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.
Related
Say I have three collections in Mongo: flavor, color, and cupcake. Each collection has its own _id (obviously) and the cupcake collection references the _ids in flavor and cupcake, like so:
{
"_id": ObjectId("123"),
"flavorId": ObjectId("234"),
"colorId": ObjectId("345"),
"moreData": {}
}
This is a toy example, of course, and there is more stuff in these collections. That's not important to this question, except that it's the moreData that I'm really looking for when I query.
I want to be able to look up cupcake objects by flavorId and by colorId (and they are appropriately indexed for such lookups). However, both fields are ObjectId, and I want to avoid somebody accidentally looking for a colorId with a flavorId. How can I design the object and a repository class such that colorId and flavorId will be different types so that the compiler will not allow interchanging them, but still store both ids as ObjectId?
My first thought was to extend ObjectId and pass the extended object around, but ObjectId is a struct which cannot be extended.
You won't be able to prevent those errors, but you can use number intervals to make it easier for "someone" to find the problem.
If I'm not mistaken you can set the ids, so you can use a "prefix" for every kind.
Colors could start with 1000, flavors with 2000 and so on...
Hmm, it is a kind of soft problems, because in most repositories ID is something common (like integers). So having this in mind we could enforce passing an extra parameter instead of changing base object, like this bulletproof solution
cupcakeRepository.Find(ObjectId flavorId, ÒbjectType ÒbjectType.Flavor)
or just extend repository to be more verbose
cupcakeRepository.FindByColor(ObjectId id)
cupcakeRepository.FindByFlavor(ObjectId id)
So I ended up biting the bullet on building the Mongo-specific junk to make a custom class work for this. So here is my drop-in replacement for ObjectId:
public struct DocumentId<T> : IEquatable<DocumentId<T>>
{
static DocumentId()
{
BsonSerializer.RegisterSerializer(typeof(DocumentId<T>), DocumentIdSerializer<T>.Instance);
BsonSerializer.RegisterIdGenerator(typeof(DocumentId<T>), DocumentIdGenerator<T>.Instance);
}
public static readonly DocumentId<T> Empty = new DocumentId<T>(ObjectId.Empty);
public readonly ObjectId Value;
public DocumentId(ObjectId value)
{
Value = value;
}
public static DocumentId<T> GenerateNewId()
{
return new DocumentId<T>(ObjectId.GenerateNewId());
}
public static DocumentId<T> Parse(string value)
{
return new DocumentId<T>(ObjectId.Parse(value));
}
public bool Equals(DocumentId<T> other)
{
return Value.Equals(other.Value);
}
public override bool Equals(object obj)
{
if (ReferenceEquals(null, obj)) return false;
return obj is DocumentId<T> && Equals((DocumentId<T>)obj);
}
public static bool operator ==(DocumentId<T> left, DocumentId<T> right)
{
return left.Value == right.Value;
}
public static bool operator !=(DocumentId<T> left, DocumentId<T> right)
{
return left.Value != right.Value;
}
public override int GetHashCode()
{
return Value.GetHashCode();
}
public override string ToString()
{
return Value.ToString();
}
}
public class DocumentIdSerializer<T> : StructSerializerBase<DocumentId<T>>
{
public static readonly DocumentIdSerializer<T> Instance = new DocumentIdSerializer<T>();
public override DocumentId<T> Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
{
return new DocumentId<T>(context.Reader.ReadObjectId());
}
public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, DocumentId<T> value)
{
context.Writer.WriteObjectId(value.Value);
}
}
public class DocumentIdGenerator<T> : IIdGenerator
{
public static readonly DocumentIdGenerator<T> Instance = new DocumentIdGenerator<T>();
public object GenerateId(object container, object document)
{
return DocumentId<T>.GenerateNewId();
}
public bool IsEmpty(object id)
{
var docId = id as DocumentId<T>? ?? DocumentId<T>.Empty;
return docId.Equals(DocumentId<T>.Empty);
}
}
The type parameter T can be anything; it is never used. It should be the type of your object, like so:
public class Cupcake {
[BsonId]
public DocumentId<Cupcake> Id { get; set; }
// ...
}
This way, your Flavor class has an Id of type DocumentId<Flavor> and your Color class has an Id of type DocumentId<Color>, and never shall the two be interchanged. Now I can create a CupcakeRepository with the following unambiguous methods as well:
public interface ICupcakeRepository {
IEnumerable<Cupcake> Find(DocumentId<Flavor> flavorId);
IEnumerable<Cupcake> Find(DocumentId<Color> colorId);
}
This should be safe with existing data as well because the serialized representation is exactly the same, just an ObjectId("1234567890abcef123456789").
Is there a way I can overload primitives, for example addition with doubles? I want to automatically round the doubles whenever an operation is performed. My current code is this:
class Test{
public static double operator +(double x, double y){
return Math.Round(x + y)
}
}
but there's an unfortunate error that says "One of the parameters of a binary operator must be the containing type".
No, and this would be horrible. Users using your library would suddenly get different behaviors from their double variables!
