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Function overloading by return type?
(14 answers)
Closed 9 years ago.
I am want to dig in that whether it is an ambiguity or an extra feature that is provided:
public class Foo
{
public int Bar(){
//code
}
public string Bar(int a){
//code
}
}
Any one having any experience with this, overloading on return type with different parameters should be a bad practice, is it?
But if the overloading was done on the basis of return type then why this is not working for.
public class Foo
{
public int Bar(int a){
//code
}
public string Bar(int a){
//code
}
}
As it will be unable to decide which function to call 1st or second, if we call obj.Bar(); , it should end in error do any one have any idea about it why it allows first code snippet to run.
The C# specification (section 10.6) states that overloaded members may not differ by only return type and as per http://msdn.microsoft.com/en-us/library/ms229029.aspx
As per your question regarding creating parameters simply to support differing return types? I personally believe that is a terrible solution to the problem. Code maintenance will become difficult and unused parameters are a definite code smell. Does the method really need to be overloaded in that case? Or does it belong in that class? Should something else be created to convert from one return type to another? All things you should ask to derive a more idiomatic solution.
This is logically impossible. Consider the following call:
object o = Bar(42);
or even
var o = Bar(42);
How would the compiler know which method to call?
Edit:
Now that I understand what you're actually asking, I think overloading by meaningless parameters is bad practice, and diminished readability, it is much preferable to distinguish by method name:
string BarToStr()
{
}
int BarToInt()
{
}
Others already explained the situation. I only would like to add this: You can do what you have in mind by using a generic type parameter:
public T Bar<T>(int a) {
// code
}
And call it like this:
int i = Bar<int>(42);
string s = Bar<string>(42);
The problem is that it is often difficult to do something meaningful with a generic type, e.g. you cannot apply arithmetic operations to it (at least before C# 11). Sometimes generic type constraints can help.
Starting with C# 11 you can declare abstract static interface members:
public interface IFactory<T> where T : IFactory<T>
{
static abstract T Create(int input);
}
Applied in the Bar method:
public T Bar<T>(int a) where T : IFactory<T> {
return T.Create(a);
}
You can not overload the function by differing only their return type.
You can only overload the function in the following ways
Parameter types
Number of parameters
Order of the parameters declared in the method
You can not come to know which function is actually called (if it was possible).
One more thing I would like to add is
function overloading is providing a function with the same name, but with a different signature. but the return type of a method is not considered as a part of the method's signature.
So this is another way to understand why method overloading can't be done only by return type.
any one having any experience with this, overloading on return type
with different parameters should be a bad practice, is it.
I'm taking that to mean - "is it bad practice to use differing parameter combinations to facilitate different return types" - if that is indeed the question, then just imagine someone else coming across this code in a few months time - effectively "dummy" parameters to determine return type...it would be quite hard to understand what's going on.
Edit - as ChrisLava points out "the way around this is to have better names for each function (instead of overloading). Names that have clear meaning within the application. There is no reason why Bar should return an int and a string. I could see just calling ToString() on the int"
C# does not allow it.
C# on the other hand does not support method resolution based on the
return type; this was a conscious decision by the language designers
not to expose the feature of the CLR which allows it. There's no
technical reason why they couldn't have done, they just felt it better
not to. Because the return value is not used for method selection a
C# method signature does not include it.
Check this.. You can't have method with same signature and just different return type. Recommended is mentioned in this question.
C# Overload return type - recommended approach
Your code will result in a Compiler Error. You can not have a Method of the same name with the same parameters and a different return type, the caller would not know which Method to call (will not be able to resolve the location in Memory of the Method to call, why the Compiler would not allow it). Your alternative would be to return an Object and Cast it based on what the Caller knows. Even then, that appears to be Bad design.
This will Work (meaning Compile), but still bad design.
public class Foo
{
public object Bar(int a)
{
return a;
}
}
You can't create method with same name, same number and types of parameters.
For more details refer this link.
I thought I'd offer this softball to whomever would like to hit it out of the park. What are generics, what are the advantages of generics, why, where, how should I use them? Please keep it fairly basic. Thanks.
Allows you to write code/use library methods which are type-safe, i.e. a List<string> is guaranteed to be a list of strings.
As a result of generics being used the compiler can perform compile-time checks on code for type safety, i.e. are you trying to put an int into that list of strings? Using an ArrayList would cause that to be a less transparent runtime error.
Faster than using objects as it either avoids boxing/unboxing (where .net has to convert value types to reference types or vice-versa) or casting from objects to the required reference type.
Allows you to write code which is applicable to many types with the same underlying behaviour, i.e. a Dictionary<string, int> uses the same underlying code as a Dictionary<DateTime, double>; using generics, the framework team only had to write one piece of code to achieve both results with the aforementioned advantages too.
I really hate to repeat myself. I hate typing the same thing more often than I have to. I don't like restating things multiple times with slight differences.
Instead of creating:
class MyObjectList {
MyObject get(int index) {...}
}
class MyOtherObjectList {
MyOtherObject get(int index) {...}
}
class AnotherObjectList {
AnotherObject get(int index) {...}
}
I can build one reusable class... (in the case where you don't want to use the raw collection for some reason)
class MyList<T> {
T get(int index) { ... }
}
I'm now 3x more efficient and I only have to maintain one copy. Why WOULDN'T you want to maintain less code?
