I have a class like this:
public class SomeClass
{
public static IEnumerable<SomeClass> GetOutput(IEnumerable<SomeClass> items)
{
//Do stuff
}
}
This class is not static, but I want to make GetOutput an extension method for IEnumerable<SomeClass>. As such, I create a new static class:
public static class SomeClassExtensionMethods
{
public static IEnumerable<SomeClass> GetOutput(this IEnumerable<SomeClass> items)
{
return SomeClass.GetOutput(items);
}
}
Is there any more elegant way to do this? Why aren't we allowed to make SomeClass.GetOutput an extension method?
There is no more elegant way to do this unfortunately.
Why they aren't allowed was already answered here: Why are extension methods only allowed in non-nested, non-generic static class?
GetOutput() has to be an instance method, and cannot be static if you wish to extend it.
Why aren't we allowed to make SomeClass.GetOutput an extension method?
You can but not with the standard C# tools.
To the C# compiler, an extension method for a type is a static method that takes an instance of that type as its first parameter and is marked with System.Runtime.CompilerServices.ExtensionAttribute.
When defining an extension method in C#, the C# compiler won't allow the application of that attribute at all, suggesting that you use the this syntax, which does have the cited requirements. So, you could define the SomeClass in some other language or use a tool that adds the attribute after the C# compiler is done.
PostSharp (non-free edition) can do that. Just mark GetOutput with a different attribute and write code to replace it with System.Runtime.CompilerServices.ExtensionAttribute.
public class SomeClass
{
[ExtensionAspect]
public static IEnumerable<SomeClass> GetOutput(IEnumerable<SomeClass> items)
{
return items;
}
}
GetOutput is marked with ExtensionAspectAttribute, which is derived from a PostSharp aspect. Post-processing during the build runs the ProvideAspect method, which adds the desired attribute.
[AttributeUsage(AttributeTargets.Method)]
public class ExtensionAspectAttribute : MethodLevelAspect, IAspectProvider
{
public IEnumerable<AspectInstance> ProvideAspects(object targetElement)
{
var constructorInfo = typeof (System.Runtime.CompilerServices.ExtensionAttribute).GetConstructor(Type.EmptyTypes);
var objectConstruction = new ObjectConstruction(constructorInfo);
var aspectInstance = new CustomAttributeIntroductionAspect(objectConstruction);
yield return new AspectInstance(targetElement, aspectInstance);
}
}
So, in another project that references the binary assembly for SomeClass, this works:
var items = new [] { new SomeClass() };
var results = items.GetOutput();
That satisfies the C# compiler, however Intellisense doesn't see it as an extension method and ReSharper colors it as an error.
Of course, this is an academic exercise because there is little reason to not define the extension method in SomeClassExtensionMethods especially since it can be done in the same namespace and even the same .cs file as SomeClass.
Related
Say I have a static class that defines a bunch of delegates that looks like the following:
namespace MyNamespace {
public static class Delegates<T> {
public delegate T Create(T input);
public delegate T Update(T input);
public delegate T Get(Guid id);
public delegate bool Delete(Guid id);
}
}
I then have a generic class that uses those delegates, where the generic type of the dependent class is the same as the generic type on the static class.
namespace MyNamespace {
public class MyClass<T> {
public void MyMethod(Delegates<T>.Create create) {
T foo = GenerateSomeT();
create(foo);
}
}
}
I'd love to get rid of the Delegates<T>. prefix in the MyMethod implementation because I believe it improves the code's readability. Normally I've removed the static class as a prefix via the "using static" directive from C# 6. But in this situation T is not a "fully-qualified-type-name", so I don't see a way to accomplish this. I'd love to do something like the following example, but this does not work as one would expect. The reason is because T is still undefined outside of the MyClass's scope.
using static MyNamespace.Delegates<T>;
namespace MyNamespace {
public class MyClass<T> {
public void MyMethod(Create create) {
T foo = GenerateSomeT();
create(foo);
}
}
}
The compiler does not allow me to move the "using static" directive inside of the class either.
Is is possible to employ the using static directive, or some other trick, to avoid explicitly stating a generic static class's with an unqualified generic type every time a consumer wants to use one of its properties or methods?
Can’t you simply make Delegates non generic, and make the delegates themselves generic?
public static class Delegates
{
public delegate T Create<T> Create(T input);
//etc
}
And now T in MyClass<T> should be in scope:
using static Delegates;
public class MyClass<T>
{
public void MyMethod(Create<T> create)
{
//...
}
}
But once you get to this point, you'd have to realize that this is pointless. Delegates are types themselves, they don't need to be contained inside a class, so you can simply do:
public delegate T Create<T>(T t);
public class MyClass<T>
{
public void MyMethod(Create<T> create)
{
//...
