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I've searched around and haven't really found a clear answer as to when you'd want to use .First and when you'd want to use .FirstOrDefault with LINQ.
When would you want to use .First? Only when you'd want to catch the exception if no results where returned?
var result = List.Where(x => x == "foo").First();
And when would you want to use .FirstOrDefault? When you'd always want the default type if no result?
var result = List.Where(x => x == "foo").FirstOrDefault();
And for that matter, what about Take?
var result = List.Where(x => x == "foo").Take(1);
I would use First() when I know or expect the sequence to have at least one element. In other words, when it is an exceptional occurrence that the sequence is empty.
Use FirstOrDefault() when you know that you will need to check whether there was an element or not. In other words, when it is legal for the sequence to be empty. You should not rely on exception handling for the check. (It is bad practice and might hurt performance).
Finally, the difference between First() and Take(1) is that First() returns the element itself, while Take(1) returns a sequence of elements that contains exactly one element.
.First will throw an exception when there are no results. .FirstOrDefault won't, it will simply return either null (reference types) or the default value of the value type. (e.g like 0 for an int.) The question here is not when you want the default type, but more: Are you willing to handle an exception or handle a default value? Since exceptions should be exceptional, FirstOrDefault is preferred when you're not sure if you're going to get results out of your query. When logically the data should be there, exception handling can be considered.
Skip() and Take() are normally used when setting up paging in results. (Like showing the first 10 results, and the next 10 on the next page, etc.)
.First() will throw an exception if there's no row to be returned, while .FirstOrDefault() will return the default value (NULL for all reference types) instead.
So if you're prepared and willing to handle a possible exception, .First() is fine. If you prefer to check the return value for != null anyway, then .FirstOrDefault() is your better choice.
But I guess it's a bit of a personal preference, too. Use whichever makes more sense to you and fits your coding style better.
First()
Returns first element of a sequence.
It throw an error when There is no element in the result or source is null.
you should use it,If more than one element is expected and you want only first element.
FirstOrDefault()
Returns first element of a sequence, or a default value if no element is found.
It throws an error Only if the source is null.
you should use it, If more than one element is expected and you want only first element.
Also good if result is empty.
We have an UserInfos table, which have some records as shown below. On the basis of this table below I have created example...
How to use First()
var result = dc.UserInfos.First(x => x.ID == 1);
There is only one record where ID== 1. Should return this record
ID: 1 First Name: Manish Last Name: Dubey Email: xyz#xyz.com
var result = dc.UserInfos.First(x => x.FName == "Rahul");
There are multiple records where FName == "Rahul". First record should be return.
ID: 7 First Name: Rahul Last Name: Sharma Email: xyz1#xyz.com
var result = dc.UserInfos.First(x => x.ID ==13);
There is no record with ID== 13. An error should be occur.
InvalidOperationException: Sequence contains no elements
How to Use FirstOrDefault()
var result = dc.UserInfos.FirstOrDefault(x => x.ID == 1);
There is only one record where ID== 1. Should return this record
ID: 1 First Name: Manish Last Name: Dubey Email: xyz#xyz.com
var result = dc.UserInfos.FirstOrDefault(x => x.FName == "Rahul");
There are multiple records where FName == "Rahul". First record should be return.
ID: 7 First Name: Rahul Last Name: Sharma Email: xyz1#xyz.com
var result = dc.UserInfos.FirstOrDefault(x => x.ID ==13);
There is no record with ID== 13. The return value is null
Hope it will help you to understand when to use First() or FirstOrDefault().
First of all, Take is a completely different method. It returns an IEnumerable<T> and not a single T, so that's out.
Between First and FirstOrDefault, you should use First when you're sure that an element exists and if it doesn't, then there's an error.
By the way, if your sequence contains default(T) elements (e.g. null) and you need to distinguish between being empty and the first element being null, you can't use FirstOrDefault.
First:
Returns the first element of a sequence
Throws exception: There are no elements in the result
Use when: When more than 1 element is expected and you want only the first
FirstOrDefault:
Returns the first element of a sequence, or a default value if no element is found
Throws exception: Only if the source is null
Use when: When more than 1 element is expected and you want only the first. Also it is ok for the result to be empty
From: http://www.technicaloverload.com/linq-single-vs-singleordefault-vs-first-vs-firstordefault/
Another difference to note is that if you're debugging an application in a Production environment you might not have access to line numbers, so identifying which particular .First() statement in a method threw the exception may be difficult.