This is also true for non-collection classes such as a Callable<T> or a Reference<T> that has to interact with other classes. Do you really want to extend Callable<T> and Future<T> and every other associated class to create type-safe versions?
I don't.
Not needing to typecast is one of the biggest advantages of Java generics, as it will perform type checking at compile-time. This will reduce the possibility of ClassCastExceptions which can be thrown at runtime, and can lead to more robust code.
But I suspect that you're fully aware of that.
Every time I look at Generics it gives
me a headache. I find the best part of
Java to be it's simplicity and minimal
syntax and generics are not simple and
add a significant amount of new
syntax.
At first, I didn't see the benefit of generics either. I started learning Java from the 1.4 syntax (even though Java 5 was out at the time) and when I encountered generics, I felt that it was more code to write, and I really didn't understand the benefits.
Modern IDEs make writing code with generics easier.
Most modern, decent IDEs are smart enough to assist with writing code with generics, especially with code completion.
Here's an example of making an Map<String, Integer> with a HashMap. The code I would have to type in is:
Map<String, Integer> m = new HashMap<String, Integer>();
And indeed, that's a lot to type just to make a new HashMap. However, in reality, I only had to type this much before Eclipse knew what I needed:
Map<String, Integer> m = new Ha Ctrl+Space
True, I did need to select HashMap from a list of candidates, but basically the IDE knew what to add, including the generic types. With the right tools, using generics isn't too bad.
In addition, since the types are known, when retrieving elements from the generic collection, the IDE will act as if that object is already an object of its declared type -- there is no need to casting for the IDE to know what the object's type is.
A key advantage of generics comes from the way it plays well with new Java 5 features. Here's an example of tossing integers in to a Set and calculating its total:
Set<Integer> set = new HashSet<Integer>();
set.add(10);
set.add(42);
int total = 0;
for (int i : set) {
total += i;
}
In that piece of code, there are three new Java 5 features present:
Generics
Autoboxing and unboxing
For-each loop
First, generics and autoboxing of primitives allow the following lines:
set.add(10);
set.add(42);
The integer 10 is autoboxed into an Integer with the value of 10. (And same for 42). Then that Integer is tossed into the Set which is known to hold Integers. Trying to throw in a String would cause a compile error.
Next, for for-each loop takes all three of those:
for (int i : set) {
total += i;
}
First, the Set containing Integers are used in a for-each loop. Each element is declared to be an int and that is allowed as the Integer is unboxed back to the primitive int. And the fact that this unboxing occurs is known because generics was used to specify that there were Integers held in the Set.
Generics can be the glue that brings together the new features introduced in Java 5, and it just makes coding simpler and safer. And most of the time IDEs are smart enough to help you with good suggestions, so generally, it won't a whole lot more typing.
And frankly, as can be seen from the Set example, I feel that utilizing Java 5 features can make the code more concise and robust.
Edit - An example without generics
The following is an illustration of the above Set example without the use of generics. It is possible, but isn't exactly pleasant:
Set set = new HashSet();
set.add(10);
set.add(42);
int total = 0;
for (Object o : set) {
total += (Integer)o;
}
(Note: The above code will generate unchecked conversion warning at compile-time.)
When using non-generics collections, the types that are entered into the collection is objects of type Object. Therefore, in this example, a Object is what is being added into the set.
set.add(10);
set.add(42);
In the above lines, autoboxing is in play -- the primitive int value 10 and 42 are being autoboxed into Integer objects, which are being added to the Set. However, keep in mind, the Integer objects are being handled as Objects, as there are no type information to help the compiler know what type the Set should expect.
for (Object o : set) {
This is the part that is crucial. The reason the for-each loop works is because the Set implements the Iterable interface, which returns an Iterator with type information, if present. (Iterator<T>, that is.)
However, since there is no type information, the Set will return an Iterator which will return the values in the Set as Objects, and that is why the element being retrieved in the for-each loop must be of type Object.
Now that the Object is retrieved from the Set, it needs to be cast to an Integer manually to perform the addition:
total += (Integer)o;
Here, a typecast is performed from an Object to an Integer. In this case, we know this will always work, but manual typecasting always makes me feel it is fragile code that could be damaged if a minor change is made else where. (I feel that every typecast is a ClassCastException waiting to happen, but I digress...)
The Integer is now unboxed into an int and allowed to perform the addition into the int variable total.
I hope I could illustrate that the new features of Java 5 is possible to use with non-generic code, but it just isn't as clean and straight-forward as writing code with generics. And, in my opinion, to take full advantage of the new features in Java 5, one should be looking into generics, if at the very least, allows for compile-time checks to prevent invalid typecasts to throw exceptions at runtime.
If you were to search the Java bug database just before 1.5 was released, you'd find seven times more bugs with NullPointerException than ClassCastException. So it doesn't seem that it is a great feature to find bugs, or at least bugs that persist after a little smoke testing.
For me the huge advantage of generics is that they document in code important type information. If I didn't want that type information documented in code, then I'd use a dynamically typed language, or at least a language with more implicit type inference.
Keeping an object's collections to itself isn't a bad style (but then the common style is to effectively ignore encapsulation). It rather depends upon what you are doing. Passing collections to "algorithms" is slightly easier to check (at or before compile-time) with generics.
