I'm getting really confused by entity properties and what they mean. I understand that get and set allow the application to interact with protected and private properties, but what about add and remove?
When running the command
php app/console doctrine:generate:entities bundle:entity
Sometimes it will generate getters and setters and other times it will generate add and remove properties (and usually a get method as well). I've noticed that it also sometimes depends on the relationship with other entities (i.e. OnetoMany), but not always :-S
Nowhere in the Symfony documentation that I can find is this addressed, and it's causing me to see the message "Neither the property "x" nor one of the methods exist and have public access" way too often. Can anybody provide a succinct explanation of this?
add and remove are used to deal with collections. If for example your Entity (Let's say A) contains a collection of B elements, then the command will provide an addB() and a removeB() public methods to help you add and remove elements from your collection. It'll also provide a getter which returns the whole collection.
The command generates methods based on the type of attributes you're working with (ArrayCollection, string, ...)
For xxxToMany associations, Doctrine will generate a "adder" and a "remover" instead of a plain setter. The idea is to easily add and/or remove a single object from the collection without needing to pass around the entire collection everytime to the setter.
Note though that these generated methods are an implementation detail you are free to revise. If you prefer a single setter method for example, feel free to implement that one yourself.
I personally don't rely on the accessor generation of Doctrine anymore. Doing it manually allows greater control of your entity's API, and is also quite easy in an IDE like Netbeans or PHPStorm.
I'm working on a Symfony 2.3 Project that utilizes the Doctrine 2 ORM. As is to be expected functionality is split and grouped into mostly independent bundles to allow for code-reuse in other projects.
I have a UserBundle and a ContactInfoBundle. The contact info is split off because other entities could have contact information associated, however it is not inconcievable that a system may be built where users do not require said contact information. As such I'd very much prefer these two do not share any hard links.
However, creating the association mapping from the User entity to the ContactInfo entity creates a hard dependency on the ContactInfoBundle, as soon as the bundle is disabled Doctrine throws errors that ContactInfo is not within any of its registered namespaces.
My investigations have uncovered several strategies that are supposed to counter this, but none of them seem fully functional:
Doctrine 2's ResolveTargetEntityListener
This works, as long as the interface is actually replaced at runtime. Because the bundle dependency is supposed to be optional, it could very well be that there is NO concrete implementation available (i.e. contactInfoBundle is not loaded)
If there is no target entity, the entire configuration collapses onto itself because the placeholder object is not an entity (and is not within the /Entity namespace), one could theoretically link them to a Mock entity that doesn't really do anything. But this entity then gets its own table (and it gets queried), opening up a whole new can of worms.
Inverse the relation
For the ContactInfo it makes the most sense for User to be the owning side, making ContactInfo the owning side successfully sidesteps the optional part of the dependency as long as only two bundles are involved. However, as soon as a third (also optional) bundle desires an (optional) link with ContactInfo, making ContactInfo the owning side creates a hard dependency from ContactInfo on the third bundle.
Making User the owning side being logical is a specific situation. The issue however is universal where entity A contains B, and C contains B.
Use single-table inheritance
As long as the optional bundles are the only one that interacts with the newly added association, giving each bundle their own User entity that extends UserBundle\Entities\User could work. However having multiple bundles that extend a single entity rapidly causes this to become a bit of a mess. You can never be completely sure what functions are available where, and having controllers somehow respond to bundles being on and/or off (as is supported by Symfony 2's DependencyInjection mechanics) becomes largely impossible.
Any ideas or insights in how to circumvent this problem are welcome. After a couple of days of running into brick walls I'm fresh out of ideas. One would expect Symfony to have some method of doing this, but the documentation only comes up with the ResolveTargetEntityListener, which is sub-optimal.
I have finally managed to rig up a solution to this problem which would be suited for my project. As an introduction, I should say that the bundles in my architecture are laid out "star-like". By that I mean that I have one core or base bundle which serves as the base dependency module and is present in all the projects. All other bundles can rely on it and only it. There are no direct dependencies between my other bundles. I'm quite certain that this proposed solution would work in this case because of the simplicity in the architecture. I should also say that I fear there could be debugging issues involved with this method, but it could be made so that it is easily switched on or off, depending on a configuration setting, for instance.
