In my project I have a class that represents a certain document type. Each class has its own properties and methods although there are some similarities between them.
I am trying to implement an extension method for each class that has the same name (method overloading) but I am getting an ambiguous call error
Code to explain further
Document Type A class representation:
static class DocType_A
{
private static XElement baseDocType_A = XmlFiles[0].Root;
// Extension Method
internal static IEnumerable<ViewData> AsViewData(this IEnumerable<XElement> result)
{
return
from doc in result select new ViewData { };
}
// Contains methods for querying through the XML file of this [DocType_A] type
internal static class Query
{
internal static IEnumerable<XElement> ByStatusOrAny(byte status = 0)
{
return baseDocType_A.Descendants("DocType_A").Select(doc => doc);
}
internal static IEnumerable<XElement> Expired()
{
return baseDocType_A.Descendants("DocType_A").Where("some conditions").Select(doc => doc);
}
}
// Represents the data needed to be displayed to the user via the DGV
internal class ViewData
{
// [DocType_A] related Property
}
internal class Counter
{
// some property
}
}
Document Type B class representation:
static class DocType_B
{
private static XElement baseDocType_B = XmlFiles[1].Root;
// Extension Method
internal static IEnumerable<ViewData> AsViewData(this IEnumerable<XElement> result)
{
return
from doc in result select new ViewData { };
}
// Contains methods for querying through the XML file of this [DocType_A] type
internal static class Query
{
internal static IEnumerable<XElement> ByStatusOrAny(byte status = 0)
{
return baseDocType_B.Descendants("DocType_B").Select(doc => doc);
}
internal static IEnumerable<XElement> Expired()
{
return baseDocType_B.Descendants("DocType_B").Where("some conditions").Select(doc => doc);
}
}
// Represents the data needed to be displayed to the user via the DGV
internal class ViewData
{
// [DocType_B] related Property
}
internal class Counter
{
// some property
}
}
Usage:
class SomeApplicationClass
{
void Method()
{
IEnumerable<DocType_A.ViewData> query = DocType_A.Query.ByStatusOrAny().AsViewData();
}
void SomeOtherMethod()
{
IEnumerable<DocType_B.ViewData> query = DocType_B.Query.ByStatusOrAny().AsViewData();
}
}
But I am getting ambiguous call error.
Is is possible to make extension method for each class with the same name?
The extension methods that you are creating aren't for Doctype_A or Doctype_B, you are creating them for the IEnumerable < XElement >. So you do actually have two extension methods with the same signature.
If you want one specific to A or B, you'd do it like this
internal static IEnumerable<XElement> ByStatusOrAny(this DocType_A doc, byte status = 0)
{
return doc.Query.Descendants("DocType_A").Select(doc => doc);
}
and then call it like
DocType_A.ByStatusOrAny().AsViewData();
I know you want it to be scoped, but that's not how it works. Whatever has the [this] qualifier is what the extension method is going to be applied to, regardless of the class it's in. You might be able to do what you want if you keep each extension method in a different namespace and only reference the namespace you want in the specific files you want, but I've never tried that, so your mileage may very.
Also, as others have pointed out, your example doesn't seem to fit the normal use case for extension methods. So if that's really what you are doing you might want to rework something.
You are thinking about extension Methods as if they were class Methods. They are not.
What they actually are is syntax sugar for a bunch of static Methods, so the compiler & Intellisense can help you calling them.
You have to functions that require a parameter of type IEnumerable<XElement>. Wich one is the compiler suppsoed to call on a IEnumerable<XElement>? For all intents of overload checking, the "this" parameters is ignored. After after that you have two functions with the same name and with any parameter types to differentiate on.
I believe that your problem is here:
internal static IEnumerable<ViewData> AsViewData(this IEnumerable<XElement> result)
You have one metod like this in DocType_B and DocType_A ... if you remove static, it should work, but I don't think this is a way to go.
Related
I have an Address class:
public class Address
{
//Some stuff
}
and there's a corresponding *Wrapper class to enforce certain rules on how to
use the Address class:
public class AddressWrapper : IWrapped<Address>
{
private Address _wrapped;
public Address GetWrapped()
{
return _wrapped;
}
//And some more
}
where IWrapped is defined as:
public interface IWrapped<T>
{
T GetWrapped();
}
I have the following generic class for saving these entities (there are other
entities that follow this pattern of Entity and EntityWrapper):
public class GenericRepository
{
private GenericRepository() { }
public static void Add<T>(IWrapped<T> entity)
{
//Do something
}
public static void AddList<T>(IList<IWrapped<T>> entities)
{
//Do something
}
}
and I have this test code:
[Test]
public void UseGenericRepository()
{
AddressWrapper addrW = new AddressWrapper();
addrW.AddrLine1 = "x";
addrW.AddrLine2 = "y";
addrW.AddrLine3 = "z";
addrW.City = "Starling City";
//This works as expected
GenericRepository.Add<Address>(addrW);
IList<AddressWrapper> addrList = new List<AddressWrapper>();
//Fill up the addrList
//This gives error: best overloaded method match has some invalid
//arguments
GenericRepository.AddList<Address>(addrList);
}
AddressWrapped is of type IWrapped<Address> (i.e., it implements it) and
Address is the type parameter being given to the AddList method, so the
types should line up. I know that this is due to my limited knowledge of C#
generics (familiar with Java generics), but can't figure out what's wrong here
--- it should work.
