Hash B: 45592480 <-- these values change each time I rerun the app?

Is there a more proper way to make it so I don't break how Object.Equals works for my objects, but am still able to have my own equality checking based on the parameter values?

The reason I ask is because I have a service: SynchronizeUsers() which downloads an array of users. Instead of clearing out my cache of users, I'd rather just update the ones that need to be updated, remove the ones that the synch says to, and add the new ones. But, I can't just do Object.Equals() on these objects.

Answer 1

Too late to answer, but someone might end up here and I need to know my idea is right or wrong. If strictly values are the consideration, then why not make the objects JSON and compare the JSON strings? Like:

if (JsonConvert.SerializeObject(obj1) == JsonConvert.SerializeObject(obj2)) continue;

Answer 2

If you have ReSharper installed (it's worth it!) then all you have to do is:

Alt+Insert

With your cursor inside the class. A partial class is good for hiding the boilerplate.

It'll automatically implement an equality check for each property.

(Select all properties w/ Ctrl+A, and you can check all w/ Space!)

Answer 3

Have you tried implementing your own IEqualityComparer? You can pass this to an .Equals() overload to define your own custom equality logic, as in

User A = User B even if they are distinct instances, if properties x, y, z are the same.

See this: MSDN

Edit: I should have written you can instantiate your EqualityComparer and pass two instances to its Equals() method and get a bool. Basic console app... will show true, false, false. Thing is trivial, has the two properties shown.

var comparer = new ThingEqualityComparer();

Console.WriteLine(comparer.Equals(new Thing() { Id = 1, Name = "1" }, new Thing() { Id = 1, Name = "1" }));
Console.WriteLine(comparer.Equals(new Thing() { Id = 1, Name = "1" }, new Thing() { Id = 2, Name = "2" }));
Console.WriteLine(comparer.Equals(new Thing() { Id = 1, Name = "1" }, null));


class ThingEqualityComparer : IEqualityComparer<Thing>
{
    public bool Equals(Thing x, Thing y)
    {
        if (x == null || y == null)
            return false;

        return (x.Id == y.Id && x.Name == y.Name);
    }

    public int GetHashCode(Thing obj)
    {
        return obj.GetHashCode();
    }
}

Answer 4

It is recommended to over-ride GetHashCode() when over-riding GetEquals().

http://msdn.microsoft.com/en-us/library/ms173147%28v=vs.80%29.aspx

eg

public override int GetHashCode()
{
    return this.Username.GetHashCode() * this.Id;
}

Answer 5

Here's a solution that does not require any custom logic in the class and uses generics to ensure both arguments (obj1 and obj2) are the same type at compile time:

  public static class ObjectComparerUtility
  {
    public static bool ObjectsAreEqual<T>(T obj1, T obj2)
    {
      var obj1Serialized = JsonConvert.SerializeObject(obj1);
      var obj2Serialized = JsonConvert.SerializeObject(obj2);

      return obj1Serialized == obj2Serialized;
    }
  }

Usage:

  var c1 = new ConcreteType { Foo = "test1" };
  var c2 = new ConcreteType { Foo = "test1" };
  var areEqual = ObjectComparerUtility.ObjectsAreEqual(c1, c2);
  Assert.IsTrue(areEqual);

Answer 6

Why not write your own Equality method? ie,

User a = new User();
a.Id = 1;
a.Username = "Hello";
User b = new User();
b.Id = 1;
b.Username = "Hello";
a.IsEqualTo(b);

where IsEqualTo is defined within your user class as:

Public bool IsEqualTo(user compareTo)
{
  return (UserName == compareTo.UserName && Id == compareTo.Id);
}

Answer 7

Add a method:

public class User
{
    public int UserID { get; set; }

    public bool IsUser(object obj) 
    {
        return (obj is User && ((User)obj).UserID == this.UserID);
    }
}

Answer 8

If you just want to hash things look into using an extension method, to generate the hash.

public static int GenerateHash(this User myUser){
    return myUser.UserName.GetHashCode() ^ ... other properties....
}

Then in your code you could do:

Console.WriteLine("Hash A: {0} | Hash B: {1}", a.GenerateHash(), b.GenerateHash());

This would leave everything intact, and should not break anything else. If you are looking for a way to compare objects, you could do the same using an extension method:

public static int AreEqual(this User myUser, User someOther){
    return myUser.UserName == someOther.UserName && ...  other properties.
}

Usage would be like:

if(a.AreEqual(b)){
    // these are equal have fun.
}

Answer 9

The string array is the exclude list for the compare. It uses reflection but the performance is very good. Please check apache commons lang 3 library

CompareToBuilder.reflectionCompare(arg0, arg1, new String[]{"UID", "uidcount"})