i need to compare two objects but compare a number of their properties in one hit.
this is not for sorting, but instead to confirm whether anything has changed; as one is the old saved instance, and the second is a newly imported instance of the same thing
i assume this is best served by writing a custom comparer. just am a bit confused as to whether to do IComparer, or IComparable, or what tbh.
thanks
nat
If you only have a single definition of equality for your class, you don't really need to implement any interface: simply override the Equalsmethod. Best practice though, would be to implement IEquatable<T>and to override GetHashCode sensibly (if you don't override the hash-code, equality will misbehave when collection classes, LINQ methods etc. use it as a pre-condition for equality). Here's a sample implementation:
public class Person : IEquatable<Person>
{
public string Name { get; set; }
public int Age { get; set; }
public override int GetHashCode()
{
return (Name == null ? 0 : Name.GetHashCode()) ^ Age;
}
public override bool Equals(object obj)
{
return Equals(obj as Person);
}
public bool Equals(Person other)
{
return other != null && other.Name == Name && other.Age == Age;
}
}
This will allow you to do:
Person savedPerson = ...
Person importedPerson = ...
bool hasChanged = !savedPerson.Equals(importedPerson);
If, on the other hand, you do have lots of different definitions of equality for different circumstances, your best bet would be to write up different IEqualityComparer<T>implementations. Here's a sample implementation:
public class AgeComparer : IEqualityComparer<Person>
{
public bool Equals(Person x, Person y)
{
return (x == null || y == null) ? x == y : x.Age == y.Age;
}
public int GetHashCode(Person obj)
{
return obj == null ? 0 : obj.Age;
}
}
In this case, the check will look like:
Person savedPerson = ...
Person importedPerson = ...
IEqualityComparer<Person> comparer = ...
bool hasChanged = !comparer.Equals(savedPerson, importedPerson);
As you have alluded to IComparable is typically used for sorting.
In this case you would want to overload the comparison operator: ==
Because the reverse comparison should also be valid, this implies you must also overload the != operator.
When you overload == and !- you are also expected to override Equals and GetHashCode.
A decent C# book should explain the details.
An alternative to implementing the IComparable interface would be to override the Equals and GetHashCode functions.
That is about how you want to perform compare operation. You can do the operation in both ways:
public void Method()
{
Foo p1 = new Foo();
Foo p2 = new Foo();
p1.CompareTo(p2);
FooComparer c = new FooComparer();
c.Compare(p1, p2);
}
class Foo : IComparable
{
public int CompareTo(object obj)
{
throw new NotImplementedException();
}
}
class FooComparer : IComparer<Foo>
{
public int Compare(Foo x, Foo y)
{
throw new NotImplementedException();
}
}
I prefer using IComparer as seperation of concerns. Foo is my class, I have some busines needs on it. If I want to compare some Foos, I use FooComparer. For your situation you can return the number of changed properties from Compare method. If Compare method return 0. Then two Foos are same.
As I said. That is completely how you want to perform the action I think. Overriding Equality Operators are also good solution. Implementing IEqualityComparer<T> is another solution.
Yes you will need your type to implement IComparable. Here is a sample on how to do it.
Related
I have a class like this
public class TestData
{
public string Name {get;set;}
public string type {get;set;}
public List<string> Members = new List<string>();
public void AddMembers(string[] members)
{
Members.AddRange(members);
}
}
I want to know if it is possible to directly compare to instances of this class to eachother and find out they are exactly the same? what is the mechanism? I am looking gor something like if(testData1 == testData2) //Do Something And if not, how to do so?
You should implement the IEquatable<T> interface on your class, which will allow you to define your equality-logic.
