So this question is about .Select() statements on a static collection (ie not the result of a Select(), Where() or other LINQ operation, for example a List of array).
I was under the impression that when using .Select() or other non-filtering, non-sorting methods, a .ElementAt() would take the element from the original collection and run it through the .Select. I saw this as the best way as the .ElementAt() only returns one element and LINQ does not cache anything, so the other generated items get thrown away.
To provide an example:
var original = Enumerable.Range(0, 1000);
var listWithADifficultSelect = original.Select(aMethod);
var onlyOneItem = listWithADifficultSelect.ElementAt(898);
object aMethod(int number) {
// Gets the item from some kind of database, difficult operation
// Takes at least a few milliseconds
return new object();
}
To see this in the bigger picture, if I have a list of 20K items and I only need the nth item, but I perform a pretty heavy .Select(), I would expect the .Select() to only project that one item from the list.
So I have a two-fold question here:
Why is this built this way?
Is there a way to build an improved .Select() that does what I want it to do?
A universal solution that would even translate well to SQL (if that's an issue) would be to use Skip and Take. You can skip the first n-1 items and then take 1 from your original IEnumerable (or IQueryable).
var original = Enumerable.Range(0, 1000);
var onlyOneItem = original.Skip(898 - 1).Take(1).Select(aMethod);
Skip and Take are Linq's equivalent of SQL's OFFSET and LIMIT.
In a simplified case like your example, you won't see any improvements to performance, but if you have an expensive query in your actual application, this way you can avoid fetching any unnecessary elements
If I understand your problem correctly, you don't want LINQ to call aMethod for the first 897 elements if you only need the 898th.
So why don't you call it like that:
var onlyOneItem = aMethod(original.ElementAt(898));
If you want to get several specific elements and just don't want LINQ to re-evaluate aMethod all the time, then turn your result into a List or array:
var listWithADifficultSelect = original.Select(aMethod).ToList(); // or ToArray();
So the Select with all its aMethod calls is only executed once and you can access all your elements without re-calling aMethod.
If you want to write your own LINQ methods that do more what you want than LINQ already does, you can easily implement your own extensions:
public static class MyLinq
{
public static IEnumerable<TResult> MySelect<TSource,TResult>(this IEnumerable<TSource>, Func<TSource,TResult> selector)
{
// implement yourself
}
public static TSource MyElementAt<TSource>(this IEnumerable<TSource>, int index)
{
// implement yourself
}
}
Related
I have some doubts over how Enumerators work, and LINQ. Consider these two simple selects:
List<Animal> sel = (from animal in Animals
join race in Species
on animal.SpeciesKey equals race.SpeciesKey
select animal).Distinct().ToList();
or
IEnumerable<Animal> sel = (from animal in Animals
join race in Species
on animal.SpeciesKey equals race.SpeciesKey
select animal).Distinct();
I changed the names of my original objects so that this looks like a more generic example. The query itself is not that important. What I want to ask is this:
foreach (Animal animal in sel) { /*do stuff*/ }
I noticed that if I use IEnumerable, when I debug and inspect "sel", which in that case is the IEnumerable, it has some interesting members: "inner", "outer", "innerKeySelector" and "outerKeySelector", these last 2 appear to be delegates. The "inner" member does not have "Animal" instances in it, but rather "Species" instances, which was very strange for me. The "outer" member does contain "Animal" instances. I presume that the two delegates determine which goes in and what goes out of it?
I noticed that if I use "Distinct", the "inner" contains 6 items (this is incorrect as only 2 are Distinct), but the "outer" does contain the correct values. Again, probably the delegated methods determine this but this is a bit more than I know about IEnumerable.
Most importantly, which of the two options is the best performance-wise?
The evil List conversion via .ToList()?
Or maybe using the enumerator directly?
If you can, please also explain a bit or throw some links that explain this use of IEnumerable.
IEnumerable describes behavior, while List is an implementation of that behavior. When you use IEnumerable, you give the compiler a chance to defer work until later, possibly optimizing along the way. If you use ToList() you force the compiler to reify the results right away.