Generics in Java facilitate parametric polymorphism. By means of type parameters, you can pass arguments to types. Just as a method like String foo(String s) models some behaviour, not just for a particular string, but for any string s, so a type like List<T> models some behaviour, not just for a specific type, but for any type. List<T> says that for any type T, there's a type of List whose elements are Ts. So List is a actually a type constructor. It takes a type as an argument and constructs another type as a result.
Here are a couple of examples of generic types I use every day. First, a very useful generic interface:
public interface F<A, B> {
public B f(A a);
}
This interface says that for some two types, A and B, there's a function (called f) that takes an A and returns a B. When you implement this interface, A and B can be any types you want, as long as you provide a function f that takes the former and returns the latter. Here's an example implementation of the interface:
F<Integer, String> intToString = new F<Integer, String>() {
public String f(int i) {
return String.valueOf(i);
}
}
Before generics, polymorphism was achieved by subclassing using the extends keyword. With generics, we can actually do away with subclassing and use parametric polymorphism instead. For example, consider a parameterised (generic) class used to calculate hash codes for any type. Instead of overriding Object.hashCode(), we would use a generic class like this:
public final class Hash<A> {
private final F<A, Integer> hashFunction;
public Hash(final F<A, Integer> f) {
this.hashFunction = f;
}
public int hash(A a) {
return hashFunction.f(a);
}
}
This is much more flexible than using inheritance, because we can stay with the theme of using composition and parametric polymorphism without locking down brittle hierarchies.
Java's generics are not perfect though. You can abstract over types, but you can't abstract over type constructors, for example. That is, you can say "for any type T", but you can't say "for any type T that takes a type parameter A".
I wrote an article about these limits of Java generics, here.
One huge win with generics is that they let you avoid subclassing. Subclassing tends to result in brittle class hierarchies that are awkward to extend, and classes that are difficult to understand individually without looking at the entire hierarchy.
Wereas before generics you might have classes like Widget extended by FooWidget, BarWidget, and BazWidget, with generics you can have a single generic class Widget<A> that takes a Foo, Bar or Baz in its constructor to give you Widget<Foo>, Widget<Bar>, and Widget<Baz>.
Generics avoid the performance hit of boxing and unboxing. Basically, look at ArrayList vs List<T>. Both do the same core things, but List<T> will be a lot faster because you don't have to box to/from object.
The best benefit to Generics is code reuse. Lets say that you have a lot of business objects, and you are going to write VERY similar code for each entity to perform the same actions. (I.E Linq to SQL operations).
With generics, you can create a class that will be able to operate given any of the types that inherit from a given base class or implement a given interface like so:
public interface IEntity
{
}
public class Employee : IEntity
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int EmployeeID { get; set; }
}
public class Company : IEntity
{
public string Name { get; set; }
public string TaxID { get; set }
}
public class DataService<ENTITY, DATACONTEXT>
where ENTITY : class, IEntity, new()
where DATACONTEXT : DataContext, new()
{
public void Create(List<ENTITY> entities)
{
using (DATACONTEXT db = new DATACONTEXT())
{
Table<ENTITY> table = db.GetTable<ENTITY>();
foreach (ENTITY entity in entities)
table.InsertOnSubmit (entity);
db.SubmitChanges();
}
}
}
public class MyTest
{
public void DoSomething()
{
var dataService = new DataService<Employee, MyDataContext>();
dataService.Create(new Employee { FirstName = "Bob", LastName = "Smith", EmployeeID = 5 });
var otherDataService = new DataService<Company, MyDataContext>();
otherDataService.Create(new Company { Name = "ACME", TaxID = "123-111-2233" });
}
}
Notice the reuse of the same service given the different Types in the DoSomething method above. Truly elegant!
There's many other great reasons to use generics for your work, this is my favorite.
I just like them because they give you a quick way to define a custom type (as I use them anyway).
So for example instead of defining a structure consisting of a string and an integer, and then having to implement a whole set of objects and methods on how to access an array of those structures and so forth, you can just make a Dictionary
Dictionary<int, string> dictionary = new Dictionary<int, string>();
And the compiler/IDE does the rest of the heavy lifting. A Dictionary in particular lets you use the first type as a key (no repeated values).
Typed collections - even if you don't want to use them you're likely to have to deal with them from other libraries , other sources.
Generic typing in class creation:
public class Foo < T> {
public T get()...
Avoidance of casting - I've always disliked things like
new Comparator {
public int compareTo(Object o){
if (o instanceof classIcareAbout)...
Where you're essentially checking for a condition that should only exist because the interface is expressed in terms of objects.
My initial reaction to generics was similar to yours - "too messy, too complicated". My experience is that after using them for a bit you get used to them, and code without them feels less clearly specified, and just less comfortable. Aside from that, the rest of the java world uses them so you're going to have to get with the program eventually, right?
To give a good example. Imagine you have a class called Foo
public class Foo
{
public string Bar() { return "Bar"; }
}
Example 1
Now you want to have a collection of Foo objects. You have two options, LIst or ArrayList, both of which work in a similar manner.
Arraylist al = new ArrayList();
List<Foo> fl = new List<Foo>();
//code to add Foos
al.Add(new Foo());
f1.Add(new Foo());
In the above code, if I try to add a class of FireTruck instead of Foo, the ArrayList will add it, but the Generic List of Foo will cause an exception to be thrown.