You can write and use a wrapper object however:
public struct MyDouble
{
public double Value {get; set;}
public MyDouble(double initValue)
{
Value = initValue;
}
public static double operator +(MyDouble x, MyDouble y){
return Math.Round(x.Value + y.Value)
}
}
You can also make it castable to/from a double, among other options. This way users know they are using your object and won't be surprised when their math operations are rounded.
If you want to assign from a simple double, you would need to define an implicit operator, similar to that of Nullable<T> (source):
public static implicit operator MyDouble(double value) {
return new MyDouble(value);
}
You can't overload operators on primitive types. This would cause havoc in your codebase.
What you can do instead, is to create a simple wrapper around the primitive type, let's say RoundedDouble:
public struct RoundedDouble : IEquatable<RoundedDouble>, IComparable<RoundedDouble>
{
public readonly double Value;
public RoundedDouble(double value)
{
Value = Math.Round(value); // Or anything else
}
public static implicit operator RoundedDouble(double value)
{
return new RoundedDouble(value);
}
public static implicit operator double(RoundedDouble wrapper)
{
return wrapper.Value;
}
public int GetHashCode()
{
return Value.GetHashCode();
}
public bool Equals(object other)
{
if (other is RoundedDouble)
return ((RoundedDouble)other).Value == Value;
return false;
}
public string ToString()
{
return Value.ToString();
}
// Add your operators here, and implement the interfaces
}
This is a structure. It has the same value semantics as a double.
Extend it by adding the operators, and by implementing at least IEquatable<RoundedDouble> and IComparable<RoundedDouble>.
I am creating a Volume value type and so far so good but when I overrode the multiplication operator and wrote a unit test, if the test fails instead of getting the expected and actual amounts I get the full qualified type name.
There's not much in the code:
private decimal amount;
public Volume(decimal value)
{
amount = value;
}
public static implicit operator Volume(decimal value)
{
return new Volume(value);
}
//... continue same methods with all number types
public static Volume operator *(Volume left, decimal right)
{
return new Volume(left.amount * right);
}
With this code if I write a failing test instead of getting the expected and actual amount failing message I get:
Message: Assert.AreEqual failed. Expected: <MyTypes.Utilities.Volume>. Actual: <MyTypes.Utilities.Volume>.
I tried adding the following:
public static implicit operator Decimal(Volume value)
{
return value.amount;
}
Not only this did not work but now the test that proves the type can be initialized with a decimal amount fails with the same message:
[TestMethod]
public void VolumeTypeGetsInitializedByDecimalValue()
{
Decimal value = 123456781.1235657789464356m;
Volume volume = value;
Assert.AreEqual(value, volume);
}
This is the first time I try to do this so I am not sure why it is behaving this way. Any guidance is appreciated.
First way is to replace
Assert.AreEqual(value, volume);
with:
Assert.AreEqual((Volume)value, volume);
another way is to replace assert string with:
Assert.IsTrue(value.Equals(volume), string.Format("It was expected to get '{0}' but got '{1}'.", volume, value));
and override ToString to be like:
public override String ToString(){
return amount.ToString();
}
Update
To make things "right", additionally to overriding ToString method I also do recommend to override Equals and GetHashCode methods with marking amount field as readonly:
private readonly decimal amount;
public bool Equals(Volume other)
{
return amount == other.amount;
}
public override bool Equals(object obj)
{
return obj is Volume ? Equals((Volume)obj) : base.Equals(obj);
}
public override int GetHashCode()
{
return amount.GetHashCode();
}
To see the decimal value enclosed by your Volume class when it is printed by your test, just implement an override of ToString():
public override String ToString(){
return amount.ToString();
}
Assert.AreEqual calls object.Equals, implimented like this. First it compares 2 objects with object.ReferenceEquals, if they are not equal it checks if one of them null and if not it calls Equals method of one of objects.
static bool Equals(object a, object b)
{
if(ReferenceEquals(a, b)
return true;
if(a == null || b == null)
return false;
return a.Equals(b);
}
In c# all value (struct) type inherit from ValueType, which overrides Object Equals method. First it compares types, in your case it is Volume and decimal, so Equals returns false. For equal types it compares field values via reflection. So if you want Assert.AreEqual to compare Volume and decimal, you'll have to override Equals like this:
public override bool Equals(object obj)
{
if (obj is decimal)
{
return amount == (decimal) obj;
}
if (obj is Volume)
{
return amount == ((Volume) obj).amount;
}
return false;
}
Still it is possible that Assert.AreEqual(Decimal, Volume) and Assert.AreEqual(Volume, Decimal) will produce different results, so I'd recomend you to test with Assert.IsTrue(Volume.Equals(Decimal)).
The problem is with the command Assert.AreEqual. Although I cannot speak directly to how the implementation works (documentation), I believe that the function is using the operator ==.
In order for your volume to work correctly with Assert.AreEqual you will need to define
the operators == and !=.
EDIT: Found documentation that suggests that Assert.AreEqual implements IEquatable
See here
Such as
public static bool operator !=(Volume valA, Volume valB)
{
return valA.amount != valB.amount;
}
public static bool operator ==(Volume valA, Volume valB)
{
return valA.amount == valB.amount;
}
Note: You will also need to implement Equals & GetHashCode (or use IEquatable)
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
}
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.