Part A: An IEnumerable is returned from a query expression. Part B: On an IEnumerable variable, we can use the foreach-loop. This loop iterates with simple syntax. Next: Display receives an IEnumerable argument. We can pass Lists or arrays to it. Also: We can implement IEnumerable on a type to provide support for the foreach-loop. This is done through the GetEnumerator method. C program that uses IEnumerable argument using System; using System. IEnumerable works with a 2D array.
It enables a foreach-loop over the values in a 2D or jagged array. We create a custom method that returns IEnumerable. We can abstract the loop itself out. Generic: The return value is a generic IEnumerable collection of ints. We must specify the int in angle brackets. So: We can implement IEnumerable to fix this error on a class. This provides the GetEnumerator method. This example implements the IEnumerable interface on an Example class.
So: We forward the details of the implementation to the List. Our IEnumerable implementation relies on another. Foreach: The foreach-loop in the Main method implicitly secretly calls the GetEnumerator method. So "HERE" is written. Result: The program compiles.
The foreach loop refers to the List's Enumerator and loops over the elements of the List. C program that implements IEnumerable using System; using System. Collections; using System. It then uses the List's Enumerator.
Some methods, like Enumerable. Rangemake it easier to create IEnumerable collections. We do not need to create a separate array. Program: We test Enumerable. Range, which returns an IEnumerable collection. We enumerate its return value with foreach. Info: Other methods, like Enumerable. Repeat and Empty, are available.
They can be useful in certain programs.
Code Review Stack Exchange is a question and answer site for peer programmer code reviews. It only takes a minute to sign up. RemoveI figured out that there is even a simpler way:. Returns true if item is successfully removed; otherwise, false.
This method also returns false if item was not found in the List. It's a short enough method that there's not much to comment upon. You could shorten these two lines:. Consider the case where source and exceptions each consist of one value, repeated n times, e. If you don't need to preserve the order of the elements a simple LINQ query like this one would give you the same results:.
I guess am late to the party!!!! There are two things I would like to add here is. Note, using the default Except may result in your duplicate being removed e. The implementation of Except ensures duplicates are removed as a set class is used for the implementation. Hence the second Enumerable is added to the set and duplicates are removed when the first Enumerable is added. A snippet of the implementation is given below. Sign up to join this community. The best answers are voted up and rise to the top.
Home Questions Tags Users Unanswered. Removing exact instances of elements in one list from another Ask Question. Asked 3 years, 9 months ago.
Active 3 years, 9 months ago. Viewed 3k times. RemoveAt tExceptions. ExceptExact b ; Console. Der Kommissar. Der Kommissar Der Kommissar The number 6 does not occur in the source but in the result. Fixed, thanks for catching that! Arent all stack variables temporary? Active Oldest Votes. RemoveI figured out that there is even a simpler way: Removes the first occurrence of a specific object from the List.I not only want to do this within this article, but I also want to give you some further information to understand the things more deeply.
If you only want to know when to use which type, scroll down and have a look at the table providing the scenarios and the relevant types.
I strongly recommend reading of the entire article to get a deeper understanding. First of all, it is important to understand, that there are two different interfaces defined in the. NET base class library. The IEnumerable interface is located in the System. Collections namespace and contains only a single method definition. The interface definition looks like this:. The GetEnumerator method must return an instance of an object of a class which implements the IEnumerator interface.
It is important to know that the C language foreach keyword works with all types that implement the IEnumerable interface.
I believe you have been using the foreach keyword many times and without worrying about the reason why and how it worked with that type. Generic namespace :. Please take a look at the official msdn documentation if you would like to get some more information. As you can imagine, there are also two versions of ICollection which are System.
ICollection and the generic version System. ICollection inherits from IEnumerable.
How C# IEnumerable Can Kill Your Site’s Performance
You therefore have all members from the IEnumerable interface implemented in all classes that implement the ICollection interface. I just want to let you know about the official description from the msdn documentation:. In fact, we have some more methods to add, remove and clear a collection. The way synchronization was implemented differs also. I believe that this happened because the generic version of this interface was introduced with.
NET 2. NET 1. The IList interface has of course a non-generic and a generic version. We start with looking at the non-generic IList interface:. In addition it provides method definitions for adding and removing elements and to clear the collection. It also provides methods for handling the positioning of the elements within the collection. It also provides an object indexer to allow the user to access the collection with square brackets like:.
We only have some new methods for accessing a collection with specific positioning. Take a look at the following graphic. Not every interface member is displayed, but it should be enough to give you an overview about all types we discussed. Now that we have looked at all of the interfaces in question, we are ready to decide which interface we should depend on in a certain situation.
I am going to show you how this decision can be made very easily and what you can expect to gain if you do so. If you use a narrower interface type such as IEnumerable instead of IListyou protect your code against breaking changes.
If you use IEnumerablethe caller of your method can provide any object which implements the IEnumerable interface. These are nearly all collection types of the base class library and in addition many custom defined types. If you use a wider interface type such as IListyou are more in danger of breaking code changes.The following code example demonstrates how to use Take to return elements from the start of a sequence.
This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C or For Each in Visual Basic.
Take enumerates source and yields elements until count elements have been yielded or source contains no more elements. If count exceeds the number of elements in sourceall elements of source are returned. The Take and Skip methods are functional complements.