The basic idea is to rig up my own ResolveTargetEntityListener, which would skip relating the entities if the related entity is missing. This would allow the process of execution to continue if there is a class bound to the interface missing. There's probably no need to emphasize the implication of the typo in the configuration - the class won't be found and this can produce a hard-to-debug error. That's why I'd advise to turn it off during the development phase and then turn it back on in the production. This way, all the possible errors will be pointed out by the Doctrine.
Implementation
The implementation consists of reusing the ResolveTargetEntityListener's code and putting some additional code inside the remapAssociation method. This is my final implementation:
<?php
namespace Name\MyBundle\Core;
use Doctrine\ORM\Event\LoadClassMetadataEventArgs;
use Doctrine\ORM\Mapping\ClassMetadata;
class ResolveTargetEntityListener
{
/**
* #var array
*/
private $resolveTargetEntities = array();
/**
* Add a target-entity class name to resolve to a new class name.
*
* #param string $originalEntity
* #param string $newEntity
* #param array $mapping
* #return void
*/
public function addResolveTargetEntity($originalEntity, $newEntity, array $mapping)
{
$mapping['targetEntity'] = ltrim($newEntity, "\\");
$this->resolveTargetEntities[ltrim($originalEntity, "\\")] = $mapping;
}
/**
* Process event and resolve new target entity names.
*
* #param LoadClassMetadataEventArgs $args
* #return void
*/
public function loadClassMetadata(LoadClassMetadataEventArgs $args)
{
$cm = $args->getClassMetadata();
foreach ($cm->associationMappings as $mapping) {
if (isset($this->resolveTargetEntities[$mapping['targetEntity']])) {
$this->remapAssociation($cm, $mapping);
}
}
}
private function remapAssociation($classMetadata, $mapping)
{
$newMapping = $this->resolveTargetEntities[$mapping['targetEntity']];
$newMapping = array_replace_recursive($mapping, $newMapping);
$newMapping['fieldName'] = $mapping['fieldName'];
unset($classMetadata->associationMappings[$mapping['fieldName']]);
// Silently skip mapping the association if the related entity is missing
if (class_exists($newMapping['targetEntity']) === false)
{
return;
}
switch ($mapping['type'])
{
case ClassMetadata::MANY_TO_MANY:
$classMetadata->mapManyToMany($newMapping);
break;
case ClassMetadata::MANY_TO_ONE:
$classMetadata->mapManyToOne($newMapping);
break;
case ClassMetadata::ONE_TO_MANY:
$classMetadata->mapOneToMany($newMapping);
break;
case ClassMetadata::ONE_TO_ONE:
$classMetadata->mapOneToOne($newMapping);
break;
}
}
}
Note the silent return before the switch statement which is used to map the entity relations. If the related entity's class does not exist, the method just returns, rather than executing faulty mapping and producing the error. This also has the implication of a field missing (if it's not a many-to-many relation). The foreign key in that case will just be missing inside the database, but as it exists in the entity class, all the code is still valid (you won't get a missing method error if accidentally calling the foreign key's getter or setter).
Putting it to use
To be able to use this code, you just have to change one parameter. You should put this updated parameter to a services file which will always be loaded or some other similar place. The goal is to have it at a place that will always be used, no matter what bundles you are going to use. I've put it in my base bundle services file:
doctrine.orm.listeners.resolve_target_entity.class: Name\MyBundle\Core\ResolveTargetEntityListener
This will redirect the original ResolveTargetEntityListener to your version. You should also clear and warm your cache after putting it in place, just in case.
Testing
I have done only a couple of simple tests which have proven that this approach might work as expected. I intend to use this method frequently in the next couple of weeks and will be following up on it if the need arises. I also hope to get some useful feedback from other people who decide to give it a go.
You could create loose dependencies between ContactInfo and any other entities by having an extra field in ContactInfo to differentiate entities (e.g. $entityName). Another required field would be $objectId to point to objects of specific entities. So in order to link User with ContactInfo, you don't need any actual relational mappings.
If you want to create a ContactInfo for a $user object, you need to manually instantiate it and simply setEntityName(get_class($user)), setObjectId($user->getId()). To retrieve user ContactInfo, or that of any object, you can create a generic function that accepts $object. It could simply just return ...findBy(array('entityName' => get_class($user), 'objectId' => $object->getId());
With this approach, you could still create User form with ContactInfo (embed ContactInfo into User). Though after you process the form, you will need to persist User first and flush, and then persist ContactInfo. Of course this is only necessary for newly created User objects, just so to get user id. Put all persist/flush in a transaction if you're concerned about data integrity.
