public static class MyClass
{
public static void Add<T>(this List<T> list, T item)
{
list.Add(item);
Console.WriteLine(item.ToString());
}
}
then
List<string> list = new List<string>(){"1","2"};
list.Add("3");
But the member method would be called.
Is there anyway to call my Extension Method this way?
I don't want to call it like this:
MyClass.Add(list, item)
You can't. Instance methods always take precedence over extension methods, assuming they're applicable. Member resolution will only consider extension methods once it's failed to find a non-extension-method option.
I would suggest you simply rename your method - unless the point was to call this method transparently with existing code.
If you made it take an IList<T> instead of List<T>, you could create a wrapper type which implements IList<T> and delegates all calls onto the wrapped list, performing any extra tasks as you go. You could then also write an extension method to IList<T> which created the wrapper - which would allow for more fluent syntax in some cases. Personally I prefer the wrapper approach to deriving a new collection type, as it means you can use it with your existing collections, making the code changes potentially smaller... but it all depends on what you're trying to do.
Instance methods always take precedence over extension methods, so no.
The correct thing to do here would appear to be polymorphism - but note that List<T> doesn't provide virtual methods. Collection<T> does, though:
using System;
using System.Collections.ObjectModel;
class MyClass<T> : Collection<T> {
protected override void InsertItem(int index, T item) {
base.InsertItem(index, item);
Console.WriteLine("Added:" + item.ToString());
}
protected override void SetItem(int index, T item) {
base.SetItem(index, item);
Console.WriteLine("Set (indexer):" + item.ToString());
}
// see also ClearItems and RemoveItem
}
Related
I want to override ToString() on IEnumerable<Int32>.
I was thinking to use Extension methods.
But when I do this below, it still calls the ToString() on System.Object. When I rename my method, then it calls my method.
As my extension method is in a static class, I am not able to override.
How can I achieve this so that my ToString() implementation is called when I call .ToString() on List<Int32> for example?
public static class ExtensionMethods
{
public static new string ToString(this IEnumerable<Int32> set)
{
var sb = new StringBuilder();
// Do some modifications on sb
return sb.ToString();
}
}
How can I achieve this so that my ToString() implementation is called when I call .ToString() on List for example?
You can't, basically. Extension methods are only used if no matching instance method can be found.
I suggest you give your method a different name, avoiding the problem - and the potential confusion your method would cause.
Note that even if extension methods were matched in preference to (say) methods declared on object, it would only make a difference for your own code being compiled with an appropriate using directive - not any other code which has already bound the call to the normal one.
If you can give more information about what you're trying to achieve, we may be able to help you more - but for the moment, something like ToDelimitedString (or whatever your method does) sounds like the best bet to me.
You cannot replace a method using extension methods.
Method resolution will check for a method belonging to the type, before trying to find matching extension methods.
In other words, you cannot replace ToString, but yes, you can create your own method.
Either create your own IEnumerable<T> type with an overridden ToString method, or use a different method name. Of course, using your own type will of course only work when you're actually using that type.
It's not possible to override ToString, but you can create a wrapper, that you can call in every place, where you are using IEnumerable<Int32>
To output a collection as string I am using an extension method
public static string ToString<T>( this IEnumerable<T> messages)
{
return ToString<T>(messages, Environment.NewLine, "" );
}
ToString<T>( this IEnumerable<T>messages, string separator, string sComment)
is described in my post
ToString function for Generic List
See also similar Overriding ToString() of List<MyClass>
Similar function implemented as an extension method described in post:
Separator Delimited ToString for Array, List, Dictionary, Generic IEnumerable
You cannot override ToString but you can create a generic method.
This is a simple solution based on Michael Freidgeim's answer (its link is broken):
static public class Extensions {
// extension for arrays, lists, any Enumerable -> AsString
public static string AsString<T>(this IEnumerable<T> enumerable) {
var sb = new StringBuilder();
int inx = 0;
foreach (var item in enumerable) {
sb.Append($"{inx}: {item}\r\n");
inx++;
}
return sb.ToString();
}
}
Usage:
Console.WriteLine(arr.AsString());
Console.WriteLine(list.AsString());
Console.WriteLine(linqResult.AsString());
I have a list that contains FrameworkElements and I want to create an extension method called MoveToTop. All this will do is accept an item that is part of that list and move it to the beginning of the list. I know this could be accomplished without the use of an extension method, but I would like it to be implemented as an extension method.
I am having trouble trying to figure out the syntax for creating an extension method that accepts a generic parameter. I know this isn't correct, but if someone could give me an idea how how to accomplish this, I would appreciate it.
public static class Extensions
{
public static void MoveToTop(this ICollection<T> sequence)
{
//logic for moving the item goes here.
}
}
You were close, just need the <T> after the method name before the parenthesis. That's where the generic type parameter list for generic methods is placed. It declares the generic type parameters the method will accept, which then makes them available to be used in the arguments, return values, and method body.
public static class Extensions
{
public static void MoveToTop<T>(this ICollection<T> sequence)
{
//logic for moving the item goes here.
}
}
I had a generic extension method
public static IList<T> Replace<T>(this IList<T> source, Ilist<T> newList) where T:IStateful
which I called using
myStatefulSetOfStuff = myStatefulSetOfStuff.Replace(GetNewSetOfStuff());
I realized, though, that my method would work on all collections that implement ICollection, so I changed it to
public static ICollection<T> Replace<T>(this ICollection<T> source, ICollection<T> newList) where T:IStateful
However, now the method returns an IColllection, which forces me to write the call as:
myStatefulSetOfStuff = myStatefulSetOfStuff.Replace(GetNewSetOfStuff()).ToList();
How can I re-write my method so that I don't need the .ToList() on my call?
