Basically the title. I see a lot of the former in the code I'm working on and I was wondering why they didn't use the latter. Are there any differences between the two?
Thanks.
Straight from the documentation
Unlike the Equals method and the equality operator, the ReferenceEquals method cannot be overridden. Because of this, if you want to test two object references for equality and are unsure about the implementation of the Equals method, you can call the ReferenceEquals method. However, note that if objA and objB are value types, they are boxed before they are passed to the ReferenceEquals method.
Not really. One can overload operator ==, so you basically can get it return e.g. false always. The recommended usage for this operator is to signify value equality, so (for correcftly implemented operator) basically check against null may return true if the object is semantically null.
See this article for more details and some historical overview.
Another difference is that for value types ReferenceEquals doesn't make much sense: for example, any two "instances" of int 0 have to be considered the same in any sane circumstances. (For purists: I put quotes because, strictly speaking, we cannot talk about instances of a value type.)
The main benefit of ReferenceEquals() is that the intention is much clearer, you are trying to determine if two references are equal, not if the contents of the objects referred to are equal.
It does this by checking for operator == equality between the two references cast to object (since the params are both object), which eliminates any subclass operator == overloads that might confuse the issue (like string's).
So essentially, this:
if (ReferenceEquals(x, y))
Is the same as this:
if (((object)x) == ((object)y))
Though the former is easier to read.
There are definitely times where it comes in handy, especially avoiding infinite recursion when you are overloading operator == yourself:
public class Foo
{
public static bool operator ==(Foo first, Foo second)
{
// can't say if (first == second) here or infinite recursion...
if (ReferenceEquals(first, second))
{
// if same object, obviously same...
return true;
}
// etc.
}
}
Related
This question already has answers here:
C# difference between == and Equals()
(20 answers)
Closed 9 years ago.
What is the difference between a.Equals(b) and a == b for value types, reference types, and strings? It would seem as though a == b works just fine for strings, but I'm trying to be sure to use good coding practices.
From When should I use Equals and when should I use ==:
The Equals method is just a virtual
one defined in System.Object, and
overridden by whichever classes choose
to do so. The == operator is an
operator which can be overloaded by
classes, but which usually has
identity behaviour.
For reference types where == has not
been overloaded, it compares whether
two references refer to the same
object - which is exactly what the
implementation of Equals does in
System.Object.
Value types do not provide an overload
for == by default. However, most of
the value types provided by the
framework provide their own overload.
The default implementation of Equals
for a value type is provided by
ValueType, and uses reflection to make
the comparison, which makes it
significantly slower than a
type-specific implementation normally
would be. This implementation also
calls Equals on pairs of references
within the two values being compared.
using System;
public class Test
{
static void Main()
{
// Create two equal but distinct strings
string a = new string(new char[] {'h', 'e', 'l', 'l', 'o'});
string b = new string(new char[] {'h', 'e', 'l', 'l', 'o'});
Console.WriteLine (a==b);
Console.WriteLine (a.Equals(b));
// Now let's see what happens with the same tests but
// with variables of type object
object c = a;
object d = b;
Console.WriteLine (c==d);
Console.WriteLine (c.Equals(d));
}
}
The result of this short sample program is
True
True
False
True
Here is a great blog post about WHY the implementations are different.
Essentially == is going to be bound at compile time using the types of the variables and .Equals is going to be dynamically bound at runtime.
In the most shorthand answer:
== opertator is to check identity. (i.e: a==b are these two are the same object?)
.Equals() is to check value. (i.e: a.Equals(b) are both holding identical values?)
With one exception:
For string and predefined value types (such as int, float etc..),
the operator == will answer for value and not identity. (same as using .Equals())
One significant difference between them is that == is a static binary operator that works on two instances of a type whereas Equals is an instance method. The reason this matters is that you can do this:
Foo foo = new Foo()
Foo foo2 = null;
foo2 == foo;
But you cannot do this without throwing a NullReferenceException:
Foo foo = new Foo()
Foo foo2 = null;
foo2.Equals(foo);
At a simple level, the difference is which method is called. The == method will attempt ot bind to operator== if defined for the types in question. If no == is found for value types it will do a value comparison and for reference types it will do a reference comparison. A .Equals call will do a virtual dispatch on the .Equals method.
