I have a Class Test which has a overriden method for "Equals" method and then I have a TestCollection class which is implemented using ICollection<Test> & IEnumerable<Test> in the Collection I have implemented Remove method which just removes the item from the current TestCollection object.
Whenever I class remove method for the TestCollection object, this internally calls "Equals" method which is overridden at Test class.
For one of my scenario, I do not want this Equals to be called, what are the other ways where I can remove the item from my collection without calling Equals
Below is the sample code for better understanding.
Test Class
public class Test
{
public int Id { get; set; }
private Collection<Test> _entities = new Collection<Test>();
public bool Remove(Test item)
{
return this._entities.Remove(item);
}
public override bool Equals(object obj)
{
Console.WriteLine("Equals inside Test Object");
return true;
}
}
TestCollection class
public class TestCollection : ICollection<Test>, IEnumerable<Test>
{
public TestCollection() : base() { }
private Collection<Test> _entities = new Collection<Test>();
public TestCollection(IList<Test> entityList)
{
this._entities = new Collection<Test>(entityList);
}
public bool Remove(Test item)
{
return this._entities.Remove(item);
}
public override bool Equals(object obj)
{
Console.WriteLine("Equals inside Test Collection Object");
return true;
}
}
I think you are missing the point here. Equals method is implementing the arithmetic relation of equivalence, like having attributes of being reflexive, symmetric and transitive. There are no two distinct ways to say that two objects are equal, you see?
Solution for you is to remove implementation of the Equals method. This method is intended to be overridden if and only if there is exactly one definition of equivalence for a class - like integer equality - there is exactly one way to test whether two integers are equal.
Also, that is the reason why Remove method does not accept an additional parameter such as an IComparer or IEqualityComparer - that wouldn't make sense.
On a related note: Entities should never override Equals. There is no equality relation (in mathematical terms) defined for objects that can change their state over time, and entity is defined as an object with lifetime. The trouble there is that you can pick two versions of the same entity and ask whether they are equal. Well, they are both equal (that is the same entity) and not equal (those are two versions of it). Therefore, Equals method is not the way to check equality of entities.
The short answer is that you cannot.
The way that an item is removed from a list is done by doing an equality check for the item in question on each of the entries in the list.
There may be some way to do it, however, but I doubt it's a good practice, or even desirable code.
You could wrap the list into another list that uses a custom IEqualityComparer implementation. Allow that comparer to have two different modes (pass through to object.Equals, or don't) and switch them before remove (and switch back afterwards).
You could find the index of the item you want to remove (not use its Equal) and call RemoveAt
Related
I've read some stuff about overriding Equal and GetHashcode but do I need it when just having a simple own Equal method like below?
And if I do have to override it:
Why is that? - Should I use Id.GetHashCode() when overriding GetHashCode()?
public class Foo {
public Guid Id { get; } = new Guid();
public bool Equal(Foo other) {
if (other == null) return false;
return Id == other.Id;
}
}
Your code looks like you want to implement IEquatable<Foo> for your object. And MSDN advice to oveeride Object.Equals() method:
If you implement IEquatable, you should also override the base
class implementations of Object.Equals(Object) and GetHashCode so that
their behavior is consistent with that of the IEquatable.Equals
method. If you do override Object.Equals(Object), your overridden
implementation is also called in calls to the static
Equals(System.Object, System.Object) method on your class. In
addition, you should overload the op_Equality and op_Inequality
operators. This ensures that all tests for equality return consistent
results.
Another pointer to your solution can be found here - It explains what all are your options -- please have a read
Typically, you implement value equality when objects of the type are
expected to be added to a collection of some sort, or when their
primary purpose is to store a set of fields or properties. You can
base your definition of value equality on a comparison of all the
fields and properties in the type, or you can base the definition on a
subset.
I'm having some difficulty using Linq's .Except() method when comparing two collections of a custom object.
I've derived my class from Object and implemented overrides for Equals(), GetHashCode(), and the operators == and !=. I've also created a CompareTo() method.
In my two collections, as a debugging experiment, I took the first item from each list (which is a duplicate) and compared them as follows:
itemListA[0].Equals(itemListB[0]); // true
itemListA[0] == itemListB[0]; // true
itemListA[0].CompareTo(itemListB[0]); // 0
In all three cases, the result is as I wanted. However, when I use Linq's Except() method, the duplicate items are not removed:
List<myObject> newList = itemListA.Except(itemListB).ToList();
Learning about how Linq does comparisons, I've discovered various (conflicting?) methods that say I need to inherit from IEquatable<T> or IEqualityComparer<T> etc.
