I have a problem with generic. When I try to use less operators in generic, their call is not happening. But it works with the method Equals.
That is a some test class:
public class Test
{
public int i;
static public Boolean operator ==(Test obj1, Test obj2)
{
Console.WriteLine("operator ==");
return obj1.i == obj2.i;
}
static public Boolean operator !=(Test obj1, Test obj2)
{
Console.WriteLine("operator !=");
return obj1.i != obj2.i;
}
public override bool Equals(object obj)
{
Console.WriteLine("operator equals");
return this == (Test)obj;
}
public override int GetHashCode()
{
Console.WriteLine("HashCode");
return 5;
}
}
And class Checker:
public class Checker
{
public Boolean TestGeneric<T>(T Left, T Right) where T : class
{
return Left == Right; //not work override operators
return Left.Equals(Right); //work fine
}
}
Small testing:
Test left = new Test() { i = 4 };
Test right = new Test() { i = 4 };
var checker = new Checker();
Console.WriteLine(checker.TestGeneric<Test>(left, right));
Console.ReadKey();
How I can use less operators in class Test from generic?
Overloaded operators are static methods, so they don't participate in polymorphism; they are resolved statically at compile time, based on the known type of the operands.
In a generic method, the compiler can't know that T will be Test (since it could actually be anything else), so it uses the most general definition of ==, which is reference comparison.
Note that if you add a constraint on the generic method to force T to be Test or a subclass of Test, it will work as expected, but of course it won't work anymore for other types...
This now works in C# 11 / .NET 7 (or above):
public class Test : IEqualityOperators<Test, Test, bool>
{ /* no changes except ^^^ addition */ }
public bool TestGeneric<T>(T Left, T Right) where T : IEqualityOperators<T, T, bool>
{
return Left == Right; // does what you expect
}
Related
My class looks something like this:
public class Testclass
{
public int myValue;
}
In another context I want to simply check the value of myValue against 0.
So I would write:
Testclass tc = new Testclass();
tc.myValue = 13;
if (tc.myValue == 0)
{
}
How would it be possible to simplify this so that the Testclass object knows when it is compared to a boolean? (or used as a boolean value) to write:
Testclass tc = new Testclass();
tc.myValue = 13;
if (tc)
{
}
To be more precise, the Testclass will be the result of another method that is included in a library, so the code would look like this:
anotherClass ac =new anotherClass();
// if (ac.AMethod().myValue == 0)
// should be
if (ac.AMethod())
{
}
Where AMethod would look like this:
public Testclass AMethod()
{
return new Testclass();
}
[Edit at 2016-04-13]:
Like Dennis wrote, i am using
public static implicit operator bool(TestClass value)
to get the "Boolean Value" of my class. To be a bit more precise and to stick more to my real application i would like to change the signature to
public static implicit operator UInt64(FlexComDotNetFehler fehler)
public static implicit operator Boolean(FlexComDotNetFehler fehler)
So these two methods of class FlexComDotNetFehler return the internal UInt64 field in the first case as the real representation as UInt64and in the second one as a Boolean value, that is true, when the UInt64 value is > 0.
But now, when i code
FlexComDotNetFehler x;
FlexComDotNetFehler y;
if (x == y)
where x and y are both of type FlexComDotNetFehler
the compiler cant know if it should use the Boolean or the UInt64 operator.
So i wrote
if ((UInt64)x != (UInt64)y)
but then those two type casts are greyed out.
#Ɖiamond ǤeezeƦ: thanks for your reformatting and editing. But I think now i got i right?
Greetings Wolfgang
BTW is there a playground where i can test the formatting and its output? And how do i send private messages to other users?
Define implicit cast operator for TestClass:
class TestClass
{
public int myValue;
public static implicit operator bool(TestClass value)
{
// assuming, that 1 is true;
// somehow this method should deal with value == null case
return value != null && value.myValue == 1;
}
}
Also think about converting TestClass from class to structure (see this reference). If you'll decide to convert it, avoid mutable structures.
you can use extension methods to implement method you can use any time not only for this class Testclass
public static class IntExtension
{
public static bool IsBool(this int number)
{
bool result = true;
if (number == 0)
{
result = false;
}
return result;
}
}
and then yo can
if ((ac.AMethod()).IsBool())
{}
I know there is no such thing like Operator Overloading in Java and C#. A task is given to me by my teacher to achieve operator overloading in any of these languages. I don't know the deep concepts of these languages, only basic OOP. So can any one tell is there any other way to achieve this?