}
}
No static using directives needed. This is how all inbuilt framework delegates are defined: Func<>, Action<>, etc.
Short answer is no, we cannot do that.
you cannot write a using statement with a generic type parameter whose type is not yet resolvable to a concrete type or interface type.
From Microsoft Docs:
A generic class, such as GenericList listed in Introduction to Generics, cannot be used as-is because it is not really a type; it is more like a blueprint for a type. To use GenericList, client code must declare and instantiate a constructed type by specifying a type argument inside the angle brackets. The type argument for this particular class can be any type recognized by the compiler.
All the T parameters needs to be get resolved via calling code that are using that generic type and in case of using statement we cannot specify any way that which type should be replaced instead of generic T parameter.
You can do something like: using static MyNamespace.Delegates<SomeType> but not what you are trying to do. You need to stick with what you tried in first approach i think.
EDIT:
After doing a quick search i found a similar question here.
All of the examples for extension methods that I have seen consume the extension method in a class like:
class Program
{
static void Main()
{
...Call extension method here
}
}
These seem to work because the consuming class is static. Is there a way to consume an extension method in a non static class like below? I can't seem to find any examples like this.
I have my Extension Method class:
using System;
using System.Collections.Generic;
namespace AwesomeApp
{
public static class LinqExtensionMethods
{
public static IEnumerable<T> FindItemsBeforeAndAfter<T>(this IEnumerable<T> items, Predicate<T> matchFilling)
{
if (items == null)
throw new ArgumentNullException("items");
if (matchFilling == null)
throw new ArgumentNullException("matchFilling");
return items;
}
}
}
And I have my class that consumes the extention method
namespace AwesomeApp
{
public class Leaders : ILeaders
{
var leaders = GetAllLeaders();
var orderedleaders = leaders.OrderByDescending(o => o.PointsEarned);
var test = orderedleaders.FindItemsBeforeAndAfter(w => w.UserId == 1);
}
}
If I call the extension method from a static class I do not the the 'Extension method must be defined in a non-generic static class' error:
public class test
{
public void testfunc()
{
List<int> testlist = new List<int>() {1,2,3,4,5,6,7,8,9};
testlist.FindItemsBeforeAndAfter<int>(e => e == 5);
}
}
I have read through all the stackoverflow answers I can find on the non-generic static class error and they deal with writing your extension method but don't deal with consuming it.
So the question is: If using an extension method with a non-static class is not possible then is there any way to do something that works in a similar way? for example it can be called as .ExtensionMethod not Helper.ExtensionMethod(passedObject)??
Resolution: I thought I cut and pasted the extension method from public class Leaders : ILeaders to its own class so that I could make it static but I actually just copied it. The compiler error was pointing to the class name so I did not see the extension method still at the bottom of the file. The error message is accurate and everyone that answered is correct.
These seem to work because the consuming class is static.
No, that's incorrect. Extension methods definitely don't have to be consumed from static classes or static methods.
However, they do have to be declared in a class which is:
Non-nested
Non-generic
Static
You appear to be confusing calling with declaring - when you say:
If I call the extension method from a static class I do not the the 'Extension method must be defined in a non-generic static class' error
... you'll only get that if you try to declare the method in a class which doesn't satisfy all the above criteria. You should double click on the error to show where it's being generated - I'm sure you'll find it's the declaration, not the use of the method.
Note that your final example (class test) is not a static class, nor a static method.
I get the following compilation error at the class name.
Extension method must be defined in a non-generic static class
I am not using normal class. What could be the reason for this. I don't know and don't want to use extension methods.
As requested, here is my comment as an answer:
Without your code there isn't much we can do. My best guess is that you accidentally typed "this" somewhere in a parameter list.
Sample for extension method
public static class ExtensionMethods {
public static object ToAnOtherObject(this object obj) {
// Your operation here
}
}
I had the same problem, and solved it as follows. My code was something like this:
public static class ExtensionMethods
{
public static object ToAnOtherObject(this object obj)
{
// Your operation here
}
}
and I changed it to
public static class ExtensionMethods
{
public static object ToAnOtherObject(object obj)
{
// Your operation here
}
}
I removed the word "this" of the parameter of the method.
I'm guessing this relates to your previous list question; if so, the example I provided is an extension method, and would be:
public static class LinkedListUtils { // name doesn't matter, but must be
// static and non-generic
public static IEnumerable<T> Reverse<T>(this LinkedList<T> list) {...}
}
This utility class does not need to be the same as the consuming class, but extension methods is how it is possible to use as list.Reverse()
If you don't want it as an extension method, you can just make it a local static method - just take away the "this" from the firstparameter:
public static IEnumerable<T> Reverse<T>(LinkedList<T> list) {...}
and use as:
foreach(var val in Reverse(list)) {...}
The following points need to be considered when creating an extension method:
The class which defines an extension method must be non-generic and static
Every extension method must be a static method
The first parameter of the extension method should use the this keyword.