This probably doesn't make any difference, but here's my config:
NHibernate 4.x
.NET Framework (4.5)
This is because of the missing type variance of IList<T>. (IList<int> is not an IList<object>).
Use IEnumerable<T>, because it is covariant:
public static void AddList<T>(IEnumerable<IWrapped<T>> entities)
{
//Do something
}
Reason: If you get an instance of List<AddressWrapper>, the compiler doesn't know if it is compatible with any possible implementation of IList<IWrapped<T>>. Assume another class that implements IWrapped<T>. It wouldn't be compatible when writing to the List. Even though you don't write to the list in AddList, the compiler only accepts compatible types. IEnumerable<T> cannot be written, so it can be variant.
Not related to the question I suggest to use covariance for your own interface as well:
public interface IWrapped<out T>
to make IWrapped<Thing> compatible with IWrapped<SpecificThing>.
MSDN: https://msdn.microsoft.com/en-us/library/ee207183.aspx
To make this clear by an example. Would would you expect happen if we had two types implement IWrapped<T>?
public class AddressWrapper : IWrapped<Address>
{
private Address _wrapped;
public Address GetWrapped()
{
return _wrapped;
}
//And some more
}
public class OtherWrapper : IWrapped<MailBox>
{
public MailBox GetWrapped()
{
throw new MailBox();
}
}
And we tried to add them to a third list inside AddList<T>:
public static void AddList<T>(IList<IWrapped<T>> entities)
{
internalList = new List<IWrapped<T>>();
list.AddRange(entities); // BOOM.
}
The type system is keeping you from making a mistake. List<T> isn't covariant exactly for that reason.
At the point at which you're trying to call AddList(), for all the compiler knows, that method may add objects of any type that implements IWrapper<Address> (i.e. types that aren't AddressWrapper) to the passed in list.
That would be bad because the list you're trying to pass to the method doesn't want to contain anything other than AddressWrappers.
NB: Please see the answer by #StefanSteinegger, it is especially enlightening.
What worked for me was changing the way I was defining addrList, from:
IList<AddressWrapper> addrList = new List<AddressWrapper>();
to:
IList<IWrapped<Address>> addrList = new List<IWrapped<Address>>();
However, I am also changing the signature of the GenericRepository.AddList<T>(..) method to take an IEnumerable as that also helps indicate that the input is read-only. So:
public static void AddList<T>(IEnumerable<IWrapped<T>> entities)
{
//Do some stuff
}
I'm new to C# and I began working on a project that needed a method added to a class in C#. I found myself re examining the differences between static and instance methods and I'm unable to explain the following in a sample project.
My Core object:
namespace ExtendingObjects
{
public class MyCoreObject
{
public String name;
public String returnName()
{
return name;
}
}
}
My attempt to extend the object:
namespace ExtendingObjects
{
public static class Extensions
{
public static void addName(this MyCoreObject mco, String str)
{
mco.name=str;
}
public static String getName(this MyCoreObject mco)
{
return "test";
}
}
}
Calling program:
namespace ExtendingObjects
{
class Program
{
static void Main(string[] args)
{
MyCoreObject co = new MyCoreObject();
co.addName("test");
//Static method seems to work with instance?
String n = co.returnName();
Console.WriteLine("The name is " + n);
Console.ReadLine();
//Does not work
//Static method from a type
//String n2 = MyCoreObject.getName()
}
}
}
It was my understanding that static items stayed with the class and instance items with the instance per MSDN Static and Instance Members. However, I seem to be able to access a static method through an instance above, but not able to access a static method through a type.
Why does co.returnName() work and not MyCoreObject.getName()? I would think they would be reverse based on my reading. How can I make the getName() method available without instantiating the object first?
Thanks in advance.
Your two methods are extension methods, which are meant to look like instance methods when they're called. They can be called statically, but you need to supply the instance as the first argument, and specify the class which declares the extension method, not the type that the method "extends":
Extensions.getName(co);
When you call an extension method "as" an instance method, it's just a compiler trick. So this code:
co.addName("test");
is compiled to the exact equivalent of:
Extensions.addName(co, "test");
(As an aside, you would do well to start following normal .NET naming conventions as soon as possible. The earlier you get in the habit, the better.)
Hi I have a namespace with a lot of classes and all of them has a method Destroy(int id)
I want to call that method using dynamic word.