After "finish" studying Symfony2, I start to take a look to Sonata Admin and User bundles since, those bundles, could help me in some future works.
On the firsts, all seems to me terribly complicated and difficult to understd. But with few hours of architecture study, applied to real files,classes and so on, I suppose that I've understood perfectly what's the essence of that bundle and how this works.
My only "black hole" into whole thing understand is, where can I find the parameter used into sonata.user.admin.user, called sonata.user.admin.user.entity? As far I know, that parameter simply point to user class that is responsable for read/write operation from and to database.
I ask this because, after dozens of minutes spent looking, the only place where I found those information are
./app/cache/prod/appProdProjectContainer.php
./app/cache/dev/appDevDebugProjectContainer.xml
./app/cache/dev/appDevDebugProjectContainer.php
Is possible that this variable is defined only there?
If yes, because is placed into cache folder? Is loading time faster?
You should also look through Extensions in DependencyInjection folders - sometimes arguments are defined in yaml in one way, but after loading with concatenation they are transformed into another:
If in your parameters you have something like:
your_user: 'Somename'
And in DependencyInjection it is parsed as follows:
foreach($config as $key => $value) {
$container->setParameter('your.prepend.'.$key, $value);
}
Then in the end you'll have your parameter compiled with index your.prepend.your_user.
I guess, the same logic is used in mentioned bundles.
UPDATE:
Example of usage of described logic
I come from low level languages - C++ is the highest level I program in.
Recently I came across Reflection, and I just cannot fathom how it could be used without code smells.
The idea of inspecting a class/method/function during runtime, in my opinion, points to a flaw in design - I think most problems Reflection (tries to) solve could be used with either Polymorphism or proper use of inheritance.
Am I wrong? Do I misunderstand the concept and utility of Reflection?
I am looking for a good explanation of when to utilize Reflection where other solutions will fail or be too cumbersome to implement as well as when NOT to use it.
Please enlighten this low-level lubber.
Reflection is most commonly used to circumvent the static type system, however it also has some interesting use cases:
Let's write an ORM!
If you're familiar with NHibernate or most other ORMs, you write classes which map to tables in your database, something like this:
// used to hook into the ORMs innards
public class ActiveRecordBase
{
public void Save();
}
public class User : ActiveRecordBase
{
public int ID { get; set; }
public string UserName { get; set; }
// ...
}
How do you think the Save() method is written? Well, in most ORMs, the Save method doesn't know what fields are in derived classes, but it can access them using reflection.
Its wholly possible to have the same functionality in a type-safe manner, simply by requiring a user to override a method to copy fields into a datarow object, but that would result in lots of boilerplate code and bloat.
Stubs!
Rhino Mocks is a mocking framework. You pass an interface type into a method, and behind the scenes the framework will dynamically construct and instantiate a mock object implementing the interface.
Sure, a programmer could write the boilerplate code for the mock object by hand, but why would she want to if the framework will do it for her?
Metadata!
We can decorate methods with attributes (metadata), which can serve a variety of purposes:
[FilePermission(Context.AllAccess)] // writes things to a file
[Logging(LogMethod.None)] // logger doesn't log this method
[MethodAccessSecurity(Role="Admin")] // user must be in "Admin" group to invoke method
[Validation(ValidationType.NotNull, "reportName")] // throws exception if reportName is null
public void RunDailyReports(string reportName) { ... }
You need to reflect over the method to inspect the attributes. Most AOP frameworks for .NET use attributes for policy injection.
Sure, you can write the same sort of code inline, but this style is more declarative.
Let's make a dependency framework!
Many IoC containers require some degree of reflection to run properly. For example:
public class FileValidator
{
public FileValidator(ILogger logger) { ... }
}
// client code
var validator = IoC.Resolve<FileValidator>();
Our IoC container will instantiate a file validator and pass an appropriate implementation of ILogger into the constructor. Which implementation? That depends on how its implemented.
Let's say that I gave the name of the assembly and class in a configuration file. The language needs to read name of the class as a string and use reflection to instantiate it.
Unless we know the implementation at compile time, there is no type-safe way to instantiate a class based on its name.