Example two.
Now you have your two array lists and you want to call the Bar() function on each. Since hte ArrayList is filled with Objects, you have to cast them before you can call bar. But since the Generic List of Foo can only contain Foos, you can call Bar() directly on those.
foreach(object o in al)
{
Foo f = (Foo)o;
f.Bar();
}
foreach(Foo f in fl)
{
f.Bar();
}
Haven't you ever written a method (or a class) where the key concept of the method/class wasn't tightly bound to a specific data type of the parameters/instance variables (think linked list, max/min functions, binary search, etc.).
Haven't you ever wish you could reuse the algorthm/code without resorting to cut-n-paste reuse or compromising strong-typing (e.g. I want a List of Strings, not a List of things I hope are strings!)?
That's why you should want to use generics (or something better).
The primary advantage, as Mitchel points out, is strong-typing without needing to define multiple classes.
This way you can do stuff like:
List<SomeCustomClass> blah = new List<SomeCustomClass>();
blah[0].SomeCustomFunction();
Without generics, you would have to cast blah[0] to the correct type to access its functions.
Don't forget that generics aren't just used by classes, they can also be used by methods. For example, take the following snippet:
private <T extends Throwable> T logAndReturn(T t) {
logThrowable(t); // some logging method that takes a Throwable
return t;
}
It is simple, but can be used very elegantly. The nice thing is that the method returns whatever it was that it was given. This helps out when you are handling exceptions that need to be re-thrown back to the caller:
...
} catch (MyException e) {
throw logAndReturn(e);
}
The point is that you don't lose the type by passing it through a method. You can throw the correct type of exception instead of just a Throwable, which would be all you could do without generics.
This is just a simple example of one use for generic methods. There are quite a few other neat things you can do with generic methods. The coolest, in my opinion, is type inferring with generics. Take the following example (taken from Josh Bloch's Effective Java 2nd Edition):
...
Map<String, Integer> myMap = createHashMap();
...
public <K, V> Map<K, V> createHashMap() {
return new HashMap<K, V>();
}
This doesn't do a lot, but it does cut down on some clutter when the generic types are long (or nested; i.e. Map<String, List<String>>).
Generics allow you to create objects that are strongly typed, yet you don't have to define the specific type. I think the best useful example is the List and similar classes.
Using the generic list you can have a List List List whatever you want and you can always reference the strong typing, you don't have to convert or anything like you would with a Array or standard List.
the jvm casts anyway... it implicitly creates code which treats the generic type as "Object" and creates casts to the desired instantiation. Java generics are just syntactic sugar.
I know this is a C# question, but generics are used in other languages too, and their use/goals are quite similar.
Java collections use generics since Java 1.5. So, a good place to use them is when you are creating your own collection-like object.
An example I see almost everywhere is a Pair class, which holds two objects, but needs to deal with those objects in a generic way.
class Pair<F, S> {
public final F first;
public final S second;
public Pair(F f, S s)
{
first = f;
second = s;
}
}
Whenever you use this Pair class you can specify which kind of objects you want it to deal with and any type cast problems will show up at compile time, rather than runtime.
Generics can also have their bounds defined with the keywords 'super' and 'extends'. For example, if you want to deal with a generic type but you want to make sure it extends a class called Foo (which has a setTitle method):
public class FooManager <F extends Foo>{
public void setTitle(F foo, String title) {
foo.setTitle(title);
}
}
While not very interesting on its own, it's useful to know that whenever you deal with a FooManager, you know that it will handle MyClass types, and that MyClass extends Foo.
From the Sun Java documentation, in response to "why should i use generics?":
"Generics provides a way for you to communicate the type of a collection to the compiler, so that it can be checked. Once the compiler knows the element type of the collection, the compiler can check that you have used the collection consistently and can insert the correct casts on values being taken out of the collection... The code using generics is clearer and safer.... the compiler can verify at compile time that the type constraints are not violated at run time [emphasis mine]. Because the program compiles without warnings, we can state with certainty that it will not throw a ClassCastException at run time. The net effect of using generics, especially in large programs, is improved readability and robustness. [emphasis mine]"
Generics let you use strong typing for objects and data structures that should be able to hold any object. It also eliminates tedious and expensive typecasts when retrieving objects from generic structures (boxing/unboxing).
One example that uses both is a linked list. What good would a linked list class be if it could only use object Foo? To implement a linked list that can handle any kind of object, the linked list and the nodes in a hypothetical node inner class must be generic if you want the list to contain only one type of object.
If your collection contains value types, they don't need to box/unbox to objects when inserted into the collection so your performance increases dramatically. Cool add-ons like resharper can generate more code for you, like foreach loops.
Another advantage of using Generics (especially with Collections/Lists) is you get Compile Time Type Checking. This is really useful when using a Generic List instead of a List of Objects.
Single most reason is they provide Type safety
List<Customer> custCollection = new List<Customer>;
as opposed to,
object[] custCollection = new object[] { cust1, cust2 };
as a simple example.
In summary, generics allow you to specify more precisily what you intend to do (stronger typing).
This has several benefits for you:
Because the compiler knows more about what you want to do, it allows you to omit a lot of type-casting because it already knows that the type will be compatible.