Actually, you should override the Equals method as well.
public class TestData : IEquatable<TestData>
{
public string Name {get;set;}
public string type {get;set;}
public List<string> Members = new List<string>();
public void AddMembers(string[] members)
{
Members.AddRange(members);
}
// Overriding Equals member method, which will call the IEquatable implementation
// if appropriate.
public override bool Equals( Object obj )
{
var other = obj as TestData;
if( other == null ) return false;
return Equals (other);
}
public override int GetHashCode()
{
// Provide own implementation
}
// This is the method that must be implemented to conform to the
// IEquatable contract
public bool Equals( TestData other )
{
if( other == null )
{
return false;
}
if( ReferenceEquals (this, other) )
{
return true;
}
// You can also use a specific StringComparer instead of EqualityComparer<string>
// Check out the specific implementations (StringComparer.CurrentCulture, e.a.).
if( EqualityComparer<string>.Default.Compare (Name, other.Name) == false )
{
return false;
}
...
// To compare the members array, you could perhaps use the
// [SequenceEquals][2] method. But, be aware that [] {"a", "b"} will not
// be considerd equal as [] {"b", "a"}
return true;
}
}
One way of doing it is to implement IEquatable<T>
public class TestData : IEquatable<TestData>
{
public string Name {get;set;}
public string type {get;set;}
public List<string> Members = new List<string>();
public void AddMembers(string[] members)
{
Members.AddRange(members);
}
public bool Equals(TestData other)
{
if (this.Name != other.Name) return false;
if (this.type != other.type) return false;
// TODO: Compare Members and return false if not the same
return true;
}
}
if (testData1.Equals(testData2))
// classes are the same
You can also just override the Equals(object) method (from System.Object), if you do this you should also override GetHashCode see here
There are three ways objects of some reference type T can be compared to each other:
With the object.Equals method
With an implementation of IEquatable<T>.Equals (only for types that implement IEquatable<T>)
With the comparison operator ==
Furthermore, there are two possibilities for each of these cases:
The static type of the objects being compared is T (or some other base of T)
The static type of the objects being compared is object
The rules you absolutely need to know are:
The default for both Equals and operator== is to test for reference equality
Implementations of Equals will work correctly no matter what the static type of the objects being compared is
IEquatable<T>.Equals should always behave the same as object.Equals, but if the static type of the objects is T it will offer slightly better performance
So what does all of this mean in practice?
As a rule of thumb you should use Equals to check for equality (overriding object.Equals as necessary) and implement IEquatable<T> as well to provide slightly better performance. In this case object.Equals should be implemented in terms of IEquatable<T>.Equals.
For some specific types (such as System.String) it's also acceptable to use operator==, although you have to be careful not to make "polymorphic comparisons". The Equals methods, on the other hand, will work correctly even if you do make such comparisons.
You can see an example of polymorphic comparison and why it can be a problem here.
Finally, never forget that if you override object.Equals you must also override object.GetHashCode accordingly.
I see many good answers here but just in case you want the comparison to work like
if(testData1 == testData2) // DoSomething
instead of using Equals function you can override == and != operators:
public static bool operator == (TestData left, TestData right)
{
bool comparison = true; //Make the desired comparison
return comparison;
}
public static bool operator != (TestData left, TestData right)
{
return !(left == right);
}
You can override the equals method and inside it manually compare the objects
Also take a look at Guidelines for Overloading Equals() and Operator ==
You will need to define the rules that make object A equal to object B and then override the Equals operator for this type.
http://msdn.microsoft.com/en-us/library/ms173147(v=vs.80).aspx
First of all equality is difficult to define and only you can define as to what equality means for you
Does it means members have same value
Or they are pointing to same location.
Here is a discussion and an answer here
What is "Best Practice" For Comparing Two Instances of a Reference Type?
Implement the IEquatable<T> interface. This defines a generalized method that a value type or class implements to create a type-specific method for determining equality of instances. More information here:
http://msdn.microsoft.com/en-us/library/ms131187.aspx
I have a Metamodel that's built like this:
class ModelElement
{
string id;
}
class Package : ModelElement
{
List<Package> nestedPackages;
List<Class> ownedClasses;
}
class Class : ModelElement
{
}
Now I've built two Models and I want to check if they're identical. I'd like to compare the ID's of the Elements and I don't want to write a method for any type of Element.