The exception message will also not include any Lambda expressions you might have used which would make any problem even are harder to debug.
That's why I always use FirstOrDefault() even though I know a null entry would constitute an exceptional situation.
var customer = context.Customers.FirstOrDefault(i => i.Id == customerId);
if (customer == null)
{
throw new Exception(string.Format("Can't find customer {0}.", customerId));
}
First()
When you know that result contain more than 1 element expected and you should only the first element of sequence.
FirstOrDefault()
FirstOrDefault() is just like First() except that, if no element match the specified condition than it returns default value of underlying type of generic collection. It does not throw InvalidOperationException if no element found. But collection of element or a sequence is null than it throws an exception.
This type of the function belongs to element operators. Some useful element operators are defined below.
First/FirstOrDefault
Last/LastOrDefault
Single/SingleOrDefault
We use element operators when we need to select a single element from a sequence based on a certain condition. Here is an example.
List<int> items = new List<int>() { 8, 5, 2, 4, 2, 6, 9, 2, 10 };
First() operator returns the first element of a sequence after satisfied the condition. If no element is found then it will throw an exception.
int result = items.Where(item => item == 2).First();
FirstOrDefault() operator returns the first element of a sequence after satisfied the condition. If no element is found then it will return default value of that type.
int result1 = items.Where(item => item == 2).FirstOrDefault();
I found a website that apperars to explain the need for FirstOrDefault
http://thepursuitofalife.com/the-linq-firstordefault-method-and-null-resultsets/
If there are no results to a query, and you want to to call First() or Single() to get a single row... You will get an “Sequence contains no elements” exception.
Disclaimer: I have never used LINQ, so my apologies if this is way off the mark.
Others have very well described the difference between First() and FirstOrDefault(). I want to take a further step in interpreting the semantics of these methods. In my opinion FirstOrDefault is being overused a lot. In the majority of the cases when you’re filtering data you would either expect to get back a collection of elements matching the logical condition or a single unique element by its unique identifier – such as a user, book, post etc... That’s why we can even get as far as saying that FirstOrDefault() is a code smell not because there is something wrong with it but because it’s being used way too often. This blog post explores the topic in details. IMO most of the times SingleOrDefault() is a much better alternative so watch out for this mistake and make sure you use the most appropriate method that clearly represents your contract and expectations.
someList.First(); // exception if collection is empty.
someList.FirstOrDefault(); // first item or default(Type)
Which one to use?
It should be decided by the business logic, and not the fear of exception/programm failure.
For instance,
If business logic says that we can not have zero transactions on any working day (Just assume). Then you should not try to handle this scenario with some smart programming.
I will always use First() over such collection, and let the program fail if something else screwed up the business logic.
Code:
var transactionsOnWorkingDay = GetTransactionOnLatestWorkingDay();
var justNeedOneToProcess = transactionsOnWorkingDay.First(): //Not FirstOrDefault()
I would like to see others comments over this.
Ok let me give my two cents.
First / Firstordefault are for when you use the second constructor. I won't explain what it is, but it's when you would potentially always use one because you don't want to cause an exception.
person = tmp.FirstOrDefault(new Func<Person, bool>((p) =>
{
return string.IsNullOrEmpty(p.Relationship);
}));
linq many ways to implement single simple query on collections, just we write joins in sql, a filter can be applied first or last depending on the need and necessity.
Here is an example where we can find an element with a id in a collection.
To add more on this, methods First, FirstOrDefault, would ideally return same when a collection has at least one record. If, however, a collection is okay to be empty. then First will return an exception but FirstOrDefault will return null or default. For instance, int will return 0. Thus usage of such is although said to be personal preference, but its better to use FirstOrDefault to avoid exception handling.
I have 2 LINQ queries, one returns what I expect, the other doesn't and i'm trying to understand why. I'm trying to figure out if from all the nodes in Config, is there a node that is called "TEST" and its Selected attribute is True.