I'm confused because when I inherit from, for example, IEquatable<T>, I am required to provide a new Equals() method with a different signature from what I've already overridden. Do I need to have two such methods with different signatures, or should I no longer derive my class from Object?
My object definition (simplified) looks like this:
public class MyObject : Object
{
public string Name {get; set;}
public DateTime LastUpdate {get; set;}
public int CompareTo(MyObject other)
{
// ...
}
public override bool Equals(object obj)
{
// allows some tolerance on LastUpdate
}
public override int GetHashCode()
{
unchecked
{
int hash = 17;
hash = hash * 23 + Name.GetHashCode();
hash = hash * 23 + LastUpdate.GetHashCode();
return hash;
}
}
// Overrides for operators
}
I noticed that when I inherit from IEquatable<T> I can do so using IEquatable<MyObject> or IEquatable<object>; the requirements for the Equals() signature change when I use one or the other. What is the recommended way?
What I am trying to accomplish:
I want to be able to use Linq (Distinct/Except) as well as the standard equality operators (== and !=) without duplicating code. The comparison should allow two objects to be considered equal if their name is identical and the LastUpdate property is within a number of seconds (user-specified) tolerance.
Edit:
Showing GetHashCode() code.
It doesn't matter whether you override object.Equals and object.GetHashCode, implement IEquatable, or provide an IEqualityComparer. All of them can work, just in slightly different ways.
1) Overriding Equals and GetHashCode from object:
This is the base case, in a sense. It will generally work, assuming you're in a position to edit the type to ensure that the implementation of the two methods are as desired. There's nothing wrong with doing just this in many cases.
2) Implementing IEquatable
The key point here is that you can (and should) implement IEquatable<YourTypeHere>. The key difference between this and #1 is that you have strong typing for the Equals method, rather than just having it use object. This is both better for convenience to the programmer (added type safety) and also means that any value types won't be boxed, so this can improve performance for custom structs. If you do this you should pretty much always do it in addition to #1, not instead of. Having the Equals method here differ in functionality from object.Equals would be...bad. Don't do that.
3) Implementing IEqualityComparer
This is entirely different from the first two. The idea here is that the object isn't getting it's own hash code, or seeing if it's equal to something else. The point of this approach is that an object doesn't know how to properly get it's hash or see if it's equal to something else. Perhaps it's because you don't control the code of the type (i.e. a 3rd party library) and they didn't bother to override the behavior, or perhaps they did override it but you just want your own unique definition of "equality" in this particular context.
In this case you create an entirely separate "comparer" object that takes in two different objects and informs you of whether they are equal or not, or what the hash code of one object is. When using this solution it doesn't matter what the Equals or GetHashCode methods do in the type itself, you won't use it.
Note that all of this is entirely unrelated from the == operator, which is its own beast.
The basic pattern I use for equality in an object is the following. Note that only 2 methods have actual logic specific to the object. The rest is just boiler plate code that feeds into these 2 methods
class MyObject : IEquatable<MyObject> {
public bool Equals(MyObject other) {
if (Object.ReferenceEquals(other, null)) {
return false;
}
// Actual equality logic here
}
public override int GetHashCode() {
// Actual Hashcode logic here
}
public override bool Equals(Object obj) {
return Equals(obj as MyObject);
}
public static bool operator==(MyObject left, MyObject right) {
if (Object.ReferenceEquals(left, null)) {
return Object.ReferenceEquals(right, null);
}
return left.Equals(right);
}
public static bool operator!=(MyObject left, MyObject right) {
return !(left == right);
}
}
If you follow this pattern there is really no need to provide a custom IEqualityComparer<MyObject>. The EqualityComparer<MyObject>.Default will be enough as it will rely on IEquatable<MyObject> in order to perform equality checks
You cannot "allow some tolerance on LastUpdate" and then use a GetHashCode() implementation that uses the strict value of LastUpdate!
Suppose the this instance has LastUpdate at 23:13:13.933, and the obj instance has 23:13:13.932. Then these two might compare equal with your tolerance idea. But if so, their hash codes must be the same number. But that will not happen unless you're extremely extremely lucky, for the DateTime.GetHashCode() should not give the same hash for these two times.
Besides, your Equals method most be a transitive relation mathematically. And "approximately equal to" cannot be made transitive. Its transitive closure is the trivial relation that identifies everything.
I have a list which contains objects but these objests aren't unique in the list. I wrte this code to make unique them in another list:
foreach (CategoryProductsResult categoryProductsResult in categoryProductsResults.Where(categoryProductsResult => !resultSet.Contains(categoryProductsResult)))
{
resultSet.Add(categoryProductsResult);
}
But at the end resultSet is the same with categoryProductsResults.
categoryProductsResult's second row :
resultSet first row:
As you can see resultSet's first row and categoryProductsResult's second row is the same but it adds the second row to resultSet.