Package a; //pretend both have classes
Package b; //and nested packages
compare(a.nestedPackages, b.nestedPackages);
compare(a.ownedClasses; b.OwnedClasses);
Since Class and Package both inherit from ModelElement, both have IDs. So I want to write a Function "compare" which compares the IDs. I thought of using Generics but the generic datatype doesn't have the attribute "id". Any ideas?
You could look at Enumerable.SequenceEqual, combined with a custom comparer.
bool equal = Enumerable.SequenceEqual(a.nestedPackages, b.nestedPackages,
new ModelElementComparer());
public class ModelElementComparer : IEqualityComparer<ModelElement>
{
public bool Equals(ModelElement x, ModelElement y)
{
return x.id == y.id;
}
public int GetHashCode(ModelElement obj)
{
return x.id;
}
}
Here are MSDN links to this particular SequenceEqual override and the IEqualityComparer interface.
Instead of writing a compare method, you could override the Object.Equals() method on the ModelElement class
public override bool Equals(Object obj)
{
//Check for null and compare run-time types.
if (obj == null || GetType() != obj.GetType())
return false;
ModelElement m = (ModelElement)obj;
return (id == m.id);
}
You will also need to override Object.GetHashCode() as well if you do this.
Something like Linq's Except with a custom comparer might work here:
http://msdn.microsoft.com/en-us/library/system.linq.enumerable.except.aspx
An empty resulting enumerable will mean there are no differences.
What I've read on the HashSet is it uses the default comparer for a class. I'm expecting the code below to fail when adding the second Spork to the hash set. I think my understanding of what is happening is incomplete. From MSDN of the HashSet constructor:
The IEqualityComparer implementation to use when comparing values in the set, or null to use the default EqualityComparer implementation for the set type.
So what is the default comparer, and how can I tell .Net to use my own comparer?
public class Spork : IEquatable<Spork>
{
public int Id { get; set; }
public bool Equals(Spork other)
{
return other != null && other.Id == this.Id;
}
public override bool Equals(object obj)
{
var other = obj as Spork;
return other != null && other.Id == this.Id;
}
public override int GetHashCode()
{
return Id.GetHashCode();
}
}
public class Bjork
{
public static HashSet<Spork> Sporks { get; set; }
public static void Main()
{
Sporks = new HashSet<Spork>();
Sporks.Add(new Spork() { Id = 0 });
Sporks.Add(new Spork() { Id = 0 }); // come on, please throw an exception
}
}
It is using your equality methods - but HashSet<T>.Add doesn't throw an exception when you try to add an equal value - it just returns false.
If you change the last two lines to print out the return value of Add, you'll see it returns True the first time, then False.
If your goal is to work like the Dictionary and disallow the same entry multiple times and throw an exception, you would have to inherit from HashSet and IEquatable:
class UniqueHashSet<T> : HashSet<T>, IEquatable<T>
Then, of course, write a new .Add() method to hide the base Add.
But, I'm sure there is a better way.
Or, as #Jon says, it does maintain a unique collection.
I have a class it contains some string members, some double members and some array objects.
I create two objects of this class, is there any simplest, efficient way of comparing these objects and say their equal? Any suggestions?
I know how to write a compare function, but will it be time consuming.
The only way you can really do this is to override bool Object.Equals(object other) to return true when your conditions for equality are met, and return false otherwise. You must also override int Object.GetHashCode() to return an int computed from all of the data that you consider when overriding Equals().
As an aside, note that the contract for GetHashCode() specifies that the return value must be equal for two objects when Equals() would return true when comparing them. This means that return 0; is a valid implementation of GetHashCode() but it will cause inefficiencies when objects of your class are used as dictionary keys, or stored in a HashSet<T>.