As to what the particular methods do, it's all in the code. Users can define / override these methods and do anything they please. Ideally this methods should be equivalent (sorry for the pun) and have the same output but it is not always the case.
One simple way to help remember the difference is that a.Equals(b) is more analogous to
a == (object)b.
The .Equals() method is not generic and accepts an argument of type "object", and so when comparing to the == operator you have to think about it as if the right-hand operand were cast to object first.
One implication is that a.Equals(b) will nearly always return some value for a and b, regardless of type (the normal way to overload is to just return false if b is an unkown type). a == b will just throw an exception if there's no comparison available for those types.
"==" is an operator that can be overloaded to perform different things based on the types being compared.
The default operation performed by "==" is a.Equals(b);
Here's how you could overload this operator for string types:
public static bool operator == (string str1, string str2)
{
return (str1.Length == str2.Length;)
}
Note that this is different than str1.Equals(str2);
Derived classes can also override and redefine Equals().
As far as "best practices" go, it depends on your intent.
For strings you want to be careful of culture specific comparisons. The classic example is the german double S, that looks a bit like a b. This should match with "ss" but doesn't in a simple == comparison.
For string comparisons that are culture sensitive use: String.Compare(expected, value, StringComparison....) == 0 ? with the StringComparison overload you need.
By default, both == and .Equals() are equivalent apart from the possibility of calling .Equals() on a null instance (which would give you a NullReferenceException). You can, however, override the functionality of either of them independently (though I'm not sure that would ever be a good idea unless you're trying to work around the shortcomings of another system), which would mean you could MAKE them different.
You'll find people on both sides of the aisle as to the one to use. I prefer the operator rather than the function.
If you're talking about strings, though, it's likely a better idea to use string.Compare() instead of either one of those options.
Failing to override GetHashCode and Equals when overloading the equality operator causes the compiler to produce warnings. Why would it be a good idea to change the implementation of either? After reading Eric Lippert's blog post on GetHashCode it's seems like there probably aren't many useful alternatives to GetHashCode's base implementation, why does the compiler I encourage you to change it?
Let's suppose you are implementing a class.
If you are overloading == then you are producing a type that has value equality as opposed to reference equality.
Given that, now the question is "how desirable is it to have a class that implements reference equality in .Equals() and value equality in ==?" and the answer is "not very desirable". That seems like a potential source of confusion. (And in fact, the company that I now work for, Coverity, produces a defect discovery tool that checks to see if you are confusing value equality with reference equality for precisely this reason. Coincidentally I was just reading the spec for it when I saw your question!)
Moreover, if you are going to have a class that implements both value and reference equality, the usual way to do it is to override Equals and leave == alone, not the other way around.
Therefore, given that you have overloaded ==, it is strongly suggested that you also override Equals.
If you are overriding Equals to produce value equality then you are required to override GetHashCode to match, as you know if you've read my article that you linked to.
If you don't override Equals() when you override == you will have some amazingly bad code.
How would you feel about this happening?
if (x == y)
{
if (!x.Equals(y))
throw new InvalidOperationException("Wut?");
}
Here's an example. Given this class:
class Test
{
public int Value;
public string Name;
public static bool operator==(Test lhs, Test rhs)
{
if (ReferenceEquals(lhs, rhs))
return true;
if (ReferenceEquals(lhs, null) || ReferenceEquals(rhs, null))
return false;
return lhs.Value == rhs.Value;
}
public static bool operator!=(Test lhs, Test rhs)
{
return !(lhs == rhs);
}
}
This code will behave oddly:
Test test1 = new Test { Value = 1, Name = "1" };
Test test2 = new Test { Value = 1, Name = "2" };
if (test1 == test2)
Console.WriteLine("test1 == test2"); // This gets printed.
else
Console.WriteLine("test1 != test2");
if (test1.Equals(test2))
Console.WriteLine("test1.Equals(test2)");
else
Console.WriteLine("NOT test1.Equals(test2)"); // This gets printed!
You do NOT want this!
My guess is that the compiler takes its clues from your actions, and decides that since you find it important to provide an alternative implementation of the equality operator, then you probably want the object equality to remain consistent with your new implementation of ==. After all, you do not want the two equality comparisons to mean drastically different things, otherwise your program would be hard to understand even on a very basic level. Therefore, the compiler thinks that you should redefine Equals as well.
