I had some fun with hibernate. A function like that:
public class Key
{
public virtual bool IsEqual(Key key)
{
return this == key;
}
}
One would expect this function to always return true if the parameter was the same as the instance where IsEqual was called upon:
Assert.IsTrue(MyKey.IsEqual(MyKey));
But this is only the case as long as the instance "myKey" is not a lazily loaded object / proxy. A KeyProxy will delegate that call to the internal Key object that is wrapped, and this results in the wrapped object to be compared with the Proxy object (which will in turn fail).
Basically, it has also been discussed here : NHibernate, proxies and equality
The solution there is a little bit disappointing. Overriding equals to compare the primary key properties has the drawback that it only works for objects that already have a value, whereas new objects don´t have a primary key value until saved. I could try to force new objects to directly receive a valid primary key value, but that doesnt sound like a great way of handling this issue.
Is there a better (more general) way known to handle such situations? Would´nt overriding Equals and comparing with a unique (non-persisted) property just do the trick?
Something like that?
public object Identifier {get; private set;}
public Key()
{
Identifer = new object();
}
public override bool Equals(object obj)
{
if (obj == null)
{
return false;
}
Key k = obj as Key;
if (k == null)
{
return false;
}
return this.Identifier == key.Identifier;
}
To overcome this and other problems, such as using an identity column as the primary key, we added a GUID to the base class of our domain model, object creation is handled by factory classes that gives each entity a GUID and this is then persisted as part of the entity.
The GUID is then used to compare entities, basicly we use it in the Equals() and GetHashCode() methods.
public override int GetHashCode()
{
return this.EqualityIdentifier.GetHashCode();
}
public override bool Equals(object obj)
{
IDomainObject Obj = obj as IDomainObject
if (Obj == null)
{
return false;
}
return this.EqualityIdentifier == Obj.EqualityIdentifier;
}
To have a minimum performance impact, I decided to use a non-persisted readonly int property "Identifier" that is filled (lazily/at first access) by a small static and thread-safe number generator method.
private static int _equalityIdentifierSequence;
private static int GenerateEqualityIdentifier()
{
Interlocked.Increment(ref _equalityIdentifierSequence);
return _equalityIdentifierSequence;
}
I am quite comfortable with the fact that two objects that were loaded from different sessions but are representing the same entity are regarded as "not equal", so the GUID strategy did not look that promising to me. The original problem of proxies compared with their wrapped objects seems to be solved with that.
Related
I have a class:
public class Item
{
public string Name { get; set; }
public override int GetHashCode()
{
return Name.GetHashCode();
}
}
The purpose of overriding GetHashCode is that I want to have only one occurence of an object with specified name in Dictionary.
But is it safe to get hash code from string?
In other words, is there any chance that two objects with different values of property Name would return the same hash code?
But is it safe to get hash code from string?
Yes, it is safe. But, what you're doing isn't. You're using a mutable string field to generate your hash code. Let's imagine that you inserted an Item as a key for a given value. Then, someone changes the Name string to something else. You now are no longer able to find the same Item inside your Dictionary, HashSet, or whichever structure you use.
More-so, you should be relying on immutable types only. I'd also advise you to implement IEquatable<T> as well:
public class Item : IEquatable<Item>
{
public Item(string name)
{
Name = name;
}
public string Name { get; }
public bool Equals(Item other)
{
if (ReferenceEquals(null, other)) return false;
if (ReferenceEquals(this, other)) return true;
return string.Equals(Name, other.Name);
}
public override bool Equals(object obj)
{
if (ReferenceEquals(null, obj)) return false;
if (ReferenceEquals(this, obj)) return true;
if (obj.GetType() != this.GetType()) return false;
return Equals((Item) obj);
}
public static bool operator ==(Item left, Item right)
{
return Equals(left, right);
}
public static bool operator !=(Item left, Item right)
{
return !Equals(left, right);
}
public override int GetHashCode()
{
return (Name != null ? Name.GetHashCode() : 0);
}
}
is there any chance that two objects with different values of property
Name would return the same hash code?