EDIT:
There seemes to be some confusion, so I'll try to clear it up. I have a list. I want to perform an operation on that list with a new list. No problem. And I figured out how to return a List with an extension method.
But I realized, hey, the actual code in Replace() isn't specific to Lists, it can apply to any collection. So I modified Replace to return an ICollection. This then forces the calling method to look like
var newStuff = left.Replace(right).ToList()
or
var newStuff = left.Replace(right).ToArray()
etc.
But I don't want to say ToList, ToArray, etc., I want the method to just infer the correct return type from the source object. So I can say
var newStuff = left.Replace(right);
and newStuff will be of the same type as left. Right will be of the same type as well.
Try the following
public static TCollection Replace<TCollection, TItem>(
this TCollection source,
TCollection newList)
where TCollection : ICollection<TItem>
where TItem : IStateful
Here's a use case example
interface IStateful { }
class Foo : IStateful { }
static void Test()
{
ICollection<Foo> left = null, right= null;
left.Replace<ICollection<Foo>, Foo>(right);
}
Unfortunately the generic parameters do appear necessary in this scenario (can't get type inference to work for this specific scenario)
EDIT
My answer is based off of a bit of a misread of the question. I thought the intent was to flow the type of the source to the return type of the method. Upon further re-reading though it appears you want instead to flow any source and return an List in all cases. In which case I suggest you take a look at Reed's answer.
If you need it to always return IList<T>, just change it to:
public static IList<T> Replace<T>(this ICollection<T> source, ICollection<T> newList) where T:IStateful
And put the .ToList() call inside your extension method (unless it's already creating a list internally).
That being said, you can "nest" this by having two type parameters, if you wish to do so.
If I have these two classes:
class A {}
class B : A {}
and I make a List<A> but I want to add a List<B> to it by calling List<A>.AddRange(List<B>) but the compiler refuses:
Argument '1': cannot convert from 'System.Collections.Generic.List<A>'
to 'System.Collections.Generic.IEnumerable<B>
which I completely understand because IEnumerable<B> does not inherit from IEnumerable<A>, its generic type has the inheritance.
My solution is to enumerate through List<B> and individually add items because List<A>.Add(A item) will work with B items:
foreach(B item in listOfBItems)
{
listOfAItems.Add(item);
}
However, that's rather non-expressive because what I want is just AddRange.
I could use
List<B>.ConvertAll<A>(delegate(B item) {return (A)item;});
but that's unnecessarily convoluted and a misnomer because I'm not converting, I'm casting .
Question: If I were to write my own List-like collection what method would I add to it that would allow me to copy a collection of B's into a collection of A's as a one-liner akin to List<A>.AddRange(List<B>) and retain maximum type-safety. (And by maximum I mean that the argument is both a collection and type inhertance checking.)
Indeed, generic types are not variant right now. In C# 4.0, IEnumerable<B> will be convertible to IEnumerable<A> if B is convertible to A via a reference conversion. For some details on the design of this feature, see:
http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx
This does unfortnuately not work because generics in .net do not (yet) support covariance.
You can make a small helper method or class to overcome this issue however.
If you implement your own list class, you can add covariance using an additional generic parameter:
class MyList<T> {
void AddRange<U>(IEnumerable<U> items) where U: T {
foreach (U item in items) {
Add(item);
}
}
}
Can't you just do:
listOfAItems.AddRange(listOfBItems.Cast<A>());
I was able to achieve this using LINQ...
listOfAItems.AddRange(listOfBItems.Cast<A>());
In case you find yourself in a situation where generic types are not variant, the following extension method can make your life easier:
public static void AddRange<TList,TOther>(this List<TList> list, IEnumerable<TOther> collection) where TOther: TList {
foreach(TOther e in collection) {
list.Add(e);
}
}
Instead of having to derive from List<T> or having this method in some utility class, using it as an extension method simplifies usage. You can also profit from inference, so this formerly invalid call will become valid without any modification:
List<Animal> animals;
List<Dog> dogs;
animals.AddRange(dogs);
The only thing I can come up with is this
public class MyList<T> : List<T>
{
public void AddRange<Tother>(IEnumerable<Tother> col)
where Tother: T
{
foreach (Tother item in col)
{
this.Add(item);
}
}
}
Calling it means doing MyList<A>.AddRange<B>(MyList<B>). This fails if the argument is not enumerable or if the type inheritance doesn't work out so it satisfies my question's maximum type safety requirement.
How do I get the type of a generic typed class within the class?
An example:
I build a generic typed collection implementing ICollection< T>. Within I have methods like
public void Add(T item){
...
}
public void Add(IEnumerable<T> enumItems){
...
}
How can I ask within the method for the given type T?
The reason for my question is: If object is used as T the collection uses Add(object item) instead of Add(IEnumerable<object> enumItems) even if the parameter is IEnumerable. So in the first case it would add the whole enumerable collection as one object instead of multiple objects of the enumerable collection.
So i need something like
if (T is object) {
// Check for IEnumerable
}
but of course that cannot work in C#. Suggestions?
Thank you very much!
Michael
You can use: typeof(T)
if (typeof(T) == typeof(object) ) {
// Check for IEnumerable
}
Personally, I would side step the issue by renaming the IEnumerable<T> method to AddRange. This avoids such issues, and is consistent with existing APIs such as List<T>.AddRange.
It also keeps things clean when the T you want to add implements IEnumerable<T> (rare, I'll admit).
If you want to use the is operator in a generic class/method you have to limit T to a reference type:
public void MyMethod<T>(T theItem) where T : class
{
if (theItem is IEnumerable) { DoStuff(); }
}