Late Binding / Duck Typing
There are all kinds of reasons why you'd want to read the properties of an object at runtime. I'd pick logging as the simplest use case -- let say you were writing a logger which accepts any object and spits out all of its properties to a file.
public static void Log(string msg, object state) { ... }
You could override the Log method for all possible static types, or you could just use reflection to read the properties instead.
Some languages like OCaml and Scala support statically-checked duck-typing (called structural typing), but sometimes you just don't have compile-time knowledge of an objects interface.
Or as Java programmers know, sometimes the type system will get your way and require you to write all kinds of boilerplate code. There's a well-known article which describes how many design patterns are simplified with dynamic typing.
Occasionally circumventing the type system allows you to refactor your code down much further than is possible with static types, resulting in a little bit cleaner code (preferably hidden behind a programmer friendly API :) ). Many modern static languages are adopting the golden rule "static typing where possible, dynamic typing where necessary", allowing users to switch between static and dynamic code.
Projects such as hibernate (O/R mapping) and StructureMap (dependency injection) would be impossible without Reflection. How would one solve these with polymorphism alone?
What makes these problems so difficult to solve any other way is that the libraries don't directly know anything about your class hierarchy - they can't. And yet they need to know the structure of your classes in order to - for example - map an arbitrary row of data from a database to a property in your class using only the name of the field and the name of your property.
Reflection is particularly useful for mapping problems. The idea of convention over code is becoming more and more popular and you need some type of Reflection to do it.
In .NET 3.5+ you have an alternative, which is to use expression trees. These are strongly-typed, and many problems that were classically solved using Reflection have been re-implemented using lambdas and expression trees (see Fluent NHibernate, Ninject). But keep in mind that not every language supports these kinds of constructs; when they're not available, you're basically stuck with Reflection.
In a way (and I hope I'm not ruffling too many feathers with this), Reflection is very often used as a workaround/hack in Object-Oriented languages for features that come for free in Functional languages. As functional languages become more popular, and/or more OO languages start implementing more functional features (like C#), we will most likely start to see Reflection used less and less. But I suspect it will always still be around, for more conventional applications like plugins (as one of the other responders helpfully pointed out).
Actually, you are already using a reflective system everyday: your computer.
Sure, instead of classes, methods and objects, it has programs and files. Programs create and modify files just like methods create and modify objects. But then programs are files themselves, and some programs even inspect or create other programs!
So, why is it so OK for a Linux install to be reflexive that nobody even thinks about it, and scary for OO programs?
I've seen good usages with custom attributes. Such as a database framework.
[DatabaseColumn("UserID")]
[PrimaryKey]
public Int32 UserID { get; set; }
Reflection can then be used to get further information about these fields. I'm pretty sure LINQ To SQL does something similar...
Other examples include test frameworks...
[Test]
public void TestSomething()
{
Assert.AreEqual(5, 10);
}
Without reflection you often have to repeat yourself a lot.
Consider these scenarios:
Run a set of methods e.g. the testXXX() methods in a test case
Generate a list of properties in a gui builder
Make your classes scriptable
Implement a serialization scheme
You can't typically do these things in C/C++ without repeating the whole list of affected methods and properties somewhere else in the code.
In fact C/C++ programmers often use an Interface description language to expose interfaces at runtime (providing a form of reflection).
Judicious use of reflection and annotations combined with well defined coding conventions can avoids rampant code repetition and increase maintainability.
I think that reflection is one of these mechanisms that are powerful but can be easily abused. You're given the tools to become a "power user" for very specific purposes, but it is not meant to replace proper object oriented design (just as object oriented design is not a solution for everything) or to be used lightly.
Because of the way Java is structured, you are already paying the price of representing your class hierarchy in memory at runtime (compare to C++ where you don't pay any costs unless you use things like virtual methods). There is therefore no cost rationale for blocking it fully.
Reflection is useful for things like serialization - things like Hibernate or digester can use it to determine how to best store objects automatically. Similarly, the JavaBeans model is based on names of methods (a questionable decision, I admit), but you need to be able to inspect what properties are available to build things like visual editors. In more recent versions of Java, reflections is what makes annotations useful - you can write tools and do metaprogramming using these entities that exist in the source code but can be accessible at runtime.
It is possible to go through an entire career as a Java programmer and never have to use reflection because the problems that you deal with don't require it. On the other hand, for certain problems, it is quite necessary.