The way I implement equality is like this:
public class Foo : IEquatable<Foo>
{
public bool Equals(Foo other)
{
if (other == null)
return false;
if (other == this)
return true; // Same object reference.
// Compare this to other and return true/false as appropriate.
}
public override bool Equals(Object other)
{
return Equals(other as Foo);
}
public override int GetHashCode()
{
// Compute and return hash code.
}
}
A simple way of implementing GetHashCode() is to XOR together the hash codes of all of the data you consider for equality in Equals(). So if, for example, the properties you compare for equality are string FirstName; string LastName; int Id;, your implementation might look like:
public override int GetHashCode()
{
return (FirstName != null ? FirstName.GetHashCode() : 0) ^
(LastName != null ? LastName.GetHashCode() : 0) ^
Id; // Primitives of <= 4 bytes are their own hash codes
}
I typically do not override the equality operators, as most of the time I'm concerned with equality only for the purposes of dictionary keys or collections. I would only consider overriding the equality operators if you are likely to do more comparisons by value than by reference, as it is syntactically less verbose. However, you have to remember to change all places where you use == or != on your object (including in your implementation of Equals()!) to use Object.ReferenceEquals(), or to cast both operands to object. This nasty gotcha (which can cause infinite recursion in your equality test if you are not careful) is one of the primary reasons I rarely override these operators.
The 'proper' way to do it in .NET is to implement the IEquatable interface for your class:
public class SomeClass : IEquatable<SomeClass>
{
public string Name { get; set; }
public double Value { get; set; }
public int[] NumberList { get; set; }
public bool Equals(SomeClass other)
{
// whatever your custom equality logic is
return other.Name == Name &&
other.Value == Value &&
other.NumberList == NumberList;
}
}
However, if you really want to do it right, this isn't all you should do. You should also override the Equals(object, object) and GetHashCode(object) methods so that, no matter how your calling code is comparing equality (perhaps in a Dictionary or perhaps in some loosely-typed collection), your code and not reference-type equality will be the determining factor:
public class SomeClass : IEquatable<SomeClass>
{
public string Name { get; set; }
public double Value { get; set; }
public int[] NumberList { get; set; }
/// <summary>
/// Explicitly implemented IEquatable method.
/// </summary>
public bool IEquatable<SomeClass>.Equals(SomeClass other)
{
return other.Name == Name &&
other.Value == Value &&
other.NumberList == NumberList;
}
public override bool Equals(object obj)
{
var other = obj as SomeClass;
if (other == null)
return false;
return ((IEquatable<SomeClass>)(this)).Equals(other);
}
public override int GetHashCode()
{
// Determine some consistent way of generating a hash code, such as...
return Name.GetHashCode() ^ Value.GetHashCode() ^ NumberList.GetHashCode();
}
}
Just spent the whole day writing an extension method looping through reflecting over properties of an object with various complex bits of logic to deal with different property type and actually got it close to good, then at 16:55 it dawned on me that if you serialize the two object, you simply need compare the two strings ... duh
So here is a simple serializer extension method that even works on Dictionaries
public static class TExtensions
{
public static string Serialize<T>(this T thisT)
{
var serializer = new DataContractSerializer(thisT.GetType());
using (var writer = new StringWriter())
using (var stm = new XmlTextWriter(writer))
{
serializer.WriteObject(stm, thisT);
return writer.ToString();
}
}
}
Now your test can be as simple as
Asset.AreEqual(objA.Serialise(), objB.Serialise())
Haven't done extensive testing yet, but looks promising and more importantly, simple. Either way still a useful method to have in your utility set right ?
The best answer is to implement IEquatable for your class - it may not be the answer you want to hear, but that's the best way to implement value equivalence in .NET.
Another option would be computing a unique hash of all of the members of your class and then doing value comparisons against those, but that's even more work than writing a comparison function ;)
Since these are objects my guess is that you will have to override the Equals method for objects. Otherwise the Equals method will give you ok only if both objects refering to the same object.