Whenever I'm "stacking" LINQ expressions, I use IEnumerable, because by only specifying the behavior I give LINQ a chance to defer evaluation and possibly optimize the program. Remember how LINQ doesn't generate the SQL to query the database until you enumerate it? Consider this:
public IEnumerable<Animals> AllSpotted()
{
return from a in Zoo.Animals
where a.coat.HasSpots == true
select a;
}
public IEnumerable<Animals> Feline(IEnumerable<Animals> sample)
{
return from a in sample
where a.race.Family == "Felidae"
select a;
}
public IEnumerable<Animals> Canine(IEnumerable<Animals> sample)
{
return from a in sample
where a.race.Family == "Canidae"
select a;
}
Now you have a method that selects an initial sample ("AllSpotted"), plus some filters. So now you can do this:
var Leopards = Feline(AllSpotted());
var Hyenas = Canine(AllSpotted());
So is it faster to use List over IEnumerable? Only if you want to prevent a query from being executed more than once. But is it better overall? Well in the above, Leopards and Hyenas get converted into single SQL queries each, and the database only returns the rows that are relevant. But if we had returned a List from AllSpotted(), then it may run slower because the database could return far more data than is actually needed, and we waste cycles doing the filtering in the client.
In a program, it may be better to defer converting your query to a list until the very end, so if I'm going to enumerate through Leopards and Hyenas more than once, I'd do this:
List<Animals> Leopards = Feline(AllSpotted()).ToList();
List<Animals> Hyenas = Canine(AllSpotted()).ToList();
There is a very good article written by: Claudio Bernasconi's TechBlog here: When to use IEnumerable, ICollection, IList and List
Here some basics points about scenarios and functions:
A class that implement IEnumerable allows you to use the foreach syntax.
Basically it has a method to get the next item in the collection. It doesn't need the whole collection to be in memory and doesn't know how many items are in it, foreach just keeps getting the next item until it runs out.
This can be very useful in certain circumstances, for instance in a massive database table you don't want to copy the entire thing into memory before you start processing the rows.
Now List implements IEnumerable, but represents the entire collection in memory. If you have an IEnumerable and you call .ToList() you create a new list with the contents of the enumeration in memory.
Your linq expression returns an enumeration, and by default the expression executes when you iterate through using the foreach. An IEnumerable linq statement executes when you iterate the foreach, but you can force it to iterate sooner using .ToList().
Here's what I mean:
var things =
from item in BigDatabaseCall()
where ....
select item;
// this will iterate through the entire linq statement:
int count = things.Count();
// this will stop after iterating the first one, but will execute the linq again
bool hasAnyRecs = things.Any();
// this will execute the linq statement *again*
foreach( var thing in things ) ...
// this will copy the results to a list in memory
var list = things.ToList()
// this won't iterate through again, the list knows how many items are in it
int count2 = list.Count();
// this won't execute the linq statement - we have it copied to the list
foreach( var thing in list ) ...
Nobody mentioned one crucial difference, ironically answered on a question closed as a duplicated of this.
IEnumerable is read-only and List is not.
See Practical difference between List and IEnumerable
The most important thing to realize is that, using Linq, the query does not get evaluated immediately. It is only run as part of iterating through the resulting IEnumerable<T> in a foreach - that's what all the weird delegates are doing.
So, the first example evaluates the query immediately by calling ToList and putting the query results in a list.
The second example returns an IEnumerable<T> that contains all the information needed to run the query later on.
In terms of performance, the answer is it depends. If you need the results to be evaluated at once (say, you're mutating the structures you're querying later on, or if you don't want the iteration over the IEnumerable<T> to take a long time) use a list. Else use an IEnumerable<T>. The default should be to use the on-demand evaluation in the second example, as that generally uses less memory, unless there is a specific reason to store the results in a list.
The advantage of IEnumerable is deferred execution (usually with databases). The query will not get executed until you actually loop through the data. It's a query waiting until it's needed (aka lazy loading).