This also gets you earlier feedback about the correctnes of your program. Things that previously would have failed at runtime (e.g. because an object couldn't be casted in the desired type), now fail at compile-time and you can fix the mistake before your testing-department files a cryptical bug report.
The compiler can do more optimizations, like avoiding boxing, etc.
A couple of things to add/expand on (speaking from the .NET point of view):
Generic types allow you to create role-based classes and interfaces. This has been said already in more basic terms, but I find you start to design your code with classes which are implemented in a type-agnostic way - which results in highly reusable code.
Generic arguments on methods can do the same thing, but they also help apply the "Tell Don't Ask" principle to casting, i.e. "give me what I want, and if you can't, you tell me why".
I use them for example in a GenericDao implemented with SpringORM and Hibernate which look like this
public abstract class GenericDaoHibernateImpl<T>
extends HibernateDaoSupport {
private Class<T> type;
public GenericDaoHibernateImpl(Class<T> clazz) {
type = clazz;
}
public void update(T object) {
getHibernateTemplate().update(object);
}
#SuppressWarnings("unchecked")
public Integer count() {
return ((Integer) getHibernateTemplate().execute(
new HibernateCallback() {
public Object doInHibernate(Session session) {
// Code in Hibernate for getting the count
}
}));
}
.
.
.
}
By using generics my implementations of this DAOs force the developer to pass them just the entities they are designed for by just subclassing the GenericDao
public class UserDaoHibernateImpl extends GenericDaoHibernateImpl<User> {
public UserDaoHibernateImpl() {
super(User.class); // This is for giving Hibernate a .class
// work with, as generics disappear at runtime
}
// Entity specific methods here
}
My little framework is more robust (have things like filtering, lazy-loading, searching). I just simplified here to give you an example
I, like Steve and you, said at the beginning "Too messy and complicated" but now I see its advantages
Obvious benefits like "type safety" and "no casting" are already mentioned so maybe I can talk about some other "benefits" which I hope it helps.
First of all, generics is a language-independent concept and , IMO, it might make more sense if you think about regular (runtime) polymorphism at the same time.
For example, the polymorphism as we know from object oriented design has a runtime notion in where the caller object is figured out at runtime as program execution goes and the relevant method gets called accordingly depending on the runtime type. In generics, the idea is somewhat similar but everything happens at compile time. What does that mean and how you make use of it?
(Let's stick with generic methods to keep it compact) It means that you can still have the same method on separate classes (like you did previously in polymorphic classes) but this time they're auto-generated by the compiler depend on the types set at compile time. You parametrise your methods on the type you give at compile time. So, instead of writing the methods from scratch for every single type you have as you do in runtime polymorphism (method overriding), you let compilers do the work during compilation. This has an obvious advantage since you don't need to infer all possible types that might be used in your system which makes it far more scalable without a code change.
Classes work the pretty much same way. You parametrise the type and the code is generated by the compiler.
Once you get the idea of "compile time", you can make use "bounded" types and restrict what can be passed as a parametrised type through classes/methods. So, you can control what to be passed through which is a powerful thing especially you've a framework being consumed by other people.
public interface Foo<T extends MyObject> extends Hoo<T>{
...
}
No one can set sth other than MyObject now.
Also, you can "enforce" type constraints on your method arguments which means you can make sure both your method arguments would depend on the same type.
public <T extends MyObject> foo(T t1, T t2){
...
}
Hope all of this makes sense.
I once gave a talk on this topic. You can find my slides, code, and audio recording at http://www.adventuresinsoftware.com/generics/.
Using generics for collections is just simple and clean. Even if you punt on it everywhere else, the gain from the collections is a win to me.
List<Stuff> stuffList = getStuff();
for(Stuff stuff : stuffList) {
stuff.do();
}
vs
List stuffList = getStuff();
Iterator i = stuffList.iterator();
while(i.hasNext()) {
Stuff stuff = (Stuff)i.next();
stuff.do();
}
or
List stuffList = getStuff();
for(int i = 0; i < stuffList.size(); i++) {
Stuff stuff = (Stuff)stuffList.get(i);
stuff.do();
}
That alone is worth the marginal "cost" of generics, and you don't have to be a generic Guru to use this and get value.
Generics also give you the ability to create more reusable objects/methods while still providing type specific support. You also gain a lot of performance in some cases. I don't know the full spec on the Java Generics, but in .NET I can specify constraints on the Type parameter, like Implements a Interface, Constructor , and Derivation.
Enabling programmers to implement generic algorithms - By using generics, programmers can implement generic algorithms that work on collections of different types, can be customized, and are type-safe and easier to read.
Stronger type checks at compile time - A Java compiler applies strong type checking to generic code and issues errors if the code violates type safety. Fixing compile-time errors is easier than fixing runtime errors, which can be difficult to find.
Elimination of casts.
I'm having a hard time understanding when to use Object (boxing/unboxing) vs when to use generics.
For example:
public class Stack
{
int position;
object[] data = new object[10];
public void Push (object o) { data[position++] = o; }
public object Pop() { return data[--position]; }
}
VS.
public class Stack<T>
{
int position;
T[] data = new T[100];
public void Push(T obj) {data[position++] = obj; }
public T Pop() { return data[--position]; }
}
Which one should I use and under what conditions? It seems like with the System.Object way I can have objects of all sorts of types currently living within my Stack. So wouldn't this be always preferable? Thanks!
