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.
Related
Is there a recommended built-in type in C# that collections can be converted/cast to that will still allow deferred execution, but not allow the type to be cast back into IQueryable? (Like IEnumerable<T> can in some cases)
The "built in" way to guard IQueryable<T> would be Enumerable.Select like this
IQueryable<T> source = ...;
var result = source.AsEnumerable().Select(x => x);
AsEnumerable is not enough because it is a simply cast. But it's needed to ensure Enumerable.Select is used instead of Queryable.Select.
However, other than being "built in", I see no benefit of this approach and prefer the custom extension method like in another answer, although this can be used as implementation instead of iterator function (which also has no benefit, but rather a drawback due to unnecessary delegate call, so really the custom iterator function is the right way to go).
A built in type no but it's pretty easy to do :
public static class Utils
{
public static IEnumerable<T> AsDefferedEnumerable<T>(this IQueryable<T> Source)
{
foreach (var item in Source)
{
yield return item;
}
}
}
This way you're not returning a casted IQueryable (that could be casted back) but creating a new IEnumerable wrapping it , you don't even need a new type at all for that you just end up with an IEnumerable that will itself enumerate the IQueryable as it is enumerated without exposing it.
Edit : sample of a wrapper object (untested)
public class HideImplementationEnumerable<T>
{
public HideImplementationEnumerable(IEnumerable<T> Source)
{
this.Source = Source;
}
private IEnumerable<T> Source;
public IEnumerable<T> Value
{
get
{
foreach (var item in Source)
{
yield return item;
}
}
}
}
Like I see, most IEnumerable extensions eats IEnumerable and then vomits also IEnumerable. Is it possible to make an extension method, which can eat any IEnumerable (like List) and then return the same type - List?
For example, I want to iterate over collection, if something is wrong, I will throw exception, but if all is OK, I need to return same instance (same type).
You need to use two generic arguments:
public static TEnumerable Foo<TEnumerable, TItem>(this TEnumerable sequence)
where TEnumerable: IEnumerable<TItem>
{
return sequence;
}
Note that as a result of this change you're not going to be able to infer the generic arguments when invoking this method; you're going to have to specify them explicitly.
I don't see how doing this could be beneficial, but you can solve the generics part of the problem like this:
public static TEnumerable MyExtensionMethod<T, TEnumerable>(this TEnumerable enumerable)
where TEnumerable : IEnumerable<T>
{
...
}
However, you may find it difficult to actually implement such a thing because there is no generic way to create the TEnumerable you want to return.
For example, I want to iterate over collection, if something is wrong, I will throw exception, but if all is OK, I need to return same instance (same type).
Does the following extension method not accomplish what you really want? I don't understand why you want to return the same instance.
public static void Consume<T>(this IEnumerable<T> enumerable)
{
foreach (var item in enumerable)
{
// Do nothing
}
}
Got another simple question here that is eluding me.
I have 2 classes:
namespace Assets
{
public class BaseAsset
{
// Code here
}
}
And
namespace Assets
{
public class Asset : BaseAsset
{
// Code here
}
}
I have a function that returns a collection of Asset from the database and I want another function to execute that function and return a collection of BaseAsset.
I have tried this:
public static Collection<BaseAsset> GetCategoryAssets(int CategoryId, string UserId, string CompanyId)
{
return (Collection<BaseAsset>)AssetData.getAssets(CategoryId, UserId, CompanyId);
}
but as you can guess, it doesn't work.
If I was working with lists, I could do:
public static List<BaseAsset> GetCategoryAssets(int CategoryId, string UserId, string CompanyId)
{
return AssetData.getAssets(CategoryId, UserId, CompanyId).Cast<BaseAsset>().ToList();
}
But I would prefer to use a collection, can anyone come up with an elegant solution?
Cheers,
r3plica
This is a very frequently asked question. The name of the feature that you want is generic covariance; that is, the feature that says "if a giraffe is a kind of animal then a list of giraffes is a kind of list of animals."
The problem is that a list of giraffes is not a kind of list of animals. You can put a tiger into a list of animals, but you can't put a tiger into a list of giraffes, and therefore a list of giraffes cannot be used in any context where a list of animals is expected.
The reason you should use IEnumerable<T> instead of Collection<T> is because as of C# 4, IEnumerable<T> is covariant in T, provided that the type arguments provided are both reference types. That is, a sequence of strings can be used as a sequence of objects, because both are reference types. But a sequence of ints cannot be used as a sequence of objects, because one is a value type.
The reason this is safe is because there is no way to insert a tiger into an IEnumerable<Giraffe>.
If you want the ease of .ToList, just write your own .ToCollection extension method. The implementation should be straightforward - take an IEnumerable<T>, loop through it and add everything into a collection with Add.
The problem is that Collection<T> and ICollection<T> are invariant (that is, Collection<BaseAsset> is neither a subtype nor a supertype of Collection<Asset>).
The problem will be very easily solved by returning either IEnumerable<BaseAsset> or IReadOnlyList<BaseAsset> instead of Collection<BaseAsset>.
That is, you can write:
public static IEnumerable<BaseAsset> GetCategoryAssets(int CategoryId, string UserId, string CompanyId)
{
return AssetData.getAssets(CategoryId, UserId, CompanyId);
}
The cast becomes unnecessary.
In general, you should prefer interface types (such as IList<T>, IReadOnlyList<T>, ICollection<T> or IEnumerable<T>) over concrete types (Collection<T> or List<T>) when specifying return values and function parameters.
Instead of trying to cast to the base class, why not just extract an interface and use that.
Since the Collection<T> class has a constructor that takes an IList<T> as an argument, you can always do:
Collection<BaseAsset> = new Collection<BaseAsset>(
assetList.Cast<BaseAsset>().ToList());
Of course, if you need to reuse this behaviour, you could make a CastToCollection extension:
public static Collection<TResult> CastToCollection<TResult>(this IEnumerable source)
{
return new Collection<TResult>(source.Cast<TResult>().ToList());
}
OK.
I have a class MyClass and another class that is based on List. Let's call it MyCollection.
Now when someone types:
MyCollection coll = new MyCollection();
...
coll.Find(...)
They are acting on the entire collection. I want to apply some filtering - behind the scenes - so that if they write the above code, what actually executes is something like...
coll.Where(x=>x.CanSeeThis).Find(...)
What do I need to write in the definition of the MyCollection class to make this work?
Can I make this work?
You probably want to write a wrapper class that implements IList or ICollection, using a regular List internally. This wrapper class would then proxy all method calls to the internal list, applying the filter as required.
You´ve already mentioned you´ve got your own collection, probably derived from List right?
Then you´ll need to create your own method for finding:
public class MyList<T> : System.Collections.Generic.List<T>
{
public IEnumerable<T> MyFind(Predicate<T> match)
{
return this.Where(x => x.CanSeeThis).ToList().Find(match);
}
}
This unfortunatly is needed because you cannot override the Find method on List directly. You can however use the 'new' keyword to specify that If you´ve got a reference to the instance of MyList it will use that implementation of find, like below:
public new IEnumerable<T> Find(Predicate<T> match)
{
return this.Where(x => x.CanSeeThis).ToList().Find(match);
}
However the above example will yield:
MyCollection<int> collection = new ...
collection.Find(myPredicate); // <= Will use YOUR Find-method
List<int> baseTypeCollection = collection; // The above instantiated
baseTypeCollection.Find(myPredicate); // Will use List<T>.Find!
So it´s better you make you´re own method.
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(); }
}