If you call ToList, the query will be executed, or "materialized" as I like to say.
There are pros and cons to both. If you call ToList, you may remove some mystery as to when the query gets executed. If you stick to IEnumerable, you get the advantage that the program doesn't do any work until it's actually required.
I will share one misused concept that I fell into one day:
var names = new List<string> {"mercedes", "mazda", "bmw", "fiat", "ferrari"};
var startingWith_M = names.Where(x => x.StartsWith("m"));
var startingWith_F = names.Where(x => x.StartsWith("f"));
// updating existing list
names[0] = "ford";
// Guess what should be printed before continuing
print( startingWith_M.ToList() );
print( startingWith_F.ToList() );
Expected result
// I was expecting
print( startingWith_M.ToList() ); // mercedes, mazda
print( startingWith_F.ToList() ); // fiat, ferrari
Actual result
// what printed actualy
print( startingWith_M.ToList() ); // mazda
print( startingWith_F.ToList() ); // ford, fiat, ferrari
Explanation
As per other answers, the evaluation of the result was deferred until calling ToList or similar invocation methods for example ToArray.
So I can rewrite the code in this case as:
var names = new List<string> {"mercedes", "mazda", "bmw", "fiat", "ferrari"};
// updating existing list
names[0] = "ford";
// before calling ToList directly
var startingWith_M = names.Where(x => x.StartsWith("m"));
var startingWith_F = names.Where(x => x.StartsWith("f"));
print( startingWith_M.ToList() );
print( startingWith_F.ToList() );
Play arround
https://repl.it/E8Ki/0
If all you want to do is enumerate them, use the IEnumerable.
Beware, though, that changing the original collection being enumerated is a dangerous operation - in this case, you will want to ToList first. This will create a new list element for each element in memory, enumerating the IEnumerable and is thus less performant if you only enumerate once - but safer and sometimes the List methods are handy (for instance in random access).
In addition to all the answers posted above, here is my two cents. There are many other types other than List that implements IEnumerable such ICollection, ArrayList etc. So if we have IEnumerable as parameter of any method, we can pass any collection types to the function. Ie we can have method to operate on abstraction not any specific implementation.
The downside of IEnumerable (a deferred execution) is that until you invoke the .ToList() the list can potentially change. For a really simple example of this - this would work
var persons;
using (MyEntities db = new MyEntities()) {
persons = db.Persons.ToList(); // It's mine now. In the memory
}
// do what you want with the list of persons;
and this would not work
IEnumerable<Person> persons;
using (MyEntities db = new MyEntities()) {
persons = db.Persons; // nothing is brought until you use it;
}
persons = persons.ToList(); // trying to use it...
// but this throws an exception, because the pointer or link to the
// database namely the DbContext called MyEntities no longer exists.
There are many cases (such as an infinite list or a very large list) where IEnumerable cannot be transformed to a List. The most obvious examples are all the prime numbers, all the users of facebook with their details, or all the items on ebay.
The difference is that "List" objects are stored "right here and right now", whereas "IEnumerable" objects work "just one at a time". So if I am going through all the items on ebay, one at a time would be something even a small computer can handle, but ".ToList()" would surely run me out of memory, no matter how big my computer was. No computer can by itself contain and handle such a huge amount of data.
[Edit] - Needless to say - it's not "either this or that". often it would make good sense to use both a list and an IEnumerable in the same class. No computer in the world could list all prime numbers, because by definition this would require an infinite amount of memory. But you could easily think of a class PrimeContainer which contains an
IEnumerable<long> primes, which for obvious reasons also contains a SortedList<long> _primes. all the primes calculated so far. the next prime to be checked would only be run against the existing primes (up to the square root). That way you gain both - primes one at a time (IEnumerable) and a good list of "primes so far", which is a pretty good approximation of the entire (infinite) list.
This question already has answers here:
Is there a performance impact when calling ToList()?
(10 answers)
Closed 5 years ago.