Yes, there is a statistical chance that such a thing will happen. Hash codes do not guarantee uniqueness. They strive for uni-formal distribution. Why? because your upper boundary is Int32, which is 32bits. Given the Pigenhole Principle, you may happen at end up with two different strings containing the same hash code.
Your class is buggy, because you have a GetHashCode override, but no Equals override. You also don't consider the case where Name is null.
The rule for GetHashCode is simple:
If a.Equals(b) then it must be the case that a.GetHashCode() == b.GetHashCode().
The more cases where if !a.Equals(b) then a.GetHashCode() != b.GetHashCode() the better, indeed the more cases where !a.Equals(b) then a.GetHashCode() % SomeValue != b.GetHashCode() % SomeValue the better, for any given SomeValue (you can't predict it) so we like to have a good mix of bits in the results. But the vital thing is that two objects considered equal must have equal GetHashCode() results.
Right now this isn't the case, because you've only overridden one of these. However the following is sensible:
public class Item
{
public string Name { get; set; }
public override int GetHashCode()
{
return Name == null ? 0 : Name.GetHashCode();
}
public override bool Equals(object obj)
{
var asItem = obj as Item;
return asItem != null && Name == obj.Name;
}
}
The following is even better, because it allows for faster strongly-typed equality comparisons:
public class Item : IEquatable<Item>
{
public string Name { get; set; }
public override int GetHashCode()
{
return Name == null ? 0 : Name.GetHashCode();
}
public bool Equals(Item other)
{
return other != null && Name == other.Name;
}
public override bool Equals(object obj)
{
return Equals(obj as Item);
}
}
In other words, is there any chance that two objects with different values of property Name would return the same hash code?
Yes, this can happen, but it won't happen often, so that's fine. The hash-based collections like Dictionary and HashSet can handle a few collisions; indeed there'll be collisions even if the hash codes are all different because they're modulo'd down to a smaller index. It's only if this happens a lot that it impacts performance.
Another danger is that you'll be using a mutable value as a key. There's a myth that you shouldn't use mutable values for hash-codes, which isn't true; if a mutable object has a mutable property that affects what it is considered equal with then it must result in a change to the hash-code.
The real danger is mutating an object that is a key to a hash collection at all. If you are defining equality based on Name and you have such an object as the key to a dictionary then you must not change Name while it is used as such a key. The easiest way to ensure that is to have Name be immutable, so that is definitely a good idea if possible. If it is not possible though, you need to be careful just when you allow Name to be changed.
From a comment:
So, even if there is a collision in hash codes, when Equals will return false (because the names are different), the Dictionary will handle propertly?
Yes, it will handle it, though it's not ideal. We can test this with a class like this:
public class SuckyHashCode : IEquatable<SuckyHashCode>
{
public int Value { get; set; }
public bool Equals(SuckyHashCode other)
{
return other != null && other.Value == Value;
}
public override bool Equals(object obj)
{
return Equals(obj as SuckyHashCode);
}
public override int GetHashCode()
{
return 0;
}
}
Now if we use this, it works:
var dict = Enumerable.Range(0, 1000).Select(i => new SuckyHashCode{Value = i}).ToDictionary(shc => shc);
Console.WriteLine(dict.ContainsKey(new SuckyHashCode{Value = 3})); // True
Console.WriteLine(dict.ContainsKey(new SuckyHashCode{Value = -1})); // False
However, as the name suggests, it isn't ideal. Dictionaries and other hash-based collections all have means to deal with collisions, but those means mean that we no longer have the great nearly O(1) look-up, but rather as the percentage of collisions gets greater the look-up approaches O(n). In the case above where the GetHashCode is as bad as it could be without actually throwing an exception, the look-up would be O(n) which is the same as just putting all the items into an unordered collection and then finding them by looking at every one to see if it matches (indeed, due to differences in overheads, it's actually worse than that).
So for this reason we always want to avoid collisions as much as possible. Indeed, to not just avoid collisions, but to avoid collisions after the result has been modulo'd down to make a smaller hash code (because that's what happens internally to the dictionary).