Once you provide an alternative implementation Equals, however, you need to modify GetHashCode to stay consistent with the equality implementation. Hence the compiler warns you that your implementation might be incomplete, and suggests overriding both Equals and GetHashCode.
If you don't overload the Equals method too, then using it might give different results from the ones you'd get with the operator. Like, if you overload = for integers...
int i = 1;
(1 == 1) == (i.Equals(1))
Could evaluate to false.
For the same reason, you should reimplement the GetHashCode method so you don't mess up with hashtables and such other structures that rely on hash comparisons.
Notice I'm saying "might" and "could", not "will". The warnings are there just as a reminder that unexpected things might happen if you don't follow its suggestions. Otherwise you'd get errors instead of warnings.
The documentation is pretty clear about this:
The GetHashCode method can be overridden by a derived type. Value
types must override this method to provide a hash function that is
appropriate for that type and to provide a useful distribution in a
hash table. For uniqueness, the hash code must be based on the value
of an instance field or property instead of a static field or
property.
Objects used as a key in a Hashtable object must also override the
GetHashCode method because those objects must generate their own hash
code. If an object used as a key does not provide a useful
implementation of GetHashCode, you can specify a hash code provider
when the Hashtable object is constructed. Prior to the .NET Framework
version 2.0, the hash code provider was based on the
System.Collections.IHashCodeProvider interface. Starting with version
2.0, the hash code provider is based on the System.Collections.IEqualityComparer interface.
Whats the difference between these two comparison statments?
var result = EqualityComparer<T>.Default.Equals(#this, null);
var result = #this == null;
Obviously the aim is to test whether the object '#this' isnull.
Well it depends on the type of #this. If it doesn't have an overload of ==, the second line will just perform a direct reference comparison, whereas the first line will call an overridden Equals method or an implementation of IEquatable.Equals.
Any sensible implementation will give the same result for both comparisons.
The first statement calls the Equals() method between objects to see if their values are equal, assuming it has been overriden and implemented in the class T. The second statement compares the references instead, unless the == operator has been overridden like in the String class.
operator == calls ReferenceEquals on comparing objects, so compare that objects are pointing to the same memory location.
Equals, instead, is a just virtual method, so can behave differently for different types, as it can be overriden.
For example, for CLR string Equals compares content of a string and not a reference, even if string is a reference type.
I ran into this situation today. I have an object which I'm testing for equality; the Create() method returns a subclass implementation of MyObject.
MyObject a = MyObject.Create();
MyObject b = MyObject.Create();
a == b; // is false
a.Equals(b); // is true
Note I have also over-ridden Equals() in the subclass implementation, which does a very basic check to see whether or not the passed-in object is null and is of the subclass's type. If both those conditions are met, the objects are deemed to be equal.
The other slightly odd thing is that my unit test suite does some tests similar to
Assert.AreEqual(MyObject.Create(), MyObject.Create()); // Green bar
and the expected result is observed. Therefore I guess that NUnit uses a.Equals(b) under the covers, rather than a == b as I had assumed.
Side note: I program in a mixture of .NET and Java, so I might be mixing up my expectations/assumptions here. I thought, however, that a == b worked more consistently in .NET than it did in Java where you often have to use equals() to test equality.
UPDATE Here's the implementation of Equals(), as requested:
public override bool Equals(object obj) {
return obj != null && obj is MyObjectSubclass;
}
The key difference between == and Equals is that == (like all operators) is not polymorphic, while Equals (like any virtual function) is.
By default, reference types will get identical results for == and Equals, because they both compare references. It's also certainly possible to code your operator logic and Equals logic entirely differently, though that seems nonsensical to do. The biggest gotcha comes when using the == (or any) operator at a higher level than the desired logic is declared (in other words, referencing the object as a parent class that either doesn't explicitly define the operator or defines it differently than the true class). In such cases the logic for the class that it's referenced as is used for operators, but the logic for Equals comes from whatever class the object actually is.
I want to state emphatically that, based solely upon the information in your question, there is absolutely no reason to think or assume that Equals compares values versus references. It's trivially easy to create such a class, but this is not a language specification.