How about posting your code? Extension methods are declared by preceding the first parameter of a static method with this. Since you don't won't to use an extension method, I suspect you accidentally started a parameter list with this.
Look for something like:
void Method(this SomeType name)
{
}
In a recent question of mine I learned that if there are more than one extension methods with constraints that match the given type, the most specific one will be chosen. This got me thinking - how does the compiler determine which one is "more specific"? And what will the outcome be?
Let's say I have the following classes:
public MyClass : IComparable, IDisposable
{
// Implementation of members
}
public static class MyExtensions
{
public static void DoSomething<T>(this T item)
where T : IComparable
{ /* whatever */ }
public static void DoSomething<T>(this T item)
where T : IDisposable
{ /* whatever else */ }
}
If I now use the extension method as
var instance = new MyClass();
instance.DoSomething();
which method will be used? Or will the compiler throw an error?
Note: I'm not saying this is good design, or even that I have a case where I need to do this. But the term "more specific" was loose enough to make me ponder this, and now I have to know! :P
Update:
I guess I wasn't really as interested in what will happen in the above example, as in why. It came to my mind since I'd been doing stuff like
public static class CollectionExtensions
{
public static void DoSomething<T>(this T items) where T : IList { ... }
public static void DoSomething<T>(this T items) where T : IEnumerable { ... }
}
where the compiler knows to choose the first method for new List<Something>().DoSomething(), since it is "closer" to the type passed. What I was interested in then, was "what does closer in this context mean? How will the compiler react if the constraints are from two different inheritance chains? Why?"
The extensions class won't compile, in this case - you can't overload methods based solely on generic constraints.
If you put the two extension methods into different classes, then the calling code wouldn't compile - it would be an ambiguous call, as neither method would be "better" than the other... in both cases the generic type argument would be inferred as MyClass, so there'd just be two conversions from MyClass to MyClass, neither of which is better than the other.
This is basically just a special case of overloading, once you've found out that no instance methods are applicable. I wrote an article on overloading just recently which you may find useful - it doesn't call out this specific case, but it points to the relevant bits of the spec if you want to look in detail.
It will not compile at all and throw a compile time error saying call is ambiguish between the two methods.
Type 'MyExtensions' already defines a
member called 'DoSomething' with the
same parameter types.
EDIT
Here's why compiler gives such error. Extension methods are just syntactic sugars and all they do is bring fluency and readabilty on any type.
Check this code..
var instance = new MyClass();
instance.DoSomething();
Compiler replaces this code as following.
var instance = new MyClass();
MyExtensions.DoSomething(instance);
//Compiler gets confused. Which one to call IComparable or IDisposable
In your case compiler gets confused since there are two matching signatures to the method-call and it gives you the said error.
Generic constraints are not considered as part of method signature. These two methods are considered by compiler as ones with the same signature. So you will get compile error saying that method DoSomething is already defined.
public static void DoSomething<T>(this T item)
where T : IComparable
{ /* whatever */ }
public static void DoSomething<T>(this T item)
where T : IDisposable
{ /* whatever else */ }
Consider the following example:
class MyClass {}
static class MyClassExtensions
{
public static void DoSomething<T>(this T item, List<string> someList)
{
Console.WriteLine("Method with List in signature is called.");
}
public static void DoSomething<T>(this T item, IEnumerable<string> someEnumerable)
{
Console.WriteLine("Method with IEnumerable in signature is called.");
}
}
In this example, when testing with the following:
var myClass = new MyClass();
myClass.DoSomething(new List<string>());
The first method in the extensions class is called. In short, this means that the compiler determines the signature that is nearer the arguments passed, and employs that.
This question already has answers here:
Can I add extension methods to an existing static class?
(18 answers)
Closed 1 year ago.
Can extension methods be applied to the class?
For example, extend DateTime to include a Tomorrow() method that could be invoked like:
DateTime.Tomorrow();
I know I can use
static DateTime Tomorrow(this Datetime value) { //... }
Or
public static MyClass {
public static Tomorrow() { //... }
}
for a similar result, but how can I extend DateTime so that I could invoke DateTime.Tomorrow?
Use an extension method.
Ex:
namespace ExtensionMethods
{
public static class MyExtensionMethods
{
public static DateTime Tomorrow(this DateTime date)
{
return date.AddDays(1);
}
}
}
Usage:
DateTime.Now.Tomorrow();
or
AnyObjectOfTypeDateTime.Tomorrow();
You cannot add methods to an existing type unless the existing type is marked as partial, you can only add methods that appear to be a member of the existing type through extension methods. Since this is the case you cannot add static methods to the type itself since extension methods use instances of that type.