There is a thing called operator overloading in C#, check out this code snippet from MSDN:
public struct Complex
{
public int real;
public int imaginary;
public Complex(int real, int imaginary)
{
this.real = real;
this.imaginary = imaginary;
}
// Declare which operator to overload (+), the types
// that can be added (two Complex objects), and the
// return type (Complex):
public static Complex operator +(Complex c1, Complex c2)
{
return new Complex(c1.real + c2.real, c1.imaginary + c2.imaginary);
}
}
Full list of overloadable operators
As des has shown, C# does have operator overloading. Java, on the other hand, does not. The way Java compares that two objects are equal is done through overriding the method equals(Object), which is inherited from the base object java.lang.Object. Here's an example usage:
public class MyClass {
private int value;
#Override
public boolean equals(Object o) {
return o instanceof MyClass && ((MyClass)o).value == this.value;
}
}
Of course this is only a workaround for replicating overloading the == operator. For other operators, such as >= or <= there is nothing. However, you can use OO to sort of recreate it by using a common interface:
interface Overloadable<T> {
public boolean isGreaterThan(T other);
public boolean isLessThan(T other);
}
public class MyClass implements Overloadable<MyClass> {
private int value;
#Override
public boolean equals(Object o) {
return o instanceof MyClass && ((MyClass)o).value == this.value;
}
#Override
public boolean isGreaterThan(MyClass other) {
return this.value > other.value;
}
#Override
public boolean isLessThan(MyClass other) {
return this.value < other.value;
}
}
This is by no means real operator overloading because, well, you're not overloading the operators. It does however provide the ability to compare objects in the same way.
So the compiler will not allow me to overrload the == and != operators of my class. Here is what the class looks like:
public class Item
{
public string _name;
public double _weight;
public decimal _wholesalePrice;
public int _quantity;
public Item(string name, double weight, decimal wholesalePrice, int quantity)
{
_name = name;
_weight = weight;
_wholesalePrice = wholesalePrice;
_quantity = quantity;
}
public static override bool operator ==(Item left, Item right)
{
if (left._name == right._name)
{
return true;
}
return false;
}
public static override bool operator !=(Item left,Item right)
{
return !(left == right);
}
}
The compiler keeps telling me "The modifier 'override' is not valid for this item. At first I thought I might not have declared a base method as virtual, but my class does is not derived. Any ideas what's happening?
You cannot declare an override unless you have derived the class from a parent class. You also cannot declare override on a static method. Have you tried removing override all together? That seems to work for me...
public class Item
{
public string _name;
public double _weight;
public decimal _wholesalePrice;
public int _quantity;
public Item(string name, double weight, decimal wholesalePrice, int quantity)
{
_name = name;
_weight = weight;
_wholesalePrice = wholesalePrice;
_quantity = quantity;
}
public static bool operator ==(Item left, Item right)
{
if (left._name == right._name)
{
return true;
}
return false;
}
public static bool operator !=(Item left, Item right)
{
return !(left == right);
}
}
As a side note, if you override the == and != operators, it's also good practice to override the GetHashCode and Equals methods.
You are deriving your class from the class Object, which does not have an == or != operator. So you cannot override those operators.
In addition, you cannot override a static operator or method, you can only override instance methods.
Finally, note that override and overload are two very different things. An overload is where you have multiple definitions of methods with the same name but different signatures (eg. different parameters).
The short answer is that the syntax is public static bool operator ==(Item left, Item right) without the override keyword.
This is called operator overloading, not overriding.
You may think of == as a kind of static method (inside an imaginary "global" class) taking two parameters. When the compiler sees something like
xxx == yyy
it uses overload resolution to find out which == to use. This is analogous to
Meth(xxx, yyy)
where the compiler considers overloads like Meth(Object, Object), Meth(String, String), Meth(Item, Item) and finds out which of them (if any) fits best to the compile-time types of xxx and yyy.
This is just an anolgy, of course, but helps remembering why you include static and not override when you change the == operator.
What is the workaround to this problem?
class Program
{
static void Main(string[] args)
{
var a = new Test();
var b = new Test();
var eq = Check(a, b);
}
private static bool Check<T>(T a, T b) where T : class
{
return a == b; //will not call overloaded ==
}
}
public class Test
{
public override bool Equals(object obj)
{
Test other = obj as Test;
if (ReferenceEquals(other, null)) return false;
return true;
}
public static bool operator ==(Test left, Test right)
{
return Equals(left, right);
}
public static bool operator !=(Test left, Test right)
{
return !(left == right);
}
}
The == operator is not used because your generic method exists independently of the types you will use for T. It has no way of knowing that all types used as T will overload the == operator... You can use the Equals method instead:
private static bool Check<T>(T a, T b) where T : class
{
return Equals(a, b);
}
The 'solution' here is to call
private static bool Check<T>(T a, T b) where T : class
{
//return a == b; // will not call overloaded ==
return a.Equals(b); // will cal overloaded Equals
}
It's a known behavior. MSDN says:
When applying the where T : class constraint, it is recommended that
you do not use the == and != operators on the type parameter because
these operators will test for reference identity only, not for value equality. This is the case even if these operators are overloaded in
a type used as an argument.