Always use generics! Using object's results in cast operations and boxing/unboxing of value-types. Because of these reasons generics are faster and more elegant (no casting). And - the main reason - you won't get InvalidCastExceptions using generics.
So, generics are faster and errors are visible at compile-time. System.Object means runtime exceptions and casting which in general results in lower performance (sometimes MUCH lower).
A lot of people have recommended using generics, but it looks like they all miss the point. It's often not about the performance hit related to boxing primitive types or casting, it's about getting the compiler to work for you.
If I have a list of strings, I want the compiler to prove to me that it will always contain a list of strings. Generics does just that - I specify the intent, and the compiler proves it for me.
Ideally, I would prefer an even richer type system where you could say for example that a type (even if it was a reference type) could not contain null values, but C# does unfortunately not currently offer that.
While there are times when you will want to use a non-generic collection (think caching, for instance), you almost always have collections of homogenous objects not heterogenous objects. For a homogenous collection, even if it is a collection of variants of base type or interface, it's always better to use generics. This will save you from having to cast the result as the real type before you can use it. Using generics makes your code more efficient and readable because you can omit the code to do the cast.
It all depends on what you need in the long run.
Unlike most answers here, I won't say "always use generics" because sometimes you do need to mix cats with cucumbers.
By all means, try to stick with generics for all the reasons already given in the other answers, for example if you need to combine cats and dogs create base class Mammal and have Stack<Mamal>.
But when you really need to support every possible type, don't be afraid to use objects, they don't bite unless you're mistreating them. :)
With the object type, as you say you need to perform boxing and unboxing, which gets tedious very quickly. With generics, there's no need for that.
Also, I'd rather be more specific as to what kind of objects a class can work with and generics provides a great basis for that. Why mix unrelated data types in the first place? Your particular example of a stack emphasizes the benefit of generics over the basic object data type.
// This stack should only contain integers and not strings or floats or bools
Stack<int> intStack = new Stack<int>();
intStack.Push(1);
Remember that with generics you can specify interfaces so your class can interact with objects of many different classes, provided they all implement the same interface.
Use generics when you want your structure to handle a single type. For example, if you wanted a collection of strings you would want to instantiate a strongly typed List of strings like so:
List<string> myStrings = new List<string>();
If you want it to handle multiple types you can do without generics but you will incur a small performance hit for boxing/unboxing operations.
Generics are always preferred if possible.
Aside from performance, Generics allow you to make guarantees about the types of objects that you're working with.
The main reason this is preferred to casting is that the compiler knows what type the object is, and so it can give you compile errors that you find right away instead of runtime errors that might only happen under certain scenarios that you didn't test.
Generics are not golden hammer. In cases where your activity naturally is non-generic, use good old object. One such case - caching. Cache naturally can hold different types. I've recently seen this implementation of cache wrapper
void AddCacheItem<T>(string key, T item, int duration, ICacheItemExpiration expiration)
{
. . . . . . .
CacheManager.Add(cacheKey, item, .....
}
Question: what for, if CacheManager takes object?
Then there was real havoc in Get
public virtual T GetCacheItem<T>(string cacheKey)
{
return (T)CacheManager.GetData(cacheKey); // <-- problem code
}
The problem above is that value type will crash.
I mended the method by adding this
public T GetCacheItem<T>(string cacheKey) where T : class
Because I like idea of doing this
var x = GetCacheItem<Person>("X")?
string name = x?.FullName;
But I added new method, which will allow to take value types as well
public object GetCacheItem(string cacheKey)
The bottom line, there is usage for object, especially when storing different types in collection. Or when you have compositions where completely arbitrary and unrelated objects can exist when you need to consume them based on type.
This question already has answers here:
Why is there not a `fieldof` or `methodof` operator in C#? [closed]
(4 answers)
Closed 9 years ago.
There is sizeof() and typeof(), but why not a memberinfo() returning an instance of System.Reflection.MemberInfo for the part of code selected in order to aid in reflection code.
Example:
Program()
{
Type t = typeof(Foo);
Foo foo = new Foo();
PropertyInfo pi = memberinfo(Foo.Name) as PropertyInfo;
// or shall it be like this
// PropertyInfo pi = memberinfo(foo.Name) as PropertyInfo;
string name = pi.GetValue(foo, null);
}
I am trying to understand if there is a fundamental reason why this could be implemented in the C# spec.
I am not bashing anything, I am just doing some wishful thinking, so be kind please.
Eric Lippert talks about this extensively on his blog
To quote directly from that post:
Just off the top of my head, here are a few {reasons why this hasn't been done}. (1) How do you unambiguously specify that you want a method info of an specific explicit interface implementation? (2) What if overload resolution would have skipped a particular method because it is not accessible? It is legal to get method infos of methods that are not accessible; metadata is always public even if it describes private details. Should we make it impossible to get private metadata, making the feature weak, or should we make it possible, and make infoof use a subtly different overload resolution algorithm than the rest of C#? (3) How do you specify that you want the info of, say, an indexer setter, or a property getter, or an event handler adder?
There are a couple of items which make this type of feature difficult. One of the primary ones being overloaded methods.
class Example {
public void Method() {}
public void Method(int p1) {}
}
Which MethodInfo would the following return?
var info = memberinfo(Example.Method);
As Wesley has pointed out though, Eric Lippert's Blog has the full discussion on this issue.