Here is my example:
public bool DeleteElement<T>(T tElement)
{
Type t = typeof(T);
dynamic element = tElement;
var id = element.Id;
//until here everything is fine
//here I want to say
(namespace).myClassName.Destroy(id);
//the name of myClassName is like t.ToString()
}
I can avoid namespace including a using at the top. The idea is to call that static method using dynamic, not Reflection, please see that Destroy is a static method of T. I need something like this T.Destroy(id)
If Destroy(int id) is a static method, couldn't you create an instance method that would call the static one?
public void Destroy()
{
ThisClass.Destroy(this.Id);
}
You could then define an IDestroyable interface implemented by all these classes:
interface IDestroyable { void Destroy(); }
And then modify your DeleteElement method as follows:
public bool DeleteElement<T>(T tElement) where T : IDestroyable
{
tElement.Destroy();
}
No need to use dynamic here... Actually, using dynamic in this situation is often an indication of bad design. It's quite rare to actually need dynamic except in the scenarios for which it was created (e.g. interop with dynamic languages)
(If the classes are generated but they have the partial modifier, you can declare the new method in another file that is not touched by the generator)
EDIT: if the classes are generated and are not partial, you can't modify them... So another solution would be to use reflection; I know you want to avoid that (for performance reasons I assume), but you can limit the performance impact by doing the reflection only once for each type: you just need to create and cache a delegate for each type.
class DestroyHelper<T>
{
static readonly Action<int> _destroy;
static readonly Func<T, int> _getId;
static DestroyHelper()
{
var destroyMethod = typeof(T).GetMethod("Destroy", BindingFlags.Static | BindingFlags.Public);
_destroy = (Action<int>)Delegate.CreateDelegate(typeof(Action<int>), destroyMethod);
var getIdMethod = typeof(T).GetProperty("Id").GetGetMethod();
_getId = (Func<T, int>)Delegate.CreateDelegate(typeof(Func<T, int>), getIdMethod);
}
public static void Destroy(T element)
{
_destroy(_getId(element));
}
}
If I try to declare static and non-static methods with the same parameters compiler returns an error: type 'Test' already defines a member called 'Load' with the same parameter types.
class Test
{
int i = 0;
public int I
{
get { return i; }
set { i = value; }
}
public bool Load(int newValue)
{
i = newValue;
return true;
}
public static Test Load(int newValue)
{
Test t = new Test();
t.I = newValue;
return t;
}
As far as I know these two methods can not be mixed, non static method is called on object whereas static method is called on class, so why does compiler not allow something like this and is there a way to do something similar?
If your Test class had a method like this:
public void CallLoad()
{
Load(5);
}
the compiler would not know which Load() to use. Calling a static method without the class name is entirely allowed for class members.
As for how to do something similar, I guess your best bet is to give the methods similar but different names, such as renaming the static method to LoadTest() or LoadItem().
Inside the class itself, you call both instance methods and static methods without an instance or the class name, thus making the two undistinguishable if the names and parameters are the same:
class Test
{
public void Foo()
{
Load(0); // Are you trying to call the static or the instance method?
}
// ...
}
The signature of a method is the combination of name and parameters (number and types).
In your case, your 2 methods have the same identical signature. The fact that one is static and other one is not makes no difference in accepting them as valid methods for the class.
I don't think so. if a non static method in this class calls Load(intValue). which method will be called?
Both methods have the same name, defined in the same class (scope) and with the same signature. C# does not allow this.
The problem is not related with writing this or the classname. C# specs allow you to call static methods using object instances:
AClass objectA = new AClass();
objectA.CallStaticMethod();
This code is valid so the compiler never has a way to know if you're calling a static or an instance method.
In C# a method cannot be overloaded by return type. It must at least have a different set of parameters, regardless if the method is static or not.
I try to use XDocument (XML Linq) to save and load classes. For this I have two methods:
static MyClass FromXml(XElement data); //calls 0-parameter constructor inside
public XElement ToXml();
A constructor like this
public MyClass(XElement data)
{
this = MyClass.FromXml(data);
}
does not work (says this is read only).
Can this be done somehow (without creating copying each field manually from the returned value)?
Or is the very idea wrong?
Moving the code from FromXml to constructor should work, but then saving and loading would be in two places or constructors would not be all in one place...
I don't think you want a constructor; you want a static factory method that returns type MyClass. It looks like you already have that with method FromXml. You could always write a copy constructor that takes in another instance of MyClass if you really wanted.
I think you would need something like this:
public class MyClass
{
public MyClass() {}
public MyClass(XElement data)
{
loadXml(this, data);
}
public static MyClass LoadXml(data)
{
var output = new MyClass();
loadXml(output, data);
return output;
}
private static void loadXml(MyClass classToInitialize, XElement data)
{
// your loading code goes here
}
}
You could create a non-public method static MyClass FromXml(XElement data, MyClass instance) which fills the passed-in instance using data. You can then call that from the constructor, passing this as an argument.