Do you have any suggestion?
Contains uses the default comparer which is comparing references since your class does not override Equals and GetHashCode.
class CategoryProductsResult
{
public string Name { get; set; }
// ...
public override bool Equals(object obj)
{
if(obj == null)return false;
CategoryProductsResult other = obj as CategoryProductsResult;
if(other == null)return false;
return other.Name == this.Name;
}
public override int GetHashCode()
{
return Name.GetHashCode();
}
}
Now you can simply use:
resultSet = categoryProductsResults.Distinct().ToList();
List uses the comparer returned by EqualityComparer.Default and according to the documentation for that:
The Default property checks whether type T implements the
System.IEquatable(Of T) interface and, if so, returns an
EqualityComparer(Of T) that uses that implementation. Otherwise, it
returns an EqualityComparer(Of T) that uses the overrides of
Object.Equals and Object.GetHashCode provided by T.
So you can either implement IEquatable on your custom class, or override the Equals (and GetHashCode) methods to do the comparison by the properties you require. Alternatively you could use linq:
bool contains = list.Any(i => i.Id == obj.Id);
Each categoryProductsResult is different to each other. It's like something your can see here. If you want a simpler one and the ProductId is your unique identifier. Just do the code below:
foreach (CategoryProductsResult categoryProductsResult in categoryProductsResults.Where(categoryProductsResult => resultSet.ProductId !=categoryProductsResult.ProductId)
{
resultSet.Add(categoryProductsResult);
}
Reference objects in a list are indexed by their hash code. So, Contains will never find a reference object with the same hash code (unless you override the GetHashCode and Equals implementation in the class.
This SO answer explains.
You need to check if your current item is contained in your target list for each iteration. Currently you check once at the start of the loop, which means none of your items is in the target list.
I think Distinct is already doing what you want, you might want to use this extension instead of your own loop.
I have this class (a partial listing):
class CiscoSwitch
{
private string _SwitchName = string.Empty;
public SwitchName {get {return _SwitchName;} set{_SwitchName=value; }}
}
I have 2 lists of CiscoSwitch objects. I am trying to compare them to pick out the ones that are not duplicates. I only want the duplicates. I tried a Lambda expression but got a compiler error that CiscoSwitch was a non-delgate type.
I am now wondering about something like this - it would allow me to use the List.Except() method (I think):
static class SwitchComparer
{
static bool CompareSwitchNames(CiscoSwitch s1, CiscoSwitch s2)
{
if (sw1.SwitchName == s2.SwitchName) {return true;}
else {return false;}
}
}
// to find the differences
// this is a method of the CiscoSwitchClass
private List<CiscoSwitch> FindDifferences(List<CiscoSwitch> List1, List<CiscoSwitch> List2)
{
return List1.Except(List2, SwitchComparer.CompareSwitchNames();
}
this could also be done with a foreach but I think this way is a lot cleaner, if it is correct. I am also thinking there are other attributes of a CiscoSwitch I might want to compare some day so could add methods to the SwitchComparer class as I need them.
No, just having a single method like that won't help you. You need to implement an IEqualityComparer<CiscoSwitch> to pass to Enumerable.Except - and even then your code would need to be:
return List1.Except(List2, new SwitchComparer()).ToList();
Overriding Equals and GetHashCode within CiscoSwitch will do the trick more naturally though - and ideally you should implement IEquatable<CiscoSwitch> too.
However, it's worth noting that mutable types like this don't play terribly nicely with things like Dictionary<,> - if you change an object in a way which affects its hash code after you've inserted it as a key into the dictionary, you won't be able to get at it again. Consider making the type immutable if you can.
A couple of other points to note:
Any time you write:
if (condition)
{
return true;
}
else
{
return false;
}
you should instead write the far simpler:
return condition;
So your CompareSwitchNames method would be:
static bool CompareSwitchNames(CiscoSwitch s1, CiscoSwitch s2)
{
return s1.SwitchName == s2.SwitchName;
}
Your parameter names for FindDifferences should follow .NET naming conventions (e.g. list1 and list2)
Using Except will only find you the elements in the first list which aren't in the second list; if you need to find the symmetric difference, consider using HashSet<T> explicitly.