Post-question-edit edit
Your implementation of Equals will return true for any non-null instance of your class. Though the syntax makes me think that you aren't, you may be confusing the is C# keyword (which confirms type) with the is keyword in VB.NET (which confirms referential equality). If that is indeed the case, then you can make an explicit reference comparison in C# by using Object.ReferenceEquals(this, obj).
In any case, this is why you are seeing true for Equals, since you're passing in a non-null instance of your class.
Incidentally, your comment about NUnit using Equals is true for the same reason; because operators are not polymorphic, there would be no way for a particular class to define custom equality behavior if the Assert function used ==.
a == b checks if they reference the same object.
a.Equals(b) compares the contents.
This is a link to a Jon Skeet article from 2004 that explains it better.
You pretty much answered your question yourself:
I have also over-ridden Equals() in the subclass implementation, which does a very basic check to see whether or not the passed-in object is null and is of the subclass's type. If both those conditions are met, the objects are deemed to be equal.
The == operator hasn't been overloaded - so it's returning false since a and b are different objects. But a.Equals is calling your override, which is presumably returning true because neither a nor b are null, and they're both of the subclass' type.
So your question was "When can a == b be false and a.Equals(b) true?" Your answer in this case is: when you explicitly code it to be so!
In Java a ==b check if the references of the two objects are equals (rougly, if the two objects are the same object "aliased")
a.equals(b) compare the values represented by the two objects.
They both do the same unless they are specifically overloaded within the object to do something else.
A quote from the Jon Skeet Article mentioned elsewhere.
The Equals method is just a virtual
one defined in System.Object, and
overridden by whichever classes choose
to do so. The == operator is an
operator which can be overloaded by
classes, but which usually has
identity behaviour.
The keyword here is USUALLY. They can be written to do whatever the underlying class wishes and in no way do they have to do the same.
The "==" operate tests absolute equality (unless overloaded); that is, it tests whether two objects are the same object. That's only true if you assigned one to the other, ie.
MyObject a = MyObject.Create();
MyObject b = a;
Just setting all the properties of two objects equal doesn't mean the objects themselves are. Under the hood, what the "==" operator is comparing is the addresses of the objects in memory. A practical effect of this is that if two objects are truly equal, changing a property on one of them will also change it on the other, whereas if they're only similar ("Equals" equal), it won't. This is perfectly consistent once you understand the principle.
I believe that a == b will check if the referenced object is the same.
Usually to see if the value is the same a.Equals(b) is used (this often needs to be overridden in order to work).
Why use one over the other?
== is the identity test. It will return true if the two objects being tested are in fact the same object. Equals() performs an equality test, and will return true if the two objects consider themselves equal.
Identity testing is faster, so you can use it when there's no need for more expensive equality tests. For example, comparing against null or the empty string.
It's possible to overload either of these to provide different behavior -- like identity testing for Equals() --, but for the sake of anybody reading your code, please don't.
Pointed out below: some types like String or DateTime provide overloads for the == operator that give it equality semantics. So the exact behavior will depend on the types of the objects you are comparing.
See also:
http://blogs.msdn.com/csharpfaq/archive/2004/03/29/102224.aspx
#John Millikin:
Pointed out below: some value types like DateTime provide overloads for the == operator >that give it equality semantics. So the exact behavior will depend on the types of the >objects you are comparing.
To elaborate:
DateTime is implemented as a struct. All structs are children of System.ValueType.
Since System.ValueType's children live on the stack, there is no reference pointer to the heap, and thus no way to do a reference check, you must compare objects by value only.
System.ValueType overrides .Equals() and == to use a reflection based equality check, it uses reflection to compare each fields value.
Because reflection is somewhat slow, if you implement your own struct, it is important to override .Equals() and add your own value checking code, as this will be much faster. Don't just call base.Equals();
Everyone else pretty much has you covered, but I have one more word of advice. Every now and again, you will get someone who swears on his life (and those of his loved ones) that .Equals is more efficient/better/best-practice or some other dogmatic line. I can't speak to efficiency (well, OK, in certain circumstances I can), but I can speak to a big issue which will crop up: .Equals requires an object to exist. (Sounds stupid, but it throws people off.)
You can't do the following:
StringBuilder sb = null;
if (sb.Equals(null))
{
// whatever
}
It seems obvious to me, and perhaps most people, that you will get a NullReferenceException. However, proponents of .Equals forget about that little factoid. Some are even "thrown" off (sorry, couldn't resist) when they see the NullRefs start to pop up.