There are numerous reasons why compile-time member reflection has not yet been implemented in C# - but most of them basically boil down to opportunity cost - there are many other languages features and enhancements that offer more benefit to more users. There's also the consideration that an infoof syntax could be complicated, confusing, and ultimately less powerful than using string-based reflection. It also wouldn't be a complete replacement for reflection since in many instances the metadata being manipulated isn't known at compile time.
However, all is not lost, there are a number of tricks that you can employ to perform slightly safer reflection that leverages capabilities of the C# language. For instance, we can take advantage of lambda expressions and expression trees to extract MemberInfo information. A simple example is:
public static class MethodExt {
static MethodInfo MemberInfo(Action d) {
return d.Method;
}
// other overloads ...
}
which works when you pass in a (non-anonymous) action delegate:
MethodInfo mi = MethodExt.MemberInfo( Object.ToString );
An implementation of the above using expression trees can more robust and flexible, but also substantially more complicated. It could be used to represent member and property access, indexers, etc.
The main issue with all such "fancy" approaches, is that they are confusing to developers who are used to seeing traditional reflection code. They also can't handle all cases, which often results in an unfortunate mixture of traditional reflection code and fancy expression tree code. Personally, while such techniques are interesting and inventive, it's probably best to avoid it in production code.
I myself use an approach that reads the IL from an anonymous method (using Mono.Reflection namespace) and grabs the info of the last token found in the anonymous method. This tends to be the only way to get information about things like the add_EventHandler or set_Property or captured local variables. To actual get properties I use expression trees.
The syntax that I use is Reflect.Member<T>.InfoOf<TMember>(Func<T,TMember> memberfunc) where Member is replaced with the type I'm interested in. It's verbose sure, but it lets a user know exactly what the code is trying to do. I also have Reflect.Member styles for things like statics and constructors.
Here is the relevant code snippet::
internal static MemberInfo GetLastMemberOfType(MethodBase method, MemberTypes memberType)
{
var instructions = method.GetInstructions();
var operandtype = memberType == MemberTypes.Field ? OperandType.InlineField : OperandType.InlineMethod;
for (int i = instructions.Count-1; i > -1 ; i--)
{
if (instructions[i].OpCode.OperandType == operandtype)
{
return instructions[i].Operand as MemberInfo;
}
}
return null;
}
Does it replace string based reflection? Absolutely not. Does it make my code safer while I'm refactoring interfaces and what not? Absolutely.
Will it ship with the product I'm working on? Probably not.
Everyone knows and love String.IsNullOrEmpty(yourString) method.
I was wondering if it's going to confuse developers or make code better if we extend String class to have method like this:
yourString.IsNullOrEmpty();
Pro:
More readable.
Less typing.
Cons:
Can be confusing because yourString
variable can be null and it looks
like you're executing method on a
null variable.
What do you think?
The same question we can ask about myObject.IsNull() method.
That how I would write it:
public static class StringExt
{
public static bool IsNullOrEmpty(this string text)
{
return string.IsNullOrEmpty(text);
}
public static bool IsNull(this object obj)
{
return obj == null;
}
}
If I'm not mistaken, every answer here decries the fact that the extension method can be called on a null instance, and because of this they do not support believe this is a good idea.
Let me counter their arguments.
I don't believe AT ALL that calling a method on an object that may be null is confusing. The fact is that we only check for nulls in certain locations, and not 100% of the time. That means there is a percentage of time where every method call we make is potentially on a null object. This is understood and acceptable. If it wasn't, we'd be checking null before every single method call.
So, how is it confusing that a particular method call may be happening on a null object? Look at the following code:
var bar = foo.DoSomethingResultingInBar();
Console.Writeline(bar.ToStringOr("[null]"));
Are you confused? You should be. Because here's the implementation of foo:
public Bar DoSomethingResultingInBar()
{
return null; //LOL SUCKER!
}
See? You read the code sample without being confused at all. You understood that, potentially, foo would return a null from that method call and the ToStringOr call on bar would result in a NRE. Did your head spin? Of course not. Its understood that this can happen. Now, that ToStringOr method is not familiar. What do you do in these situations? You either read the docs on the method or examine the code of the call. Here it is:
public static class BarExtensions
{
public static string ToStringOr(this bar, string whenNull)
{
return bar == null ? whenNull ?? "[null]" : bar.ToString();
}
}
Confusing? Of course not. Its obvious that the developer wanted a shorthand method of checking if bar is null and substituting a non-null string for it. Doing this can slash your code significantly and increase readability and code reuse. Of course you could do this in other ways, but this way would be no more confusing than any other. For example:
var bar = foo.DoSomethingResultingInBar();
Console.Writeline(ToStringOr(bar, "[null]"));
When you encounter this code, what do you have to differently than the original version? You still have to examine the code, you still have to determine its behavior when bar is null. You still have to deal with this possibility.
Are extension methods confusing? Only if you don't understand them. And, quite frankly, the same can be said for ANY part of the language, from delegates to lambdas.
I think the root of this problem is what Jon Skeet has mentioned in the list of things he hates in his favorite language (C#): C# should not have imported all extension methods in a whole namespace automatically. This process should have been done more explicitly.
