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C# resharper possible multiple enumeration of ienumerable

GetNames is not called twice. The implementation of IEnumerable. GetEnumerator is called each time you want to enumerate the collection with foreach. If within the IEnumerable. GetEnumerator some expensive calculation is made this might be a reason to consider. There's an SO question on how to handle this issue, and the ReSharper site also explains things here. It has some sample code that tells you to do this instead:. My question is about this specific suggestion: won't this still result in enumerating through the collection twice in the 2 for-each loops?

Then every time you enumerate foothe GetNames method is called again not literally, I can't find a link that properly explains the details, but see IEnumerable. Resharper sees this, and suggests you to store the result of enumerating GetNames in a local variable, for example by materializing it in a list:.

This will make sure that the GetNames result is only enumerated once, as long as you refer to fooEnumerated. This does matter because you usually want to enumerate only once, for example when GetNames performs a slow database call. Because you materialized the results in a list, it doesn't matter anymore that you enumerate fooEnumerated twice; you'll be iterating over an in-memory list twice.

c# resharper possible multiple enumeration of ienumerable

Yes, you'll be enumerating it twice with no doubt. Sometimes Resharper warns about: Possible multiple enumeration of IEnumerable There's an SO question on how to handle this issue, and the ReSharper site also explains things here.

c# resharper possible multiple enumeration of ienumerable

ToList ; My question is about this specific suggestion: won't this still result in enumerating through the collection twice in the 2 for-each loops? GetNames returns an IEnumerable. ToList ; This will make sure that the GetNames result is only enumerated once, as long as you refer to fooEnumerated.

Continue Reading.I've been spinning my wheels on this one for some time and I'm hoping someone can help me out. What I'd like to do it take the first k records from each group. Taking the first k of all the records is straight forward:.

But that's not what I'm after. I need is to take records from each iteration. Can someone point me in the right direction, or construct a LINQ statement as an example on how to go about this? I get a "Cannot implicitly convert type 'System. An explicit conversion exists are you missing a cast? Any help you can give to sort this last bit out is greatly appreciated.

As I've already said, you probably don't want a list here.

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You should be just fine with an IEnumerable. Bringing all of the items into a list, unless you're sure you need to do so, will be expensive with a list containing as much data as you have there.

c# resharper possible multiple enumeration of ienumerable

As for turning the sequence of anonymous types containing a grouping into a single sequence, I've already told you how to do that in an edit of my previous answer. You can simplify the code a bit if that's all you need though, so I'll just post it:.

It's worth noting here that you're doing your filtering at the very end of your query, after the ordering and grouping. If you have to then you have to, but if it's possible for you to do the filtering before you sort the data then it might perform better.

I don't think that will work because skip and take are applied to the whole list, not to each group within the list. So I'm grouping my list by iteration and then by simulated date. So if I can grab just the first from each iteration, I'll be all set to do some summary statistics on the resulting dataset. Miguel's answer is okay, but there is no need to both order and group by Iteration, you can just do a group by on it.

With the amount of data you're dealing with pulling it into a list uses a lost of memory and so should only be done if you're sure you need it. The content you requested has been removed.

Ask a question. Quick access. Search related threads. Remove From My Forums. Answered by:.It could get even worse if GetNames results in a database query, in which case you could end up getting different values in both foreach loops if some other process changes the database between the two calls.

c# resharper possible multiple enumeration of ienumerable

This kind of problem can be easily fixed — force the enumeration at the point of variable initialization by converting the sequence to an array or a list, for example:. The rest of your code can stay the same, because both array and list types implement the IEnumerable interface. Sometimes, this inspection may render false positives if the IEnumerable object is passed to some method before being enumerated. For example:. In this case, ReSharper presumes that the method will do additional enumeration, which is true in most cases.

If the method doesn't actually enumerate the IEnumerable object, you can mark the corresponding parameter with the [NoEnumerationAttribute] to prevent the false positive:. ToList. Code Inspection: Possible infinite inheritance Code Inspection: Possible unintended reference comparison.First off, that warning does not always mean so much. I usually disabled it after making sure it's not a performance bottle neck.

It just means the IEnumerable is evaluated twice, wich is usually not a problem unless the evaluation itself takes a long time. Even if it does take a long time, in this case your only using one element the first time around. It is possible to only evaluate the IEnumerable once in this case with some hassle, but profile first and see if it's really a problem. I take care to explain in the summary comments of the methods that calling IEnumerable will perform a.

The programmer can choose to. ToList at a higher level if multiple operations are being concatenated and then call the IList overload or let my IEnumerable overload take care of that.

If you only need to check the first element you can peek on it without iterating the whole collection:. If your data is always going to be repeatable, perhaps don't worry about it. Note I changed the semantic of DoSomethingElse a bit, but this is mainly to show unrolled usage. You could re-wrap the iterator, for example. You could make it an iterator block too, which could be nice; then there is no list - and you would yield return the items as you get them, rather than add to a list to be returned.

“Possible multiple enumeration of IEnumerable” issue?

However they have a huge downside that will cause problems if you try to refactor your code to use interfaces, for instance to make it more testable and friendly to dynamic proxying. In short, this is because. NET 4. But they didn't even take the opportunity to change that in. NET core. NET collections including all collection interfaces. Any other alternative will come back to bite you as you realize that you locked yourself out from some architecture choices.