(And years before the DailyWTF posting, I did actually work with someone who mandated that all equality checks be .Equals instead of ==. Even proving his inaccuracy didn't help. We just made damn sure to break all his other rules so that no reference returned from a method nor property was ever null, and it worked out in the end.)
== is generally the "identity" equals meaning "object a is in fact the exact same object in memory as object b".
equals() means that the objects logically equal (say, from a business point of view). So if you are comparing instances of a user-defined class, you would generally need to use and define equals() if you want things like a Hashtable to work properly.
If you had the proverbial Person class with properties "Name" and "Address" and you wanted to use this Person as a key into a Hashtable containing more information about them, you would need to implement equals() (and hash) so that you could create an instance of a Person and use it as a key into the Hashtable to get the information.
Using == alone, your new instance would not be the same.
According to MSDN:
In C#, there are two different kinds of equality: reference equality (also known as identity) and value equality. Value equality is the generally understood meaning of equality: it means that two objects contain the same values. For example, two integers with the value of 2 have value equality. Reference equality means that there are not two objects to compare. Instead, there are two object references and both of them refer to the same object.
...
By default, the operator == tests for reference equality by determining whether two references indicate the same object.
Both Equals and == can be overloaded, so the exact results of calling one or the other will vary. Note that == is determined at compile time, so while the actual implementation could change, which == is used is fixed at compile time, unlike Equals which could use a different implementation based on the run time type of the left side.
For instance string performs an equality test for ==.
Also note that the semantics of both can be complex.
Best practice is to implement equality like this example. Note that you can simplify or exclude all of this depending on how you plan on using you class, and that structs get most of this already.
class ClassName
{
public bool Equals(ClassName other)
{
if (other == null)
{
return false;
}
else
{
//Do your equality test here.
}
}
public override bool Equals(object obj)
{
ClassName other = obj as null; //Null and non-ClassName objects will both become null
if (obj == null)
{
return false;
}
else
{
return Equals(other);
}
}
public bool operator ==(ClassName left, ClassName right)
{
if (left == null)
{
return right == null;
}
else
{
return left.Equals(right);
}
}
public bool operator !=(ClassName left, ClassName right)
{
if (left == null)
{
return right != null;
}
else
{
return !left.Equals(right);
}
}
public override int GetHashCode()
{
//Return something useful here, typically all members shifted or XORed together works
}
}
Another thing to take into consideration: the == operator may not be callable or may have different meaning if you access the object from another language. Usually, it's better to have an alternative that can be called by name.
The example is because the class DateTime implements the IEquatable interface, which implements a "type-specific method for determining equality of instances." according to MSDN.
use equals if you want to express the contents of the objects compared should be equal. use == for primitive values or if you want to check that the objects being compared is one and the same object. For objects == checks whether the address pointer of the objects is the same.
I have seen Object.ReferenceEquals() used in cases where one wants to know if two references refer to the same object
In most cases, they are the same, so you should use == for clarity. According to the Microsoft Framework Design Guidelines:
"DO ensure that Object.Equals and the equality operators have exactly the same semantics and similar performance characteristics."
https://learn.microsoft.com/en-us/dotnet/standard/design-guidelines/equality-operators
But sometimes, someone will override Object.Equals without providing equality operators. In that case, you should use Equals to test for value equality, and Object.ReferenceEquals to test for reference equality.
If you do disassemble (by dotPeek for example) of Object, so
public virtual bool Equals(Object obj)
described as:
// Returns a boolean indicating if the passed in object obj is
// Equal to this. Equality is defined as object equality for reference
// types and bitwise equality for value types using a loader trick to
// replace Equals with EqualsValue for value types).
//
So, is depend on type.
For example:
Object o1 = "vvv";
Object o2 = "vvv";
bool b = o1.Equals(o2);
o1 = 555;
o2 = 555;
b = o1.Equals(o2);
o1 = new List<int> { 1, 2, 3 };
o2 = new List<int> { 1, 2, 3 };
b = o1.Equals(o2);
First time b is true (equal performed on value types), second time b is true (equal performed on value types), third time b is false (equal performed on reference types).