A lot of the developers I work with feel more comfortable working with a List as opposed to IEnumerable (for example). I am wondering whether there is any performance impact for ToList() overuse. For example, or, will use ToList() after ordering to get a list back out again i.e.
private void ListThinger(List<T> input)
{
input = input.OrderBy(s => s.Thing).ToList();
foreach(var thing in input)
{
// do things
}
}
My question is:
How efficient is the ToList() method? Will it create a new list and how much memory does that take, assuming the contents are POCOs? Does this change if its a value type rather than a POCO?
Will the size of the list determine efficiency or does size of list not determine cost of ToList()?
If a list is cast to an IEnumerable and then ToList() is called on it, will it just return the original object?
P.s. I understand that a single use of ToList won't break any backs, but we are building a highly concurrent system that is currently CPU bound so I am looking for little wins that, when scaled, will add up to a big improvement
How efficient is the ToList() method? Will it create a new list and how much memory does that take, assuming the contents are POCOs? Does this change if its a value type rather than a POCO?
The ToList() method materializes the given collection by creating a new list and populating it with the items of the given collection. Linq.ToList() implementation:
public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source) {
if (source == null) throw Error.ArgumentNull("source");
return new List<TSource>(source);
}
By doing so you are not gaining the power of deffered execution if needed
Will the size of the list determine efficiency or does size of list not determine cost of ToList()?
As it calls Lists copy constructor and it creates a new list then it'll work on each of the items. So it'll run in O(n) - meaning that list's size matters. MSDNs documentation about the operation of the copy constructor:
Initializes a new instance of the List class that contains elements copied from the specified collection and has sufficient capacity to accommodate the number of elements copied.
As #Jason mentioned in the comment bellow the Copy Constructor is smart and is efficient but doing it when not needed is still an O(n) operation that doesn't have to happen
If a list is cast to an IEnumerable and then ToList() is called on it, will it just return the original object?
No. It will create a new list as seen above
As for your example code:
input = input.OrderBy(s => s.Thing).ToList();
foreach(var thing in input)
{
// do things
}
As you are getting a materialized list (rather than an IQueriable/IEnumerable that might perform in deffered execution) adding the ToList after the adding gives you no benefit.
You can look here, might also help: When to use LINQ's .ToList() or .ToArray()
Yes is creates a new list. It is hard to accurately measure the memory usage but it is likely to be class size + (system word size * element count). I recommend a memory profiler.
Algorithmic efficiency of operations will of course be impacted by the element count
Yes, you get a brand new list every time. References inside are not duplicated but primitives are.
Try it yourself:
var list = new List<int>();
bool areListsTheSame = list == ((IEnumerable<int>)list).ToList();
For example, I Have such code:
IEnumerable<RSuspectOperationCode> distinctCodes = this.Distinct(); // "this" is some collection
this.Clear()
distinctCodes.Count();
As LINQ is deferred execution query - Count gives us 0. I'm interesting - is there a approach to get a distinct collection with in-place result calculation and breaking link between source and result collection, that clearing source collection won't affect result collection?
My workaround:
List<RSuspectOperationCode> distinctCodes = new List<RSuspectOperationCode>();
distinctCodes.AddRange(this.Distinct(comparer));
this.Clear();
distinctCodes.Count();
But, I wonder, is there more elegant/shorter way?
call .ToList() whenever you want it to be executed immediately.
var distinctCodes= this.Distinct().ToList();
this.Clear();
var c1 = this.Count(); // 0
var c2 = distinctCodes.Count(); // eg. 100
Instead of calling ToList after Distinct, call it before so you have deferred execution in your distinct list:
var distinctCodes = this.ToList().Distinct();
You need to call some method that will evaluate the query. In your case, calling ToArray or ToList will do what you want by evaluating the query and placing the result in a new array or List<RSuspectOperationCode>. That new list will not be affected by any changes you make to the original list.
Note that a lot of people use ToList by default, which is wrong. Regardless of the situation, if you don't intend the change the length of a list then you should use an array rather than a collection. That means that ToArray should be your default choice and only use ToList when you specifically need a List<T>. It's not a major issue but you may as well do things the right way.