So I think it's the proper type to use in function signatures to express that you just want a read-only collection. As always this is not an absolute, if you're writing database-heavy code where accidental multiple enumeration would be a disaster, you might prefer a different trade-off. Handling warning for possible multiple enumeration of IEnumerable 5 First off, that warning does not always mean so much. In this scenario you could also exploit the powerful linq extension methods even more.Achieving Exponential Slowdown by Enumerating Twice posted by Craig Gidney on November 5, In this post: the cost of iterating over a sequence multiple times.

If you enumerate an enumerable more than once, ReSharper detects that and warns you about it. Although this warning may seem pointless at first, there are two good reasons to pay attention to it. First, sometimes enumerating an enumerable is very expensive.

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For example, an enumerable might be backed by a database. Re-enumerating may force you to wait another network round trip. A straightforward way to implement that method is to enumerate the given enumerable twice: once for the odd items, and once for the even items. Seems to work fine. In practice you tend to chain lots of transformations onto the same enumerable.

Huh, that took awhile to finish. Hold on. How many passes does that say we did over the sequence? That's not good. The root cause of those million passes is OddsThenEvens using multiple enumerations. Each layer of OddsThenEvens is doing two passes of the underlying layer. Each additional layer isn't just increasing the total number of passes by two, it's multiplying the total number of passes by two.

Resharper: Possible Multiple Enumeration of IEnumerable

By the time we get to the twentieth layer that's more than a million passes. To fix our performance-when-layering problem, we need to fix the multiple enumerations problem. In the case of OddsThenEvensall we need to do is remember the even items. We just stash them into a queue, as we're yielding the odd items, for later consumption. Using a queue to stash the even items will ensure we do only one pass, while still allowing us to trickle out items as we go the caller doesn't have to wait for us to scan the whole frickin' world before they get their first frickin' item.

Here's what it looks like:. Caching items as you enumerate them the first time is a standard trick for avoiding multiple enumerations. Another common technique is smearing all your passes together, but that wouldn't have worked for OddsThenEvens. We could also have checked if the underlying sequence was a IList i. That's a rather large improvement. We've gone from a million passes to just one pass, and the improvement is directly reflected in the elapsed time it went down by six orders of magnitude.

A bit of asymptotic analysis really highlights the improvement we've made in the running time.Tag: c. I'm getting a Possible multiple enumeration of IEnumerable with Resharper and I'm trying to find out if it's really an issue or not.

This is my method:. Should be obvious, but I'm doing a search for Contact and if the search returns no results I'm returning an array of default values. The consumer should just receive a list which can be enumerated, not modified. Where is the "multiple enumeration" here? And if there is indeed one, is this not the best type to use in the situation? The multiple enumeration potential is you calling Anywhich will cause the first enumeration, and then a potential second enumeration by the caller of that method.

The warning exists because an IEnumerable can disguise something expensive such as a database call most likely an IQueryableand as IEnumerable doesn't have caching as part of it's contract, it will re-enumerate the source fresh.

This can lead to performance issues later on we have been stung by this a surprising amount and we don't even use IQueryableours was on domain model traversal. That said, it is still only a warning, and if you are aware of the potential expense of calling an enumerable over potentially slow source multiple times then you can suppress it.

Although I do remember making an IRepeatable version of IEnumerable once that internally cached as it went along. That was lost in the depths of my gratuitous code library Callbacks are independent as they're invoked on the thread-pools IO completion workers.

Resharper: Possible Multiple Enumeration of IEnumerable

If you're interested, you can see that in the source code. This particular method is for the Socket class which TcpClient and UdpClient use internallywhere overlapped IO is used to invoke the callback see the comment on Change this: [MarshalAs UnmanagedType. Other important thing Cancellation in. Net is cooperative. That means that the one holding the CancellationTokenSource signals cancellation and the one holding the CancellationToken needs to check whether cancellation was signaled either by polling the CancellationToken or by registering a delegate to run when it is signaled.

In your Task. Run you use the Yep, you need to do the adding within the loop. Add skillNameNode. You have to decide: Either maximize the form which always makes sure the TaskBar is still visible or set the dimensions manually. I suggest you remove the WindowState line. Another failsafe way would be to hide the TaskBar from your code, for example as described here I believe you need to use: SendKeys.

Else, you have to supply the type name every time, since else it can't know that you want to use that type every time Is "User Info" and "Course Data" is a different entity. If it is so, I think you may encapsulate them in one entity.

UTF8 ; writer. WriteStartDocument true ; writer.Posted by: admin February 22, Leave a comment. In several places in my code it has underlined some text and warned me that there may be a Possible multiple enumeration of IEnumerable.

c# (Csharp) and .NET :- Difference between IEnumerable and IEnumerator.

If I add. But surely, if I convert it to a List, then it will enumerate over the entire IEnumerable to build the List in the first place, and then enumerate over the List as required to find the properties i.

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Whereas in my original code, it is only doing the 3 partial enumerations. If you invoke Contains on a IEnumerable, it will invoke the extension method which will just iterate through the items in order to find it.

IList has real implementation for Contains that probably are more efficient than a regular iteration through the values it might have a search tree with hashes? February 24, c Leave a comment. This is a legacy application that I have inherited. When attempting to Edit and Questions: Closed.

Code Inspection: Possible multiple enumeration of IEnumerable

This question needs details or clarity. It is not currently accepting answers. Want to improve this question? Add details and clarify the problem by editing this post. Closed 5 days Any ideas on how to resolve this is appreciated.

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