To check equality you have to implement IEqualityComparer (or directly call one of the Equal methods).
The workaround is to call the virtual Equals method, and override that:
private static bool Check<T>(T a, T b) where T : class
{
return a.Equals(b);
}
The overloaded == operator is resolved at compile time, so it is resolved to the System.Object implementation of the operator. A virtual method call is dispatched at run time, so the override of System.Object.Equals (if any) will get called.
In your code, the virtual Equals call is occurring in the overloaded == method, which is not called; that's why the Equals override is not called.
Another solution, if you have control of the classes being passed to the method, would be to constrain to IEquatable<T>, and implement that in your classes:
private static bool Check<T>(T a, T b) where T : class, IEquatable<T>
{
return a.Equals(b);
}
That will cause the overload to resolve to the Equals(T) method, which will save some type checking at run time.
SOLVED!
This works, I need to tell the compiler that T implements IEquatable of course...
public static bool secureEquals<T>(T obj1, object obj2)
where T: class, IEquatable<T>
{...
public static bool secureEquals<T>(T obj1, T obj2)
where T: class, IEquatable<T>
{....
Question:
I tried to put repeated functionality of IEquatable implementations and Equals overrides into a separate static class like so:
public static class EqualsHelper
{
public static bool secureEquals<T>(T obj1, object obj2)
where T : class
{
if (obj2 is T)
{
return secureEquals(obj1, obj2 as T);
}
else
{
return false;
}
}
public static bool secureEquals<T>(T obj1, T obj2)
{
if (obj1 == null)
{
if (obj2 != null)
return false;
}
else
{
if (!obj1.Equals(obj2)) //this calls Dummy.Equals(object other)!
return false;
}
return true;
}
public static bool secureEquals(double[] obj1, double[] obj2)
{
if (obj1.Length != obj2.Length)
return false;
for (int i = 0; i < obj1.Length; ++i)
{
if (obj1[i] != obj2[i])//ok for doubles if they are generated in the same way? I guess so!
return false;
}
return true;
}
public class Dummy : IEquatable<Dummy>
{
public Dummy(string member)
{
_member = member;
}
private string _member;
public virtual bool Equals(Dummy other)
{
return this._member == other._member;
}
public override bool Equals(object other)
{
return EqualsHelper.secureEquals(this, other);
}
}
static void Main(string[] args)
{
Dummy d1 = new Dummy("Hugo");
Dummy d2 = new Dummy("Hugo");
object d2obj = (object)d2;
bool ret = d1.Equals(d2obj);
}
The idea was:
d1.Equals(d2obj) calls Dummy.Equals(object) calls EqualsHelper.secureEquals(T, obj) calls EqualsHelper.secureEquals(T, T) calls Dummy.Equals(Dummy).
The last call however calls Dummy.Equals(object), even though everything is typed to T there.
What am I missing?
PS: I know that replacing the call with:
if (!((IEquatable<T>)obj1).Equals(obj2)) //this calls Dummy.Equals(object other)!
does the trick, but why is it not working otherwise?
Why: because method invoke here is static typed, and the only available Equals involving a T with no generic constraint is object.Equals(object). The exact same IL has to be able to handle every T - C# generics are not like C++ templates; no per-T overload resolution occurs.
As an aside: you might also want to look at EqualityComparer<T>.Default.Equals(obj1,obj2), which will handle IEquatable<T>, Nullable<T>, etc automatically:
public static bool secureEquals<T>(T obj1, object obj2) where T : class
{
return EqualityComparer<T>.Default.Equals(obj1, obj2 as T);
}
The Equals overload within EqualsHelper.secureEquals is resolved when EqualsHelper is compiled - and that code doesn't know whether T implements IComparable<T> or not, so all that's left is Equals(Object). You can add a constraint to T which would make it use the right overload:
public static bool SecureEquals<T>(T obj1, T obj2) where T : IEquatable<T>
Of course, this would limit the classes with which you could use it.
(As an aside, note that I've renamed secureEquals to SecureEquals to comply with .NET naming conventions. I'd also not use the word "secure" here at all personally - there's nothing security-sensitive here.)
Because in your secureEquals you have no constraints, and compiler always assumes Object.Equals exist. Add interface constraint for your T.