I use to add .ToList() at the end of a Linq expression to get the enumeratio done, and to have a copy of the collection.
In your case,
... = this.Distinct().ToList()
should do the trick.
I have some code that filters through a collection of sorted objects according to a filter value. For instance, I want to find the objects where Name=="searchquery". Then I want to take the top X values from that collection.
My questions:
My collection is a List<T>. Does this collection guarantee the sort order?
If so, is there a built-in way to find the the top X objects that satisfy the condition? I'm looking for something like
collection.FindAll(o=>o.Name=="searchquery",100);
That would give me the top 100 objects that satisfy the condition. The reason is performance, once I've found my 100 objects, I don't want to keep checking the entire collection.
If i write:
collection.FindAll(o=>o.Name=="searchquery").Take(100);
will the runtime be intelligent enough to stop checking once it hits 100?
I can of course implement this myself, but if there is a built-in way (like a LInQ method) I'd prefer to use it.
collection.Where(o=>o.Name=="searchquery").Take(100)
The order should be in the same order as the original list, and it will stop checking once it takes 100 elements (Where returns an enumeration which is only evaluated as you take elements). From the documentation:
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.
If you need a different sort order, you will have to specify it (this of course means you have no choice but to examine all elements though).
Ok,
My collection is a List<T>. Does this collection guarantee the sort order?
No, but it will preserve the order of insertion.
If so, is there a built-in way to find the the top X objects that satisfy the condition?
someEnumerable.Where(r => r.Name == "searchquery").Take(100)
If i write:
// Some linq that works
will the runtime be intelligent enough to stop checking once it hits 100?
Yes, probably
Now, if you have a IList that has been sorted and you want to quickly iterate the top 100 items do this.
var list = sourceEnumerable.OrderBy(r => r.Name).ToList();
foreach(var r in list.Where(r => r.Name == "searchquery").Take(100))
{
// Do something
}
collection.Where(o=>o.Name=="searchquery").Take(100)
Is the most correct answer, because behind the scene Where is deferred execution, below is how Where method is implemented:
Where(this IEnumerable<T>, Func<T, bool> func)
{
foreach (var item in collection)
{
if (func(item))
{
yield return item;
}
}
}
So when calling Take(100), the loop just finds first 100 items which satisfy the criteria.
If you know for sure that the objects in your collection are not repeated (e.g.like a primary key), then you can use SortedList instead of List<T>. This will guarantee, that your list will be sorted when you filter it using a certain criteria. Have a look here for sorted list example:
http://msdn.microsoft.com/en-us/library/system.collections.sortedlist(v=vs.100).aspx
I have some doubts over how Enumerators work, and LINQ. Consider these two simple selects:
List<Animal> sel = (from animal in Animals
join race in Species
on animal.SpeciesKey equals race.SpeciesKey
select animal).Distinct().ToList();
or
IEnumerable<Animal> sel = (from animal in Animals
join race in Species
on animal.SpeciesKey equals race.SpeciesKey
select animal).Distinct();
I changed the names of my original objects so that this looks like a more generic example. The query itself is not that important. What I want to ask is this:
foreach (Animal animal in sel) { /*do stuff*/ }
I noticed that if I use IEnumerable, when I debug and inspect "sel", which in that case is the IEnumerable, it has some interesting members: "inner", "outer", "innerKeySelector" and "outerKeySelector", these last 2 appear to be delegates. The "inner" member does not have "Animal" instances in it, but rather "Species" instances, which was very strange for me. The "outer" member does contain "Animal" instances. I presume that the two delegates determine which goes in and what goes out of it?
I noticed that if I use "Distinct", the "inner" contains 6 items (this is incorrect as only 2 are Distinct), but the "outer" does contain the correct values. Again, probably the delegated methods determine this but this is a bit more than I know about IEnumerable.
Most importantly, which of the two options is the best performance-wise?
The evil List conversion via .ToList()?
Or maybe using the enumerator directly?