Given a sequence coll and an integer nconcatenating the results of coll. Take n and coll. Skip n yields the same sequence as coll. In Visual Basic query expression syntax, a Take clause translates to an invocation of Take. Skip to main content. Exit focus mode.
Linq Assembly: System. Returns a specified number of contiguous elements from the start of a sequence. TSource The type of the elements of source.
Is this page helpful? Yes No. Any additional feedback? Skip Submit.Part A: This Take call returns the first 2 elements in the List. This could display the 2 oldest, first elements. It operates on the result of Reverse.
Part C: The final Take call uses the top elements, which is nonsense as there are not elements in the List. Info: TakeWhile operates from the beginning of an IEnumerable collection. It can be called on the result of another method like Skip. Part 1: An integer array of 5 elements is declared. The first 3 numbers in it are odd, while the last 2 are even. Part 2: The TakeWhile extension method is invoked. It returns the first 3 odd numbers, but not the even numbers at the end. Take returns the first specified number of elements.
It acts upon an IEnumerable sequence. It returns another IEnumerable sequence containing the specified elements. Method details. Take returns the first elements of an IEnumerable. And TakeWhile, a more complex form of Take, accepts the first elements while a predicate method returns true. Extension Skip, SkipWhile.Part 3 Why and when should we use an abstract class
We use the Take method. Take is a method in the System. Linq namespace that allows you to get the first several elements from a sequence.
It receives an element count. Reverse Part C: The final Take call uses the top elements, which is nonsense as there are not elements in the List.Welcome to my blog!
I'm a Sr. All thoughts and opinions expressed in my blog and my comments are my own and do not represent the thoughts of my employer. Once again, in this series of posts I look at the parts of the. NET Framework that may seem trivial, but can help improve your code by making it easier to write and maintain. The index of all my past little wonders posts can be found here. The Skip family of methods is used to ignore items in a sequence until either a certain number are passed, or until a certain condition becomes false.
This makes the methods great for starting a sequence at a point possibly other than the first item of the original sequence. The Skip family of methods contains the following methods shown below in extension method syntax :. If you do use the form of SkipWhile which also passes an index into the predicate, then you should keep in mind that this is the index of the item in the sequence you are calling SkipWhile fromnot the index in the original collection.
For this example the result above is 2. In the code above, because Skip 2 skips 1. This same logic applies when using any of the extension methods that have an overload that allows you to pass an index into the delegate, such as SkipWhileTakeWhileSelectWhereetc.
When To Use IEnumerable, ICollection, IList And List
So why would it ever be useful to return Skip 0 deliberately? This code snippet shows three among many ways to return an internal sequence in varying levels of immutability. Finally, you can return the internal list as a sequence using Skip 0 which skips no items and just runs an iterator through the list. That is, while Skip ignores the first X items and returns the rest, Take returns a sequence of the first X items and ignores the rest.
Since they are somewhat of an inverse of each other, it makes sense that their calling signatures are identical beyond the method name obviously :. The same considerations for SkipWhile with index apply to TakeWhile with index, of course. First of all GetRange will throw if the starting index or the count are greater than the number of items in the list, but Skip and Take do not. So each has their pros and cons. Skip will ignore the specified number of items and return the rest of the sequence, whereas Take will return the specified number of items and ignore the rest of the sequence.
Similarly, the SkipWhile and TakeWhile methods can be used to skip or take items, respectively, until a given predicate returns false.
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Skip 1 ; 5: Console. WriteLine string. Join ", "afterFirstDoubleDigit. Skip 2 Skip 2.Keep up to date with the coolest technology news, analysis and reviews from industry experts. We will not share your email address. When people come up to speed in Ccollection types present options.
And, sometimes, these options confuse. This type of wondering generally brings you to the official documentation for one of the types. And, for the sake of performance, we should probably use only what we need. A good rule of thumb then would be to see if you can easily use IEnumerable, and then switch to IList when you need to do something IEnumerable fails to do.
Seems reasonable. Well, it seems reasonable until you really, truly understand both types. I think most people in the industry can relate to differences between behavior in development or test and in production.
It works on my machine! When we think of this phrase, we usually think in terms of behavior. Turned out you referenced an environment variable set on your machine but not elsewhere. But it can also apply to performance concerns.
You code something up, run it against your local database, and all seems fine. So you ship it to pre-prod or even production. Consider, for starters, the humble array. Arrays date back about as far as programming, and they represent grouped values.
They have declared and fixed capacity, so if you make a 4 element array, it will always store 4 elements. You can set those elements to different values and you can iterate over the array if you so choose, performing an operation on each element. As I mentioned, arrays are old. People have used them forever and, during that time, have bumped up against their limitations.
The property of having fixed length causes annoyance, and people find it convenient to perform quick operations like sorting and filtering duplicates. That interface demands that its implementations support operations such as adding, removing and clearing. Many implementations of IList will involve arrays at their core, decorating them with convenience functionality. So you can think of them as convenient, heavyweight arrays. No, it turns out. Not at all. When using the C IEnumerable construct, things get conceptually weird in short order.
In a sense, arrays and lists are tangible. You have a bunch of strings or integers sitting there in memory in a row, waiting for you to do things to them. Easy to work with and easy to reason about. When I return a List of fruit to you, you ask me for fruit and I hand it to you in an orderly fashion.