My personal opinion about this specific question is (since we can't do anything about the above fact) to use the extension method if you want. I don't say it won't be confusing, but this fact about extension methods (that can be called on null references) is a global thing and doesn't affect only String.IsNullOrEmpty, so C# devs should get familiar with it.
By the way, it's fortunate that Visual Studio clearly identifies extension methods by (extension) in the IntelliSense tooltip.
I'm personally not a fan of doing this. The biggest problem with extension methods right now is discoverability. Unleses you flat out know all of the methods which exist on a particular type, it's not possible to look at a method call and know that it's an extension method. As such I find it problematic to do anything with an extension method that would not be possible with a normal method call. Otherwise you will end up confusing developers.
A corollary to this problem exist in C++. In several C++ implementations it's possible to call instance methods on NULL pointers as long as you don't touch any fields or virtual methods on the type. I've worked with several pieces of code that do this intentionally and give methods differentt behavior when "this==NULL". It's quite maddening to work with.
This is not to say that I don't like extension methods. Quite the contrary, I enjoy them and use them frequently. But I think there are 2 important rules you should follow when writing them.
Treat the actual method implementation as if it's just another static method because it in fact is. For example throw ArgumentException instead of NullReference exception for a null this
Don't let an extension method perform tricks that a normal instance method couldn't do
EDIT Responding to Mehrdad's comment
The problem with taking advantage of it is that I don't see str.IsNullOrEmpty as having a significant functional advantage over String.IsNullOrEmpty(str). The only advantage I see is that one requires less typing than the other. The same could be said about extension methods in general. But in this case you're additionally altering the way people think about program flow.
If shorter typing is what people really want wouldn't IsNullOrEmpty(str) be a much better option? It's both unambiguous and is the shortest of all. True C# has no support for such a feature today. But imagine if in C# I could say
using SomeNamespace.SomeStaticClass;
The result of doing this is that all methods on SomeStaticClass were now in the global namespace and available for binding. This seems to be what people want, but they're attaching it to an extension method which I'm not a huge fan of.
I really like this approach AS LONG AS the method makes it clear that it is checking the object is null. ThrowIfNull, IsNull, IsNullOrEmpty, etc. It is very readable.
Personally, I wouldn't create an extension that does something that already exists in the framework unless it was a significant improvement in usability. In this instance, I don't think that's the case. Also, if I were to create an extension, I would name it in a way as to reduce confusion, not increase it. Again, I think this case fails that test.
Having said all that, I do have a string extension that tests, not only if the string is null or empty, but also if it only contains whitespace. I call it IsNothing. You can find it here.
It doesn't just look like you're calling a method on a null variable. You /are/ calling a method on a null variable, albeit one implemented through a static extension method. I had no idea extension methods (which I was already leery of) supported that. This even allows you to do crazy things like:
public static int PowerLength(this string obj)
{
return obj == null ? 0 : obj.Length;
}
From where I'm standing now, I would classify any use of an extension method on a null reference under considered harmful.
Let's look at the pro:s and con:s...
More readable.
Yes, slightly, but the improvement in readability is outweighed by the fact that it looks like you are calling an instance method on something that doesn't have to be an instance. In effect it's easier to read, but it's harder to understand, so the improved readability is really just an illusion.
Less typing.
Yes, but that is really not a strong argument. If typing is the main part of your programming, you are just not doing something that is remotely challenging enough for you to evolve as a developer.
Can be confusing because yourString variable can be null and it looks like
you're executing method on a null variable.
True, (as mentioned above).
Yes, it will confuse.
I think that everyone who knows about IsNullOrEmpty() perceives the use of it as quite natural. So your suggested extension method will not add more readability.
Maybe for someone new to .NET this extension method might be easier to deal with, but there is the danger possibility that she/he doesn't understand all facets of extension methods (like that you need to import the namespace and that it can be invoked on null). She/he will might wonder why this extension method does not exist in another project. Anyway: Even if someone is new to .NET the syntax of the IsNullOrEmpty() method might become natural quite fast. Also here the benefits of the extension methods will not outweight the confusion caused by it.
Edit: Tried to rephrase what I wanted to say.
I think extending any object with an "IsNull" type call is a little confusing and should be avoided if possible.
It's arguable that the IsEmptyString method might be useful for the String type, and that because you'd usually combine this with a test for null that the IsNullOrEmpty might be useful, but I'd avoid this too due to the fact that the string type already has a static method that does this and I'm not sure you're saving yourself that much typing (5 characters at most).
Calling a method on a variable that is null usually results in a NullReferenceException. The IsNullOrEmpty()-Method deviates from this behaviour in a way that is not predictable from just looking at the code. Therefore I would advise against using it since it creates confusion and the benefit of saving a couple of characters is minimal.
In general, I'm only ok with extension methods being safe to call on null if they have the word 'null' or something like that in their name. That way, I'm clued in to the fact that they may be safe to call with null. Also, they better document that fact in their XML comment header so I get that info when I mouse-over the call.
When comparing class instance to null using ".IsNull()" is not even shorter than using " == null".
Things change in generic classes when the generic type argument without constraint can be a value type.
In such case comparison with default type is lengthy and I use the extension below:
public static bool IsDefault<T>(this T x)
{
return EqualityComparer<T>.Default.Equals(x, default(T));
}