Operator overloading ==, !=, Equals

Question

I've already gone through question

I understand that, it is necessary to implement ==, != and Equals().

public class BOX { double height, length, breadth; // this is first one '==' public static bool operator== (BOX obj1, BOX obj2) { return (obj1.length == obj2.length && obj1.breadth == obj2.breadth && obj1.height == obj2.height); } // this is second one '!=' public static bool operator!= (BOX obj1, BOX obj2) { return !(obj1.length == obj2.length && obj1.breadth == obj2.breadth && obj1.height == obj2.height); } // this is third one 'Equals' public override bool Equals(BOX obj) { return (length == obj.length && breadth == obj.breadth && height == obj.height); } }

I assume, I've written code properly to override ==,!=,Equals operators. Though, I get compilation errors as follows.

'myNameSpace.BOX.Equals(myNameSpace.BOX)' is marked as an override but no suitable method found to override.

So, question is - How to override above operators & get rid of this error?

isn't the signature for Equals like public override bool Equals(object o)? — bansi
– bansi, Commented Aug 23, 2014 at 11:53
Resharper suggests writing != like return !(obj1 == obj2) which should take advantage of what you already wrote for the == overload. — drzaus
– drzaus, Commented Jul 18, 2017 at 19:44

AustinWBryan · Accepted Answer · 2022-02-02 04:20:27Z

As Selman22 said, you are overriding the default object.Equals method, which accepts an object obj and not a safe compile time type.

In order for that to happen, make your type implement IEquatable<Box>:

public class Box : IEquatable<Box> { double height, length, breadth; public static bool operator ==(Box obj1, Box obj2) { if (ReferenceEquals(obj1, obj2)) return true; if (ReferenceEquals(obj1, null)) return false; if (ReferenceEquals(obj2, null)) return false; return obj1.Equals(obj2); } public static bool operator !=(Box obj1, Box obj2) => !(obj1 == obj2); public bool Equals(Box other) { if (ReferenceEquals(other, null)) return false; if (ReferenceEquals(this, other)) return true; return height.Equals(other.height) && length.Equals(other.length) && breadth.Equals(other.breadth); } public override bool Equals(object obj) => Equals(obj as Box); public override int GetHashCode() { unchecked { int hashCode = height.GetHashCode(); hashCode = (hashCode * 397) ^ length.GetHashCode(); hashCode = (hashCode * 397) ^ breadth.GetHashCode(); return hashCode; } } }

Another thing to note is that you are making a floating point comparison using the equality operator and you might experience a loss of precision.

public override bool Equals(object obj) => obj is Box && Equals(obj as Box);
use: public override int GetHashCode() => Hashcode.Combine(height, length, breadth); instead of the "unchecked" section to simplify the code
Regarding the note about the "loss of precision": If you decide to implement a fuzzy Equals which accounts for loss of precision and declares two boxes to be equal with regards to a certain epsilon, you will violate the GetHashCode contract: GetHashCode should never return different results for objects that are "Equal".

Selman Genç · Accepted Answer · 2014-08-23 11:51:08Z

I think you declared the Equals method like this:

public override bool Equals(BOX obj)

Since the object.Equals method takes an object, there is no method to override with this signature. You have to override it like this:

public override bool Equals(object obj)

If you want type-safe Equals, you can implement IEquatable<BOX>.

epox · Accepted Answer · 2019-10-03 12:37:33Z

In fact, this is a "how to" subject. So, here is the reference implementation:

 public class BOX { double height, length, breadth; public static bool operator == (BOX b1, BOX b2) { if ((object)b1 == null) return (object)b2 == null; return b1.Equals(b2); } public static bool operator != (BOX b1, BOX b2) { return !(b1 == b2); } public override bool Equals(object obj) { if (obj == null || GetType() != obj.GetType()) return false; var b2 = (BOX)obj; return (length == b2.length && breadth == b2.breadth && height == b2.height); } public override int GetHashCode() { return height.GetHashCode() ^ length.GetHashCode() ^ breadth.GetHashCode(); } }

REF: https://msdn.microsoft.com/en-us/library/336aedhh(v=vs.100).aspx#Examples

UPDATE: the cast to (object) in the operator == implementation is important, otherwise, it would re-execute the operator == overload, leading to a stackoverflow. Credits to @grek40.

This (object) cast trick is from Microsoft String == implementaiton. SRC: https://github.com/Microsoft/referencesource/blob/master/mscorlib/system/string.cs#L643

see also: github.com/Microsoft/referencesource/blob/master/mscorlib/…
Don't write if (null == b1), it will re-execute the operator == overload, leading to a stackoverflow. Same for b2.
Yes, this should do the trick. Alternative would be using object.ReferenceEquals like the other answer.
For HashCode generation, it is better to use a tupple.GetHashCode(): (height, length, breadth).GetHashCode(). The problem with using XOR is that height, length, breadth probably use only the lower bits and the higher bits are unused. A hash ideally uses all 32 bits of the integer. This is achieved by bit-shifting the parameters by different values, which GetHashCode() does.

Marko Pavic · Accepted Answer · 2020-04-02 12:59:24Z

public class BOX { double height, length, breadth; public static bool operator == (BOX b1, BOX b2) { if (b1 is null) return b2 is null; return b1.Equals(b2); } public static bool operator != (BOX b1, BOX b2) { return !(b1 == b2); } public override bool Equals(object obj) { if (obj == null) return false; return obj is BOX b2? (length == b2.length && breadth == b2.breadth && height == b2.height): false; } public override int GetHashCode() { return (height,length,breadth).GetHashCode(); } }

using is instead of == eliminates the workaround of casting to object

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