As mentioned above, reflection is mostly used to implement code that needs to deal with arbitrary objects. ORM mappers, for instance, need to instantiate objects from user-defined classes and fill them with values from database rows. The simplest way to achieve this is through reflection.
Actually, you are partially right, reflection is often a code smell. Most of the time you work with your classes and do not need reflection- if you know your types, you are probably sacrificing type safety, performance, readability and everything that's good in this world, needlessly. However, if you are writing libraries, frameworks or generic utilities, you will probably run into situations best handled with reflection.
This is in Java, which is what I'm familiar with. Other languages offer stuff that can be used to achieve the same goals, but in Java, reflection has clear applications for which it's the best (and sometimes, only) solution.
Unit testing software and frameworks like NUnit use reflection to get a list of tests to execute and executes them. They find all the test suites in a module/assembly/binary (in C# these are represented by classes) and all the tests in those suites (in C# these are methods in a class). NUnit also allows you to mark a test with an expected exception in case you're testing for exception contracts.
Without reflection, you'd need to specify somehow what test suites are available and what tests are available in each suite. Also, things like exceptions would need to be tested manually. C++ unit testing frameworks I've seen have used macros to do this, but some things are still manual and this design is restrictive.
Paul Graham has a great essay that may say it best:
Programs that write programs? When
would you ever want to do that? Not
very often, if you think in Cobol. All
the time, if you think in Lisp. It
would be convenient here if I could
give an example of a powerful macro,
and say there! how about that? But if
I did, it would just look like
gibberish to someone who didn't know
Lisp; there isn't room here to explain
everything you'd need to know to
understand what it meant. In Ansi
Common Lisp I tried to move things
along as fast as I could, and even so
I didn't get to macros until page 160.
concluding with . . .
During the years we worked on Viaweb I
read a lot of job descriptions. A new
competitor seemed to emerge out of the
woodwork every month or so. The first
thing I would do, after checking to
see if they had a live online demo,
was look at their job listings. After
a couple years of this I could tell
which companies to worry about and
which not to. The more of an IT flavor
the job descriptions had, the less
dangerous the company was. The safest
kind were the ones that wanted Oracle
experience. You never had to worry
about those. You were also safe if
they said they wanted C++ or Java
developers. If they wanted Perl or
Python programmers, that would be a
bit frightening-- that's starting to
sound like a company where the
technical side, at least, is run by
real hackers. If I had ever seen a job
posting looking for Lisp hackers, I
would have been really worried.
It is all about rapid development.
var myObject = // Something with quite a few properties.
var props = new Dictionary<string, object>();
foreach (var prop in myObject.GetType().GetProperties())
{
props.Add(prop.Name, prop.GetValue(myObject, null);
}
Plugins are a great example.
Tools are another example - inspector tools, build tools, etc.
I will give an example of a c# solution i was given when i started learning.
It contained classes marked with the [Exercise] attribute, each class contained methods which were not implemented (throwing NotImplementedException). The solution also had unit tests which all failed.
The goal was to implement all the methods and pass all the unit tests.
The solution also had a user interface which it would read all class marked with Excercise, and use reflection to generate a user interface.
We were later asked to implement our own methods, and later still to understand how the user interface 'magically' was changed to include all the new methods we implemented.
Extremely useful, but often not well understood.
The idea behind this was to be able to query any GUI objects properties, to provide them in a GUI to get customized and preconfigured. Now it's uses have been extended and proved to be feasible.
EDIT: spelling
It's very useful for dependency injection. You can explore loaded assemblies types implementing a given interface with a given attribute. Combined with proper configuration files, it proves to be a very powerful and clean way of adding new inherited classes without modifying the client code.
Also, if you are doing an editor that doesn't really care about the underlying model but rather on how the objects are structured directly, ala System.Forms.PropertyGrid)
Without reflection no plugin architecture will work!
Very simple example in Python. Suppose you have a class that have 3 methods:
class SomeClass(object):
def methodA(self):
# some code
def methodB(self):
# some code
def methodC(self):
# some code
Now, in some other class you want to decorate those methods with some additional behaviour (i.e. you want that class to mimic SomeClass, but with an additional functionality).