There is nothing stopping you from creating your own static helper method like this:
static class DateTimeHelper
{
public static DateTime Tomorrow
{
get { return DateTime.Now.AddDays(1); }
}
}
Which you would use like this:
DateTime tomorrow = DateTimeHelper.Tomorrow;
Extension methods are syntactic sugar for making static methods whose first parameter is an instance of type T look as if they were an instance method on T.
As such the benefit is largely lost where you to make 'static extension methods' since they would serve to confuse the reader of the code even more than an extension method (since they appear to be fully qualified but are not actually defined in that class) for no syntactical gain (being able to chain calls in a fluent style within Linq for example).
Since you would have to bring the extensions into scope with a using anyway I would argue that it is simpler and safer to create:
public static class DateTimeUtils
{
public static DateTime Tomorrow { get { ... } }
}
And then use this in your code via:
WriteLine("{0}", DateTimeUtils.Tomorrow)
The closest I can get to the answer is by adding an extension method into a System.Type object. Not pretty, but still interesting.
public static class Foo
{
public static void Bar()
{
var now = DateTime.Now;
var tomorrow = typeof(DateTime).Tomorrow();
}
public static DateTime Tomorrow(this System.Type type)
{
if (type == typeof(DateTime)) {
return DateTime.Now.AddDays(1);
} else {
throw new InvalidOperationException();
}
}
}
Otherwise, IMO Andrew and ShuggyCoUk has a better implementation.
I would do the same as Kumu
namespace ExtensionMethods
{
public static class MyExtensionMethods
{
public static DateTime Tomorrow(this DateTime date)
{
return date.AddDays(1);
}
}
}
but call it like this new DateTime().Tomorrow();
Think it makes more seens than DateTime.Now.Tomorrow();
They provide the capability to extend existing types by adding new methods with no modifications necessary to the type. Calling methods from objects of the extended type within an application using instance method syntax is known as ‘‘extending’’ methods. Extension methods are not instance members on the type.
The key point to remember is that extension methods, defined as static methods, are in scope only when the namespace is explicitly imported into your application source code via the using directive. Even though extension methods are defined as static methods, they are still called using instance syntax.
Check the full example here
http://www.dotnetreaders.com/articles/Extension_methods_in_C-sharp.net,Methods_in_C_-sharp/201
Example:
class Extension
{
static void Main(string[] args)
{
string s = "sudhakar";
Console.WriteLine(s.GetWordCount());
Console.ReadLine();
}
}
public static class MyMathExtension
{
public static int GetWordCount(this System.String mystring)
{
return mystring.Length;
}
}
I was looking for something similar - a list of constraints on classes that provide Extension Methods. Seems tough to find a concise list so here goes:
You can't have any private or protected anything - fields, methods, etc.
It must be a static class, as in public static class....
Only methods can be in the class, and they must all be public static.
You can't have conventional static methods - ones that don't include a this argument aren't allowed.
All methods must begin:
public static ReturnType MethodName(this ClassName _this, ...)
So the first argument is always the this reference.
There is an implicit problem this creates - if you add methods that require a lock of any sort, you can't really provide it at the class level. Typically you'd provide a private instance-level lock, but it's not possible to add any private fields, leaving you with some very awkward options, like providing it as a public static on some outside class, etc. Gets dicey. Signs the C# language had kind of a bad turn in the design for these.
The workaround is to use your Extension Method class as just a Facade to a regular class, and all the static methods in your Extension class just call the real class, probably using a Singleton.
Unfortunately, you can't do that. I believe it would be useful, though. It is more natural to type:
DateTime.Tomorrow
than:
DateTimeUtil.Tomorrow
With a Util class, you have to check for the existence of a static method in two different classes, instead of one.
We have improved our answer with detail explanation.Now it's more easy to understand about extension method
Extension method: It is a mechanism through which we can extend the behavior of existing class without using the sub classing or modifying or recompiling the original class or struct.
We can extend our custom classes ,.net framework classes etc.
Extension method is actually a special kind of static method that is defined in the static class.
As DateTime class is already taken above and hence we have not taken this class for the explanation.
Below is the example
//This is a existing Calculator class which have only one method(Add)
public class Calculator
{
public double Add(double num1, double num2)
{
return num1 + num2;
}
}
// Below is the extension class which have one extension method.
public static class Extension
{
// It is extension method and it's first parameter is a calculator class.It's behavior is going to extend.
public static double Division(this Calculator cal, double num1,double num2){
return num1 / num2;
}
}
// We have tested the extension method below.
class Program
{
static void Main(string[] args)
{
Calculator cal = new Calculator();
double add=cal.Add(10, 10);
// It is a extension method in Calculator class.
double add=cal.Division(100, 10)
}
}