In your case though because string.GetHashCode() is reasonably good at avoiding collisions, and because that one string is the only thing that equality is defined by, your code would in turn be reasonably good at avoiding collisions. More collision-resistant code is certainly possible, but comes at a cost to performance in the the code itself* and/or is more work than can be justified.
*(Though see https://www.nuget.org/packages/SpookilySharp/ for code of mine that is faster than string.GetHashCode() on large strings on 64-bit .NET and more collision-resistant, though it is slower to produce those hash codes on 32-bit .NET or when the string is short).
Instead of using GetHashCode to prevent duplicates to be added to a dictionary, which is risky in your case as explained already, I would recommend to use a (custom) equality comparer for your dictionary.
If the key is an object, you should create an own equality comparer that compares the string Name value. If the key is the string itself, you can use StringComparer.CurrentCulture for example.
Also in this case it is key to make the string immutable, since else you might invalidate your dictionary by changing the Name.
There is a passage from NHibernate documentation:
Note: if you define an ISet of composite elements, it is very important to implement Equals() and GetHashCode() correctly.
What does correctly mean there? Is it neccessary to implement those methods for all value objects in domain?
EXTENDING MY QUESTION
In the article Marc attached user Albic states:
It's actually very hard to implement GetHashCode() correctly because, in addition to the rules Marc already mentioned, the hash code should not change during the lifetime of an object. Therefore the fields which are used to calculate the hash code must be immutable.
I finally found a solution to this problem when I was working with NHibernate. My approach is to calculate the hash code from the ID of the object. The ID can only be set though the constructor so if you want to change the ID, which is very unlikely, you have to create a new object which has a new ID and therefore a new hash code. This approach works best with GUIDs because you can provide a parameterless constructor which randomly generates an ID.
I suddenly realized what I've got inside my AbstractEntity class:
public abstract class AbstractEntity<T> where T : AbstractEntity<T> {
private Nullable<Int32> hashCode;
public virtual Guid Id { get; protected set; }
public virtual Byte[] Version { get; set; }
public override Boolean Equals(Object obj) {
var other = obj as T;
if(other == null) {
return false;
}
var thisIsNew = Equals(this.Id, Guid.Empty);
var otherIsNew = Equals(other.Id, Guid.Empty);
if(thisIsNew && otherIsNew) {
return ReferenceEquals(this, other);
}
return this.Id.Equals(other.Id);
} // public override Boolean Equals(Object obj) {
public override Int32 GetHashCode() {
if(this.hashCode.HasValue) {
return this.hashCode.Value;
}
var thisIsNew = Equals(this.Id, Guid.Empty);
if(thisIsNew) {
this.hashCode = base.GetHashCode();
return this.hashCode.Value;
}
return this.Id.GetHashCode();
} // public override Int32 GetHashCode() {
public static Boolean operator ==(AbstractEntity<T> l, AbstractEntity<T> r) {
return Equals(l, r);
}
public static Boolean operator !=(AbstractEntity<T> l, AbstractEntity<T> r) {
return !Equals(l, r);
}
} // public abstract class AbstractEntity<T>...
As all components are nested within entities should I then implement Equals() and GetHashCode() for them?
Correctly means that GetHashCode returns the same hash code for the entities that are expected to be equal. Because equality of 2 entities is made by comparison of that code.
On the other side, that means that for entities that are not equal, the uniqueness of hash code has to be guaranteed, as much as it possible.
The documentation for Equals and GetHashCode explain this well and include specific guidance on implementation for value objects. For value objects, Equals is true if the objects are the same type and the public and private fields are equal. However, this explanation applies to framework value types and you are free to create your own Equals by overriding it.
GetHashCode has two rules that must be followed:
If two objects compare as equal, the GetHashCode method for each object must return the same value. However, if two objects do not
compare as equal, the GetHashCode methods for the two object do not
have to return different values.
The GetHashCode method for an object must consistently return the same hash code as long as there is no modification to the object state
that determines the return value of the object's Equals method. Note
that this is true only for the current execution of an application,
and that a different hash code can be returned if the application is
run again.
Please explain the technique used in this code to test Object Equality and Identity.