This is as simple as:
class SomeOtherClass(object):
def __getattr__(self, attr_name):
# do something nice and then call method that caller requested
getattr(self.someclass_instance, attr_name)()
With reflection, you can write a small amount of domain independent code that doesn't need to change often versus writing a lot more domain dependent code that needs to change more frequently (such as when properties are added/removed). With established conventions in your project, you can perform common functions based on the presence of certain properties, attributes, etc. Data transformation of objects between different domains is one example where reflection really comes in handy.
Or a more simple example within a domain, where you want to transform data from the database to data objects without needing to modify the transformation code when properties change, so long as conventions are maintained (in this case matching property names and a specific attribute):
///--------------------------------------------------------------------------------
/// <summary>Transform data from the input data reader into the output object. Each
/// element to be transformed must have the DataElement attribute associated with
/// it.</summary>
///
/// <param name="inputReader">The database reader with the input data.</param>
/// <param name="outputObject">The output object to be populated with the input data.</param>
/// <param name="filterElements">Data elements to filter out of the transformation.</param>
///--------------------------------------------------------------------------------
public static void TransformDataFromDbReader(DbDataReader inputReader, IDataObject outputObject, NameObjectCollection filterElements)
{
try
{
// add all public properties with the DataElement attribute to the output object
foreach (PropertyInfo loopInfo in outputObject.GetType().GetProperties())
{
foreach (object loopAttribute in loopInfo.GetCustomAttributes(true))
{
if (loopAttribute is DataElementAttribute)
{
// get name of property to transform
string transformName = DataHelper.GetString(((DataElementAttribute)loopAttribute).ElementName).Trim().ToLower();
if (transformName == String.Empty)
{
transformName = loopInfo.Name.Trim().ToLower();
}
// do transform if not in filter field list
if (filterElements == null || DataHelper.GetString(filterElements[transformName]) == String.Empty)
{
for (int i = 0; i < inputReader.FieldCount; i++)
{
if (inputReader.GetName(i).Trim().ToLower() == transformName)
{
// set value, based on system type
loopInfo.SetValue(outputObject, DataHelper.GetValueFromSystemType(inputReader[i], loopInfo.PropertyType.UnderlyingSystemType.FullName, false), null);
}
}
}
}
}
}
// add all fields with the DataElement attribute to the output object
foreach (FieldInfo loopInfo in outputObject.GetType().GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance))
{
foreach (object loopAttribute in loopInfo.GetCustomAttributes(true))
{
if (loopAttribute is DataElementAttribute)
{
// get name of field to transform
string transformName = DataHelper.GetString(((DataElementAttribute)loopAttribute).ElementName).Trim().ToLower();
if (transformName == String.Empty)
{
transformName = loopInfo.Name.Trim().ToLower();
}
// do transform if not in filter field list
if (filterElements == null || DataHelper.GetString(filterElements[transformName]) == String.Empty)
{
for (int i = 0; i < inputReader.FieldCount; i++)
{
if (inputReader.GetName(i).Trim().ToLower() == transformName)
{
// set value, based on system type
loopInfo.SetValue(outputObject, DataHelper.GetValueFromSystemType(inputReader[i], loopInfo.FieldType.UnderlyingSystemType.FullName, false));
}
}
}
}
}
}
}
catch (Exception ex)
{
bool reThrow = ExceptionHandler.HandleException(ex);
if (reThrow) throw;
}
}
One usage not yet mentioned: while reflection is generally thought of as "slow", it's possible to use Reflection to improve the efficiency of code which uses interfaces like IEquatable<T> when they exist, and uses other means of checking equality when they do not. In the absence of reflection, code that wanted to test whether two objects were equal would have to either use Object.Equals(Object) or else check at run-time whether an object implemented IEquatable<T> and, if so, cast the object to that interface. In either case, if the type of thing being compared was a value type, at least one boxing operation would be required. Using Reflection makes it possible to have a class EqualityComparer<T> automatically construct a type-specific implementation of IEqualityComparer<T> for any particular type T, with that implementation using IEquatable<T> if it is defined, or using Object.Equals(Object) if it is not. The first time one uses EqualityComparer<T>.Default for any particular type T, the system will have to go through more work than would be required to test, once, whether a particular type implements IEquatable<T>. On the other hand, once that work is done, no more run-time type checking will be required since the system will have produced a custom-built implementation of EqualityComparer<T> for the type in question.