If you can, please also explain a bit or throw some links that explain this use of IEnumerable.
IEnumerable describes behavior, while List is an implementation of that behavior. When you use IEnumerable, you give the compiler a chance to defer work until later, possibly optimizing along the way. If you use ToList() you force the compiler to reify the results right away.
Whenever I'm "stacking" LINQ expressions, I use IEnumerable, because by only specifying the behavior I give LINQ a chance to defer evaluation and possibly optimize the program. Remember how LINQ doesn't generate the SQL to query the database until you enumerate it? Consider this:
public IEnumerable<Animals> AllSpotted()
{
return from a in Zoo.Animals
where a.coat.HasSpots == true
select a;
}
public IEnumerable<Animals> Feline(IEnumerable<Animals> sample)
{
return from a in sample
where a.race.Family == "Felidae"
select a;
}
public IEnumerable<Animals> Canine(IEnumerable<Animals> sample)
{
return from a in sample
where a.race.Family == "Canidae"
select a;
}
Now you have a method that selects an initial sample ("AllSpotted"), plus some filters. So now you can do this:
var Leopards = Feline(AllSpotted());
var Hyenas = Canine(AllSpotted());
So is it faster to use List over IEnumerable? Only if you want to prevent a query from being executed more than once. But is it better overall? Well in the above, Leopards and Hyenas get converted into single SQL queries each, and the database only returns the rows that are relevant. But if we had returned a List from AllSpotted(), then it may run slower because the database could return far more data than is actually needed, and we waste cycles doing the filtering in the client.
In a program, it may be better to defer converting your query to a list until the very end, so if I'm going to enumerate through Leopards and Hyenas more than once, I'd do this:
List<Animals> Leopards = Feline(AllSpotted()).ToList();
List<Animals> Hyenas = Canine(AllSpotted()).ToList();
There is a very good article written by: Claudio Bernasconi's TechBlog here: When to use IEnumerable, ICollection, IList and List
Here some basics points about scenarios and functions:
A class that implement IEnumerable allows you to use the foreach syntax.
Basically it has a method to get the next item in the collection. It doesn't need the whole collection to be in memory and doesn't know how many items are in it, foreach just keeps getting the next item until it runs out.
This can be very useful in certain circumstances, for instance in a massive database table you don't want to copy the entire thing into memory before you start processing the rows.
Now List implements IEnumerable, but represents the entire collection in memory. If you have an IEnumerable and you call .ToList() you create a new list with the contents of the enumeration in memory.
Your linq expression returns an enumeration, and by default the expression executes when you iterate through using the foreach. An IEnumerable linq statement executes when you iterate the foreach, but you can force it to iterate sooner using .ToList().
Here's what I mean:
var things =
from item in BigDatabaseCall()
where ....
select item;
// this will iterate through the entire linq statement:
int count = things.Count();
// this will stop after iterating the first one, but will execute the linq again
bool hasAnyRecs = things.Any();
// this will execute the linq statement *again*
foreach( var thing in things ) ...
// this will copy the results to a list in memory
var list = things.ToList()
// this won't iterate through again, the list knows how many items are in it
int count2 = list.Count();
// this won't execute the linq statement - we have it copied to the list
foreach( var thing in list ) ...
Nobody mentioned one crucial difference, ironically answered on a question closed as a duplicated of this.
IEnumerable is read-only and List is not.
See Practical difference between List and IEnumerable
The most important thing to realize is that, using Linq, the query does not get evaluated immediately. It is only run as part of iterating through the resulting IEnumerable<T> in a foreach - that's what all the weird delegates are doing.
So, the first example evaluates the query immediately by calling ToList and putting the query results in a list.
The second example returns an IEnumerable<T> that contains all the information needed to run the query later on.
In terms of performance, the answer is it depends. If you need the results to be evaluated at once (say, you're mutating the structures you're querying later on, or if you don't want the iteration over the IEnumerable<T> to take a long time) use a list. Else use an IEnumerable<T>. The default should be to use the on-demand evaluation in the second example, as that generally uses less memory, unless there is a specific reason to store the results in a list.