EDIT: If you wanted to have multiple ways of comparing, you could have something like:
public static class SwitchComparers
{
public static readonly IEqualityComparer<CiscoSwitch> ByName =
new ByNameComparer();
public static readonly IEqualityComparer<CiscoSwitch> ByCost =
new ByCostComparer();
private sealed class ByNameComparer : IEqualityComparer<CiscoSwitch>
{
// Implementation
}
private sealed class ByCostComparer : IEqualityComparer<CiscoSwitch>
{
// Implementation
}
}
You could implement a custom comparer:
public class CiscoSwitchComparer : IEqualityComparer<CiscoSwitch>
{
public bool Equals(CiscoSwitch x, CiscoSwitch y)
{
return x.SwitchName.Equals(y.SwitchName));
}
public int GetHashCode(CiscoSwitch obj)
{
return obj.SwitchName.GetHashCode();
}
}
And then:
private List<CiscoSwitch> FindDifferences(List<CiscoSwitch> List1, List<CiscoSwitch> List2)
{
return List1.Concat(List2)
.Distinct(new CiscoSwitchComparer())
.ToList();
}
You should implement IEqualityComparer[T] interface.
This interface allows the implementation of customized equality
comparison for collections. That is, you can create your own
definition of equality for type T, and specify that this definition be
used with a collection type that accepts the IEqualityComparer
generic interface. In the .NET Framework, constructors of the
Dictionary generic collection type accept this
interface.
A default implementation of this interface is provided by the Default
property of the EqualityComparer generic class. The StringComparer
class implements IEqualityComparer of type String.
This interface supports only equality comparisons. Customization of
comparisons for sorting and ordering is provided by the IComparer
generic interface.
We recommend that you derive from the EqualityComparer class
instead of implementing the IEqualityComparer interface, because
the EqualityComparer class tests for equality using the
IEquatable.Equals method instead of the Object.Equals method. This
is consistent with the Contains, IndexOf, LastIndexOf, and Remove
methods of the Dictionary class and other generic
collections.
class infoContact
{
private string contacts_first_nameField;
private string contacts_middle_nameField;
private string contacts_last_nameField;
private Phonenumber[] phone_numbersField;
private Emailaddress[] emailField;
}
I have a List<infoContact> The list contains almost 7000 which I get from some other program. In the list out of 7000, 6500 are duplicates. I am looking for a way how to eliminate duplicates.
A infoContact is duplicate if first_name, last_name, emailaddresses, phone numbers are same.
I thought of using a HashSet<infoContact> and override getHashCode() of infoContact.
I am just curious to know if that is the best way to do. If this is not a good way what is the better way?
You can use the Distinct extension method that takes an IEqualityComparer<T>. Just write a class that implements that interface, and does the comparison, and then you can just do something like this:
var filteredList = oldList.Distinct(new InfoContactComparer());
override an equals method with the parametres you want so you can compare objects through equals
i created a remove deducted items from list class before here is the key for it ,
List<string> list = new List<string>();
foreach (string line in File.ReadAllLines(somefile.txt))
{
if (!list.Contains(line))
{
list.Add(line);
}
}
Firstly think of extracting the unique values. You could use the Distinct() Linq method with a comparer like:
public class infoContactComparer : IEqualityComparer<infoContact>
{
public bool Equals(infoContact x, infoContact y)
{
return x.contacts_first_nameField == y.contacts_first_nameField
&& x.contacts_last_nameField == y.contacts_last_nameField
&& ...
}
public int GetHashCode(infoContact obj)
{
return obj.contacts_first_nameField.GetHashCode();
}
}
Two options: override GetHashCode and Equals if you control the source of infoContact and your overrides will be true for any particular use of the class.
Otherwise, define a class implementing IEqualityComparer<infoContact>, which also allows you to define proper Equals and GetHashCode methods, and then pass an instance of this into a HashSet<infoContact> constructor or into a listOfContacts.Distinct method call (using Linq).
Note: your question seems to be based on the idea that GetHashCode should determine equality or uniqueness. It shouldn't! It's part of the tool that allows a HashSet to do its job, but it is not required to return unique values for unequal instances. The values should be well distributed, but they can ultimately overlap.
In short, two equal instances should have the same hash code, but two instances sharing the same hash code are not necessarily equal. For more on guidelines for GetHashCode, please visit this blog.
The right way is to ovveride the equals method!
In this way, when you add new element in the list, the element don't will be added!
Implement your class infoContact as a derivate of IEquatable<infoContact>:
class InfoContact : IEquatable<InfoContact> {
string contacts_first_nameField;
string contacts_last_nameField;
object[] phone_numbersField;
object[] emailField;
// other fields
public bool Equals(InfoContact other) {
return contacts_first_nameField.Equals(other.contacts_first_nameField)
&& contacts_last_nameField.Equals(other.contacts_last_nameField)
&& phone_numbersField.Equals(other.phone_numbersField)
&& emailField.Equals(other.emailField);
}
}
and use Linqs Enumerable.Distinct method in order to filter the duplicates:
var infoContacts = GetInfoContacts().Distinct();