[RESOLVED] Dictionary ContainsKey custom comparison [RESOLVED] Dictionary ContainsKey custom comparison

Iequalitycomparer equals not called dating, popular tags

Conclusion There is really nothing very special about this code. It is used by many standard methods that iequalitycomparer equals not called dating out value comparisons. Now to explain why both methods are necessary, consider this example: The method is loosely typed since it takes an object as a parameter.

GetHashCode is intended as a fast but rough estimate of equality, so many operations potentially involving large numbers of comparisons start by checking this result instead of Equals, and only use Equals when necessary.

For example, take a dictionary that is indexed by a case-insensitive key, and say your implementation of GetHashCode returns the value of the first character.

Your Answer

Yes, it will cause a Dictionary to effectively be a linked list. Take the following for example. As we only care about the two ID values being the same, the method can return the result of a comparison of the two globally unique identifiers GUIDs.

Distinct customComparer ; The best part about this solution is that it will work for any property and any type, pociatok online dating instead of writing a custom IEqualityComparer, we can just reuse our generic PropertyComparer.

Custom Equals function not being called when using IEqualityComparer

It just computes the hashcode and jumps to that location. Distinct customComparer ; This will cause the Distinct method to compare our objects based our custom Equals implementation which uses the EmailAddress property of the Contact.

Rather than checking that the references for two Customer instances match, we will compare a unique ID property. IEqualityComparer in the namespace System. GetHashCode ; if obj. Second, notice the last printed line - it shows that our custom Equals method is actually called.


I have implement both GetHashCode and Equals methods. This method accepts two values or objects to be compared. The Equals method is a better test of equality, but cannot be used to map an object into an address space.

With a better hash code, Equals is called fewer times. This means that the comparison of the first and third customers returns false and the two objects' hash codes differ.

I hope this helps.

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So we need a way to provide a key selector to our ActorComparer. In more formal terms, GetHashCode must always return the same value for two objects that compare equal.

So our second George Clooney is in fact an independent object since it was created independently and its reference differs from the reference of the first one - therefore it appears in the output twice: WriteLine dict["a"] ; then the second line will likely fail with a KeyNotFoundException even though the keys "A" and "a" are equal.

If you then do something like: GetHashCode method to return a constant We will extend our IEqualityComparer to use reflection to extract the value of a specified property, rather than restricting our class to one property.

Also, there is an easy way to test how selective your hash numbers are. Suppose, we do something with actors of a movie: You can simply use a hashing function, with no new strings.

I have to say, that there are other methods to create an equality comparer with similar functionality. The parameter-less Distinct method will compare a Contact object based on the default equality comparer, but there is no quick way to specify that I want to compare them based on email address.

GetHashCode will remove this confusion. The difference between this and IComparer is IComparable will be implemented on the class that contains your field values, while IComparable derived classes are used for custom sorting.

C# - IEqualityComparer GetHashCode being called but Equals not - Stack Overflow

After inspecting multiple SO questions and answers related to the topic as well as some other articles I have both found the solution that fitted my needs for the moment and got some understanding of the way IEqualityComparer is actually used by LINQ operators.

It is a good idea to use a static instance of your IEqualityComparer. LastName ; and get a set of actors with unique last names. This class makes it a little easier, I hope you find it useful.

A Generic IEqualityComparer for Linq Distinct() - CodeProject

Some unusual cases, not shown, may not work correctly such as trailing hyphens. I thought for sure the latter would work, but no. Simple, it uses Object. So the Equals operator may not be the best solution. NET form that takes input from a user.

If you implement IEquatable you still must override Object’s Equals and GetHashCode

Because this first character is monotonous, it isn't useful for hashing because its distribution is poor. Otherwise, classes that depend on this condition will not work correctly. Add the code shown below to the class: Here's an example with sorting items in N2: To implement that interface, it must define Equals and GetHashCode.

Moreover, after going to Google and StackOverflow I have noticed that I'm not the only one to face difficulties with it. So "A" and "a" are equal but have different hash values 65 and And the problem we want to solve is that if we have a list of Contact objects where some contacts have the same email address, we want to get just a distinct list of email addresses by doing something like this: Equals y is false, so there is no reason to call Equals.

This would solve my problem at once, because it is this same method that LINQ uses when asked to extract distinct objects and perform other equality related operations see Set operators in this article.

Custom Equals function not being called when using IEqualityComparer

Confusing programmers is rarely a good idea. The resulting class is both easy to use and highly customizable, because its operation is fully defined by the key selector function provided by user. This is the custom IEqualityComparer.