Better, if you can supply me any web-link/book-reference for detailed discussion.
[Serializable]
public abstract class BusinessObject<T> : IBusinessObject where T : BusinessObject<T>
{
private int? requestedHashCode;
public virtual int ID { get; set; }
public virtual bool Equals(IBusinessObject other)
{
if (null == other || !GetType().IsInstanceOfType(other))
{
return false;
}
if (ReferenceEquals(this, other))
{
return true;
}
bool otherIsTransient = Equals(other.ID, default(T));
bool thisIsTransient = IsTransient();
if (otherIsTransient && thisIsTransient)
{
return ReferenceEquals(other, this);
}
return other.ID.Equals(ID);
}
protected bool IsTransient()
{
return Equals(ID, default(T));
}
public override bool Equals(object obj)
{
var that = obj as IBusinessObject;
return Equals(that);
}
public override int GetHashCode()
{
if (!requestedHashCode.HasValue)
{
requestedHashCode = IsTransient() ? base.GetHashCode() : ID.GetHashCode();
}
return requestedHashCode.Value;
}
}
What is a transient object?
it first checks if other is an instance of the same type as the current object. If not, they're not equal
it then performs a reference equality to check if other and the current object are the same instance. If they are, obviously they are equal
If both other and the current object are transient (i.e. not yet persisted), they don't have an ID, so they can't be compared by ID. Instead, they are compared by reference. (as noted by Marc Gravell in the comments, the test to check if the object is transient is broken; it doesn't make sense to compare an int to default(T))
Eventually, their IDs are compared; the objects are considered equal if they have the same ID
I think what the code is trying to do is say "has it got an ID yet", i.e. a "transient" object might (if I read the code's intent correctly) be one that is not yet saved to the DB. Then equality is defined as:
if it has an ID, does it match? (even for different instances of the same type)
if it doesn't have an ID, is it the same object instance (reference)
unfortunately the implementation looks completely broken, as Equals(ID, default(T)) is meaningless when T is something completely different (a BusinessObject<T>) - hence default(T) will always be null and ID will never be null (it is not nullable). So nothing will ever report as transient.
Additionally, this code:
if (null == other || !GetType().IsInstanceOfType(other))
is hugely inefficient. I suspect something involving as would be far preferable, but again: the code looks so... tortured... that I'm loathe to second-guess the intent.
I have a class it contains some string members, some double members and some array objects.
I create two objects of this class, is there any simplest, efficient way of comparing these objects and say their equal? Any suggestions?
I know how to write a compare function, but will it be time consuming.
The only way you can really do this is to override bool Object.Equals(object other) to return true when your conditions for equality are met, and return false otherwise. You must also override int Object.GetHashCode() to return an int computed from all of the data that you consider when overriding Equals().
As an aside, note that the contract for GetHashCode() specifies that the return value must be equal for two objects when Equals() would return true when comparing them. This means that return 0; is a valid implementation of GetHashCode() but it will cause inefficiencies when objects of your class are used as dictionary keys, or stored in a HashSet<T>.
The way I implement equality is like this:
public class Foo : IEquatable<Foo>
{
public bool Equals(Foo other)
{
if (other == null)
return false;
if (other == this)
return true; // Same object reference.
// Compare this to other and return true/false as appropriate.
}
public override bool Equals(Object other)
{
return Equals(other as Foo);
}
public override int GetHashCode()
{
// Compute and return hash code.
}
}
A simple way of implementing GetHashCode() is to XOR together the hash codes of all of the data you consider for equality in Equals(). So if, for example, the properties you compare for equality are string FirstName; string LastName; int Id;, your implementation might look like:
public override int GetHashCode()
{
return (FirstName != null ? FirstName.GetHashCode() : 0) ^
(LastName != null ? LastName.GetHashCode() : 0) ^
Id; // Primitives of <= 4 bytes are their own hash codes
}
I typically do not override the equality operators, as most of the time I'm concerned with equality only for the purposes of dictionary keys or collections. I would only consider overriding the equality operators if you are likely to do more comparisons by value than by reference, as it is syntactically less verbose. However, you have to remember to change all places where you use == or != on your object (including in your implementation of Equals()!) to use Object.ReferenceEquals(), or to cast both operands to object. This nasty gotcha (which can cause infinite recursion in your equality test if you are not careful) is one of the primary reasons I rarely override these operators.