I know this is not the answer you want. But since there is little number of properties in your class you can easily override the method.
I dont understand. Why does comparing my class return false? I thought .Equals checks if each element are == to eachother and me should be each to me in both classes. So whats the problem and how do i get this to be true?
public class MyTest
{
string me;
public MyTest(){}
public MyTest(string v) { me = v; }
public static implicit operator string(MyTest v){return v.me;}
public static implicit operator MyTest(string v) { return new MyTest(v); }
public override string ToString(){ return me;}
}
class Program
{
static void Main(string[] args)
{
Console.WriteLine("{0}", new MyTest("a").Equals( new MyTest("a")));
Console.WriteLine("{0}", new MyTest("a") == new MyTest("a"));
//false, false
Classes are reference types. So, the default 'Equals' method and equality operators, compare the 'references' of the instances (that is, the memory address) in order to verify if 2 instances are equal.
In other words: equality on reference types checks whether the 2 instances are the same instance.
You can override this behaviour, by overriding the Equals and GetHashCode methods. Additionally, you can overload the equality and in-equality operators.
GetHashCode method
Equals
method
== operator
!= operator
You need to override the Equals method because by default, for reference types it compares their references:
public override bool Equals(object obj)
{
var other = obj as MyTest;
if (other == null)
{
return false;
}
return other.me == me;
}
Also when overriding the Equals method it is good practice to override the GetHashCode method as well:
public override int GetHashCode()
{
return (me ?? string.Empty).GetHashCode();
}
Your class MyTest doesn't actually overload the operator Equals. So on the line where you call
Console.WriteLine("{0}", new MyTest("a").Equals( new MyTest("a")));
that is, by default, just going to do the same thing as ==.
You need to explicitly define equality in your class like this:
public override bool Equals(Object obj)
{
return (MyTest)obj.me == this.me;
}
that will actually compare the string inside this object to the one inside the other one. Now both calling .Equals and using the == operator will use your defined equality.
Edit: See #Darin Dimitrov's answer for a more comprehensive solution.
You are not comparing two classes but two different instance of class; the test returns false because you are comparing two different references.
You can override this behaviour, overriding Equals and GetHashCode.
You need to override the Equals method on the MyTest class and implement your custom comparison logic in that method. Without that the default implementation will compare only the references.
See this link - Guidelines for Overloading Equals() and Operator == (C# Programming Guide)
struct compares fields by default; class compares references by default. That doesn't mean you should simply make it a struct, as to avoid boxing etc you would also need to override GetHashCode and Equals anyway (plus ToString) - and by the time you've done that, you could have done it for the existing code.
For example:
public static bool operator ==(MyTest x, MyTest y) {
if(x == null && y == null) return true;
return x != null && y != null && x.me == y.me;
}
public static bool operator !=(MyTest x, MyTest y) {
if (x == null && y == null) return false;
return x == null || y == null || x.me != y.me;
}
public override bool Equals(object obj) {
MyTest other = obj as MyTest;
return other != null && other.me == me;
}
public override int GetHashCode() {
return me == null ? 0 : me.GetHashCode();
}
Equals and '==' for reference types looks if two objects point to the same reference. It doesn´t check the values of the class. You have to override equals (or implement IEquatable) to provide your own logic to compare to instance of a class
You have to override object.Equals (and GetHashCode). If you don't override the default implementation is to return whether they are the same object (and not compare fields).
ehy not use a struct instead (which is a value type and is compared via the sum of the comparisons of its parts
public struct MyTest
{
string me;
public MyTest(string v) { me = v; }
public static implicit operator string(MyTest v) { return v.me; }
public static implicit operator MyTest(string v) { return new MyTest(v); }
public override string ToString() { return me; }
}
As far as I know, by default "==" identify if 2 object is the same; and "Equals" compare their values.