should all c# classes override Equals and GetHashCode? For correctness
No, they already do.
Whether you have to override them, is up to how it will be used. In most cases, it is not needed.
All classes already inherit these methods from the base class, System.Object.
You can choose to override the methods in derived classes if you need to be able to compare two instances of an object beyond simple reference equality, otherwise it's not necessary.
Remember, however, that if you choose to override one of them, you also need to override the other in order to ensure that Hashtables and dictionary keys, among other things, work properly with you derived class. The GetHashCode method needs to reflect the same logic as the Equals method. See here for more explanations and examples:
http://msdn.microsoft.com/en-us/library/bsc2ak47.aspx
and
http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx
All classes to inherit this from the System.Object.
If you need to provide a specific Equals or GetHashCode for a class then you should override the methods in your classes. Otherwise just leave them..
http://msdn.microsoft.com/en-us/library/system.object.gethashcode(v=VS.71).aspx
Maybe not all, but all classes that will be put into a some kind of bag (IList, ICollection, IDictionary, Hashset, etc.) and need some simple method to differentiate them (just think about Sort(), Contains(), BinarySearch(), etc.).
If you use a class that way you should definitely implement them correct.
When you override Equals, basically.
When you want to provide a different
idea of equality than simple reference
equality.
String is a good example of this - two
strings are equal (under a simple
Equals call) if they represent the
same sequence of characters. The hash
code reflects this, such that if two
strings are equal they will have the
same hash code. (The reverse isn't
necessarily true - two unequal strings
can have the same hash code, but it's
unlikely.)
(Strings are tricky in other ways,
mind you - there are lots of different
ideas of equality based on culture and
casing, but String.Equals just looks
at the UTF-16 code points which make
up the string, and compares them in
the simplest conceivable fashion.)
by Jon Skeet
Related
Ok so I need a bit of help. I have a generic dictionary "cashdata". The keys of this dictionary are objects (Query objects, a class I have defined). Query objects have "Terms" field, which is a list of strings, and an "Operator" field, which is an enum (Either "All" or "Any").
cashdata.ContainsKey(a_query_object);
And have it yield true or false depending on if a_query_object and an object in the dictionary are identical in terms of their Terms and Operator. What is the best way to do this? A HashCode possibly? I would appreciate an example, thanks in advance.
EDIT: cashdata Dictionary is defined as such
Dictionary<Query,List<string> > cashData = new Dictionary<Query,List<string>>();
Make your object implement IEquatable Interface along with overriding Object.Equals and GetHashCode as mentioned in the remarks section in MSDN
If you implement IEquatable, you should also override the base
class implementations of Object.Equals(Object) and GetHashCode so that
their behavior is consistent with that of the IEquatable.Equals
method. If you do override Object.Equals(Object), your overridden
implementation is also called in calls to the static
Equals(System.Object, System.Object) method on your class. In
addition, you should overload the op_Equality and op_Inequality
operators. This ensures that all tests for equality return consistent
results.
Can someone please explain with example that I can understand about the difference between .Equals, IComparable and IComparer.
I was asked this in an interview.
Well first off, on the surface, Equals is a method (present in every object), while IComparable and IComparer are interfaces.
Equals is present in any class and can be overriden to provide equality testing depending on the context of the class (it's a good practice to override GetHashCode as well). By default it just tests if objects are equal in memory which is not very useful. Equals (and GetHashCode) are usually given a different implementation in the context of searching or hashing.
Implementing IComparable is a more fine-grain way of comparison, as it provides the CompareTo method, which is a greater-than/less-than comparison as opposed to Equals which is simply a is-equal-or-not comparison. For example a binary search tree structure could benefit from this method.
IComparer is similar to IComparable, except that it works from the outside. It allows you to define a "neutral" object that is used for comparing two other objects without modifying them directly, which you need to do with IComparable.
Equals is a method, when 2 other are interfaces. So look like the biggest difference.
More seriously - #ChrisSinclair gave you an answer in comments...
Equals returns true/false if the two objects are equal (or the same reference depending on your implementation) IComparable/IComparer: difference between IComparable and IComparer
.Equals() gives your class a way to test for equality against all other possible objects. This can be considered as the fallback for object equality. So this answers the question am I equivalent to the object passed in as a param.
IComparable provides for a way of comparing objects which can be ordered, possible uses include sorting. Implementing this interface puts the ordering logic into your class.
IComparer does pretty much the same as IComparable except the logic is contained in separate class.
I'm working on a simple application with a few classes. This all started when I wanted to use the Remove method on a List<Car>. This method requires that you override the Equals and the GetHashCode methods for the Car type. In this situation, I decided to implement an ID property on the Car class. That way, my Equals method simply checks for ID equality, and my GetHashCode method returns base.GetHashCode().
Is this a good approach, or is implementing a GUID for a small class too heavy-handed? There wouldn't be any need for it without the reasons I explained above. The only requirement for uniqueness for this Car type is that it be unique within the List<T> collection to which it belongs. But adding the GUID property seemed like the quickest way around the GetHashCode mess. BTW, there are no int properties on my Car type.
There wouldn't be any need for it without the reasons I explained above.
If your class doesn't logically have an ID, then it certainly seems odd to include it just for the sake of equality.
