If reference type doesn't overload an equality operator ==, then build-in equality operator on Object will be used instead. Why isn't the same true for user-defined structs:
struct A{ }
static void Main(string[] args)
{
A a = new A();
A a1 = new A();
bool equal= (a == a1); //error
}
Namely, doesn't ValueType ( from which all structs derive ) also overload == operator?
How would such a default == operator work? For reference types, comparing adresses is reasonable, but since that check will never be true for two ValueTypes (since if two ValueTypes are in scope then they are guaranteed to have different locations on the stack,) address comparison is pointless.
As the compiler has helpfully pointed out, ValueType very intentionally does not have a default == operator.
Structs probably don't provide a default == operator implementation because, unlike a class instance, a struct has no concept of reference-style identity.
From the guidelines:
Implementing the Equality Operator
(==) on Value Types
In most programming languages there is
no default implementation of the
equality operator (==) for value
types. Therefore, you should overload
== any time equality is meaningful.
You should consider implementing the
Equals method on value types because
the default implementation on
System.ValueType will not perform as
well as your custom implementation.
Implement == any time you override the
Equals method.
However, structs do provide a default Equals Method implementation which will do a memberwise compare using reflection.
Related
Equality operator (==) is a reference type and we all know that Int32 is a struct which means that is a value type. I also checked the inside of the Int32 and I couldn't find any Operator Overloading related to Equality Operator (==).
So, my question is why can we apply == on Int32?
I also checked the decimal type and I noticed that it has some Operator Overloading, so naturally we can use == on decimal types.
Per the equality operators documentation for value types:
Operands of the built-in value types are equal if their values are equal
Also, be aware that
User-defined struct types don't support the == operator by default. To support the == operator, a user-defined struct must overload it.
So, in a nutshell all user-defined structs must define the equality operator to overload it. But all built-in value types don't need to, it "just works".
I have gone to an interview where I was asked to answer a question:
Is operator overriding supported by C#?
I know that operator overloading is supported but I have no idea about overriding a operator. Is it possible?
No, operator overriding is not supported. The term Overriding is used when a method is inherited by a subclass and the subclass overrides it with its own implementation. Operators are all static in C# and cannot be overridden.
Overloading means that another method with the same name but a different signature (arguments) is defined. That's what you can do with operators.
This is extremely important to know when writing operators in C#. The operator is bound at compile time. The effective type which is passed to the operator at runtime is not important at all.
Eg. you write some comparison operator
public static bool operator==(MyClass c1, MyClass c2)
{
//...
}
And have the following code:
object myObj1 = new MyClass();
object myObj2 = new MyClass();
if (myObj1 == myObj2)
//...
object's operator will be called, not your own, because the
arguments are references of type object.
A derives directly from Object class and neither A or Object overload == operator, so why doesn't next code cause an error:
class Program
{
static void Main(string[] args)
{
A a1 = new A();
A a2 = new A();
if (a1 == a2) ... ;
}
}
class A { }
thanx
A derives directly from Object class and neither A or Object overload == operator, so why doesn't next code cause an error?
As with your other question, you seem to have some strange belief that whether an overloaded operator exists has any bearing on whether an operator can be meaningfully chosen. It does not.
Again, to resolve this situation overload resolution first attempts to determine if there is a user-defined operator defined on either of the operands. As you note, there is not.
Overload resolution then falls back on the built-in operators. As I mentioned in your other question, the built-in operators are the equality operators on int, uint, long, ulong, bool, char, float, double, decimal, object, string, all delegate types and all enum types, plus the lifted-to-nullable versions of all the value types.
Given those operators we must now determine the applicable ones. There is no implicit conversion from "A" to any of the value types, to any of the nullable value types, to string, or to any delegate type.
The only remaining applicable candidate is object.
If overload resolution chooses the equality operator that compares two objects, additional constraints must be met. In particular, both operands must either be null or a reference type, or a type parameter not constrained to be a value type. That constraint is met. Also, if the two sides have types then the operand types must have some sort of compatibility relationship; you can't do "myString == myException" because there is no relationship between string and Exception. There is a relationship between "A" and "A", namely, they are identical.
Therefore the reference equality operator is chosen, and the == means "compare these two object expressions by reference".
I am mystified as to why you believe having a user-defined == operator has anything to do with this, either in this question or your other question. The absence of such a method does not prevent the compiler from generating whatever code it likes for this expression. Can you explain?
Because by default, the == operator compares the references (memory locations) of the objects a1 and a2. And because they're different instances of class A, the expression a1 == a2 always evaluates to false in your example.
Objects have a default implementation of the == operator that checks if they refer to the same object (reference comparison). So there's no reason for it to be an error. The operator does have a meaning.
Because Object has a default implementation comparing references.
The base's == operator is called that why its not giving any error.
By default, the operator == tests for
reference equality by determining if
two references indicate the same
object, so reference types do not need
to implement operator == in order to
gain this functionality.
From: http://msdn.microsoft.com/en-us/library/ms173147(v=vs.80).aspx
The important bit concerning your question being:
reference types do not need to
implement operator == in order to gain
this functionality
In Java there are "==" and "equals" operator for reference types and "==" for value types. for reference type, "==" means both objects point to the same location and "equals" means their values are the same. does C# has similar operators for value type and reference types?
Well, == can be overloaded for reference types. For example:
string a = new string('x', 10);
string b = new string('x', 10);
Console.WriteLine(a == b); // True
Console.WriteLine(Object.ReferenceEquals(a, b)); // False
Unless it's overloaded, == means "reference equality" aka "object identity" for reference types. (As Marc says, you may override Equals without overloading ==.)
For value types, you have to overload == otherwise the C# compiler won't let you use it for comparisons. .NET itself will provide an implementation of Equals which usually does the right thing, but sometimes slowly - in most cases, if you write your own custom value type you'll want to implement IEquatable<T> and override Equals as well - and quite possibly overload various operators.
C# allows the == operator to be overloaded (and the Equals method to be overridden - although == and Equals don't have to mean the same thing).
If you want to mean "the same instance", then object.ReferenceEquals (for reference-types) is the best option. Value types default to internal equality.
Straight out of MSDN:
For predefined value types, the
equality operator (==) returns true if
the values of its operands are equal,
false otherwise. For reference types
other than string, == returns true if
its two operands refer to the same
object. For the string type, ==
compares the values of the strings.
Jon Skeet should be able to give you a perfect answer though :P
When should I use == and when should I use Equals?
http://blogs.msdn.com/csharpfaq/archive/2004/03/29/102224.aspx
This is precisely the way it works with .NET as well. The C# FAQ blog explains equals better:
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.
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.