The advantage of IEnumerable is deferred execution (usually with databases). The query will not get executed until you actually loop through the data. It's a query waiting until it's needed (aka lazy loading).
If you call ToList, the query will be executed, or "materialized" as I like to say.
There are pros and cons to both. If you call ToList, you may remove some mystery as to when the query gets executed. If you stick to IEnumerable, you get the advantage that the program doesn't do any work until it's actually required.
I will share one misused concept that I fell into one day:
var names = new List<string> {"mercedes", "mazda", "bmw", "fiat", "ferrari"};
var startingWith_M = names.Where(x => x.StartsWith("m"));
var startingWith_F = names.Where(x => x.StartsWith("f"));
// updating existing list
names[0] = "ford";
// Guess what should be printed before continuing
print( startingWith_M.ToList() );
print( startingWith_F.ToList() );
Expected result
// I was expecting
print( startingWith_M.ToList() ); // mercedes, mazda
print( startingWith_F.ToList() ); // fiat, ferrari
Actual result
// what printed actualy
print( startingWith_M.ToList() ); // mazda
print( startingWith_F.ToList() ); // ford, fiat, ferrari
Explanation
As per other answers, the evaluation of the result was deferred until calling ToList or similar invocation methods for example ToArray.
So I can rewrite the code in this case as:
var names = new List<string> {"mercedes", "mazda", "bmw", "fiat", "ferrari"};
// updating existing list
names[0] = "ford";
// before calling ToList directly
var startingWith_M = names.Where(x => x.StartsWith("m"));
var startingWith_F = names.Where(x => x.StartsWith("f"));
print( startingWith_M.ToList() );
print( startingWith_F.ToList() );
Play arround
https://repl.it/E8Ki/0
If all you want to do is enumerate them, use the IEnumerable.
Beware, though, that changing the original collection being enumerated is a dangerous operation - in this case, you will want to ToList first. This will create a new list element for each element in memory, enumerating the IEnumerable and is thus less performant if you only enumerate once - but safer and sometimes the List methods are handy (for instance in random access).
In addition to all the answers posted above, here is my two cents. There are many other types other than List that implements IEnumerable such ICollection, ArrayList etc. So if we have IEnumerable as parameter of any method, we can pass any collection types to the function. Ie we can have method to operate on abstraction not any specific implementation.
The downside of IEnumerable (a deferred execution) is that until you invoke the .ToList() the list can potentially change. For a really simple example of this - this would work
var persons;
using (MyEntities db = new MyEntities()) {
persons = db.Persons.ToList(); // It's mine now. In the memory
}
// do what you want with the list of persons;
and this would not work
IEnumerable<Person> persons;
using (MyEntities db = new MyEntities()) {
persons = db.Persons; // nothing is brought until you use it;
}
persons = persons.ToList(); // trying to use it...
// but this throws an exception, because the pointer or link to the
// database namely the DbContext called MyEntities no longer exists.
There are many cases (such as an infinite list or a very large list) where IEnumerable cannot be transformed to a List. The most obvious examples are all the prime numbers, all the users of facebook with their details, or all the items on ebay.
The difference is that "List" objects are stored "right here and right now", whereas "IEnumerable" objects work "just one at a time". So if I am going through all the items on ebay, one at a time would be something even a small computer can handle, but ".ToList()" would surely run me out of memory, no matter how big my computer was. No computer can by itself contain and handle such a huge amount of data.
[Edit] - Needless to say - it's not "either this or that". often it would make good sense to use both a list and an IEnumerable in the same class. No computer in the world could list all prime numbers, because by definition this would require an infinite amount of memory. But you could easily think of a class PrimeContainer which contains an
IEnumerable<long> primes, which for obvious reasons also contains a SortedList<long> _primes. all the primes calculated so far. the next prime to be checked would only be run against the existing primes (up to the square root). That way you gain both - primes one at a time (IEnumerable) and a good list of "primes so far", which is a pretty good approximation of the entire (infinite) list.