For example, if you have two instances which have equal properties for everything apart from ID, are they really non-equal? If they are, you should potentially just use the default implementation of Equals/GetHashCode which uses reference identity for equality. Where you would use two objects with the same ID, you just use two references to the same object instead.
It really all depends on the context, and you haven't given much of that - but adding an ID just for equality is a bit of a design smell.
Instead of implementing Equals and GetHashCode just use RemoveAll:
myList.RemoveAll(x => x.ID == myCar.ID);
This allows you to specify a predicate that indicates what items should be removed instead (it doesn't matter that you are only removing one item).
Implementing Equals and GetHashCode in the way you describe strikes me as extremely dodgey - if your Equals implementation returns true then your GetHashCode method needs to return the same value so that those two objects will be placed in the same bucket in a hashtable. Your implementation (as I understand it) doesn't match this criteria as the base GetHashCode implementation is almost certainly going to return different values for two Car instances, regardless of if they have the same ID or not.
Implementing Equals and GetHashCode isn't entirely trivial and is probably something I'd generally avoid doing if there are alternatives. If you really want to do this then take a look at these resoruces:
What is the best algorithm for an overridden System.Object.GetHashCode?
Default implementation for Object.GetHashCode().
implementing the Equals method
Also hash codes are not GUIDs
I have a number of data classes that have over 25 properties of different value types (and this may change in the future as requirements change). I would like to override equals, mostly for unit testing purposes.
Currently, the only way I know how to do this is to actually test for equality of each property hard coded. This seems bad for two reasons - first, I will have to write a lot of code to test 25 properties for equality - second, if a property in one of the classes is added at a later point in time, the Equals method will not check that, and most likely this will go unnoticed and lead to problems down the road.
Since Equals usually checks for the properties of classes, there should be a way to dynamically compare the properties of the classes being compared, which ensures that property changes to a class don't result in an incorrect implementation of Equals. Is there a way to do this?
you could write something like this using reflection - but this would be very slow.
I would stick with overriding equals but think about which part you really need for equal. I usually only check the immutable parts (like Id) for equality and just ignore the mutable fields and I think this is a good practice.
Try using reflection to compare the properties. See Comparing object properties in c# for more info!
If your class is an entity, it should have a property which allows you to uniquely identify each instance.
If your class is implemented as a value type, you'll have to check for equality by checking each property.
In the latter case, in order to prevent tedious work, you could make use of reflection to get all properties of the class at runtime, retrieve the value and use the TypeDescriptor classes to compare the values.
You can use some AOP Frameworks. If the properties you're goona to compare are much more then those ones you gonna to avoid, mark the properties to skip with special custom attribute.
Maybe T4 can help you out. With it you can generate code at a click. Within this function you can then use the slow reflection mechanism to create a hard-coded GetHashCode() function that will be called at runtime. For a first look into T4 take a look at Scotts blog about it. Or simply try searching for Text Template Transformation Toolkit with your favorite search engine.
1) Are the reasons why IEqualityComparer<T> was introduced:
a) so we would be able to compare objects (of particular type) for equality in as many different ways as needed
b) and by having a standard interface for implementing a custom equality comparison, chances are that much greater that third party classes will accept this interface as a parameter and by that allow us to inject into these classes equality comparison behavior via objects implementing IEqualityComparer<T>
2) I assume IEqualityComparer<T> should not be implemented on type T that we're trying to compare for equality, but instead we should implement it on helper class(es)?
Thank you
I'm doubtful that anyone here will be able to answer with any authority the reason that the interface was introduced (my guess--and that's all it is--would be to support one of the generic set types like Dictionary<TKey, TValue> or HashSet<T>), but its purpose is clear:
Defines methods to support the comparison of objects for equality.
If you combine this with the fact you can have multiple types implementing this interface (see StringComparer), then the answer to question a is yes.
The reason for this is threefold:
Operators (in this case, ==) are not polymorphic; if the type is upcasted to a higher level than where the type-specific comparison logic is defined, then you'll end up performing a reference comparison rather than using the logic within the == operator.
Equals() requires at least one valid reference and can provide different logic depending on whether it's called on the first or second value (one could be more derived and override the logic of the other).
Lastly and most importantly, the comparison logic provided by the type may not be what the user is after. For example, strings (in C#) are case sensitive when compared using == or Equals. This means that any container (like Dictionary<string, T> or HashSet<string>) would be case-sensitive. Allowing the user to provide another type that implements IEqualityComparer<string> means that the user can use whatever logic they like to determine if one string equals the other, including ignoring case.
As for question b, probably, though I wouldn't be surprised if this wasn't high on the list of priorities.
For your final question, I'd say that's generally true. While there's nothing stopping you from doing so, it is confusing to think that type T would provide custom comparison logic that is different from that provided on type T just because it's referenced as an IEqualiltyComparer<T>.
agreed on a and b
"should not be" is always a normative question and rarely a good metric. You do what works without getting into trouble. (Pragmatic Programmer). The fact that you can implement the interface statefull, stateless and in any which way, makes it possible to implement (alternative) comparers for all types, including value types, enums, sealed types, even abstract types; In essence it is a Strategy pattern
Sometimes there's a natural equality comparison for a type, in which case it should implement IEquatable<T>, not IEqualityComparer<T>. At other times, there are multiple possible ways of comparing objects for equality - so it makes sense to implement IEqualityComparer<T> then. It allows hash tables (and sets etc) to work in a flexible way.