The 'proper' way to do it in .NET is to implement the IEquatable interface for your class:
public class SomeClass : IEquatable<SomeClass>
{
public string Name { get; set; }
public double Value { get; set; }
public int[] NumberList { get; set; }
public bool Equals(SomeClass other)
{
// whatever your custom equality logic is
return other.Name == Name &&
other.Value == Value &&
other.NumberList == NumberList;
}
}
However, if you really want to do it right, this isn't all you should do. You should also override the Equals(object, object) and GetHashCode(object) methods so that, no matter how your calling code is comparing equality (perhaps in a Dictionary or perhaps in some loosely-typed collection), your code and not reference-type equality will be the determining factor:
public class SomeClass : IEquatable<SomeClass>
{
public string Name { get; set; }
public double Value { get; set; }
public int[] NumberList { get; set; }
/// <summary>
/// Explicitly implemented IEquatable method.
/// </summary>
public bool IEquatable<SomeClass>.Equals(SomeClass other)
{
return other.Name == Name &&
other.Value == Value &&
other.NumberList == NumberList;
}
public override bool Equals(object obj)
{
var other = obj as SomeClass;
if (other == null)
return false;
return ((IEquatable<SomeClass>)(this)).Equals(other);
}
public override int GetHashCode()
{
// Determine some consistent way of generating a hash code, such as...
return Name.GetHashCode() ^ Value.GetHashCode() ^ NumberList.GetHashCode();
}
}
Just spent the whole day writing an extension method looping through reflecting over properties of an object with various complex bits of logic to deal with different property type and actually got it close to good, then at 16:55 it dawned on me that if you serialize the two object, you simply need compare the two strings ... duh
So here is a simple serializer extension method that even works on Dictionaries
public static class TExtensions
{
public static string Serialize<T>(this T thisT)
{
var serializer = new DataContractSerializer(thisT.GetType());
using (var writer = new StringWriter())
using (var stm = new XmlTextWriter(writer))
{
serializer.WriteObject(stm, thisT);
return writer.ToString();
}
}
}
Now your test can be as simple as
Asset.AreEqual(objA.Serialise(), objB.Serialise())
Haven't done extensive testing yet, but looks promising and more importantly, simple. Either way still a useful method to have in your utility set right ?
The best answer is to implement IEquatable for your class - it may not be the answer you want to hear, but that's the best way to implement value equivalence in .NET.
Another option would be computing a unique hash of all of the members of your class and then doing value comparisons against those, but that's even more work than writing a comparison function ;)
Since these are objects my guess is that you will have to override the Equals method for objects. Otherwise the Equals method will give you ok only if both objects refering to the same object.
I know this is not the answer you want. But since there is little number of properties in your class you can easily override the method.
a class has an ID property and this property gets value from a primary key column of an SQL table.
Is it a good practice if I write
public override int GetHashCode()
{
return this.ID + GetType().GetHashCode();
}
into my class? (Equals overrided already on the same way.)
Why would you particularly want to include the type in the hashcode? I can see how that could be useful if you had a lot of different types of object with the same ID in the same map, but normally I'd just use
public override int GetHashCode()
{
return ID; // If ID is an int
// return ID.GetHashCode(); // otherwise
}
Note that ideas of equality become tricky within inheritance hierarchies - another reason to prefer composition over inheritance. Do you actually need to worry about this? If you can seal your class, it will make the equality test easier as you only need to write:
public override bool Equals(object obj)
{
MyType other = obj as other;
return other != null && other.ID == ID;
}
(You may well want to have a strongly-typed Equals method and implement IEquatable.)
Why can't you just do
public override int GetHashCode() {
return this.ID.GetHashCode();
}
I am not sure if what you are doing is good practice because I am not familiar with how the hash code is assigned to a type instance. And the purpose of the hashcode is to have a consistence representation of the object in Int32 form.