Query 1 - Which returns the right thing has the condition inside Any():
var res1 =
(from config in _config.CurrentSettings.Config let name = config.name select config).Any(
config => config.name.Equals("TEST") && config.selected == true);
Query 2, which fails, has the condition inside Select:
(_config.CurrentSettings.Config.Select(config => config.name.Equals("TEST") && config.selected))
.Any();
LINQ's Any() with no condition means "has at least one row". The first query specifies a condition, making it "has at least one row matching the condition".
To make the second query equivalent to first one, use Any(flag => flag), or replace Select with Where. Both these options are inferior to the initial Any, with the condition inside, because they are not as readable.
The whole thing here is useless.
(from config in _config.CurrentSettings.Config let name = config.name select config)
You can narrow it down to this
_config.CurrentSettings.Config.Any(config => config.name.Equals("TEST") && config.selected == true);
Which will perform same as your first block of code.
The Select methods converts an ienumerable into another form using a selector
you give.
parameter-less Any returns true if sequence contains any element. otherwise returns false.
You may want to try:
var isTestSelected = _config.CurrentSettings.Config.Any(config => config.name.Equals("TEST") && config.selected);
Looking at the documentation for the Select and Any methods, may provide some insight.
If you look at the Parameters section of each of those links, you'll see that while both methods accept a Func, their usage is actually different.
Any
predicate
Type: System.Func<TSource, Boolean> A function to test each
element for a condition.
Select
selector
Type: System.Func<TSource, TResult> A transform function to
apply to each element.
So, in your Query1 example, you're applying a transform (or map) to each item in your collection, which declares a local name variable
let name = config.name (which is not being used), and then just returns the object as is (without actually transforming anything) select config. This bit of code is superfluous, and can be removed.
Your Any() lambda is doing all the work in Query1, by filtering items that don't match your lambda predicate.
In Query2, you're passing a filtering lambda to a transform function, and then using a filtering function without a filter.
There are many different ways to get your desired result using Linq. I encourage you to look at the different mapping (selector) and filtering (predicate) functions provided by the framework (link).
I have been searching for a way to do the following with a faster EF query :
using (DAL.MandatsDatas db = new DAL.MandatsDatas())
{
if(db.ARTICLE.Any( t => t.condition == condition))
oneArticle = db.ARTICLE.First( t => t.condition == condition);
}
It works fine, but the more i add of these, the slower it feels.
It just looks like it goes through all the rows 2 times (i don't know if it's the case)
I've been searching, saw people using the count() > 0 and other irrelevant stuff...
Is there a faster way to check if someting exist and then take it.
Also i was wondering if the FirstOrDefault() could help my case, how does it work ?
Yes, FirstOrDefault is better here:
oneArticle = db.ARTICLE.FirstOrDefault(t => t.condition == condition);
Basically Any will do one select, and then First will do one more. While FirstOrDefault will do the same First does, and just return null if there was no output, thus eliminating the need to run another selection operation.
Yes, FirstOrDefault will be faster because it will only query once. The way it works is if no rows where available it will return null, if there was rows available it returns the fist row based on any ordering you applied (if any).
I've decided to take a quick look into the LINQ side of things, as opposed to just using a straight up foreach loop, but i'm having some trouble getting it to work, mainly due to datatypes i believe.
So i've got this, so far;
var selectedSiteType = from sites in siteTypeList
where sites.SiteTypeID == temp
select sites;
siteTypeList is a list of SiteTypes. I'm trying to find a particular one (Which i've denounced with variable "temp".
How do i then use this selected SiteType AS a SiteType? When i try and pass "selectedSiteType" through to another function, like so;
mSiteTypeSub.EditSitetype(selectedSiteType);
note: I tried with providing an index, as if selectedSiteType was a list / Array, but that didnt work either, i get the following error:
Argument 1: cannot convert from
'System.Collections.Generic.IEnumerable<DeviceManager_take_2.SiteType>' to
'DeviceManager_take_2.SiteType'
Am i missing something? perhaps a cast of some kind? Like i said i'm new to this and am struggling to get my head around this. Chances are i've got the whole concept wrong and bingbangbosh i've made a fool of myself!
Cheers in advance.
Use First / FirstOrDefault / Single / SingleOrDefault to get an item of the particular type from the collection.
var value = selectedSiteType.First();
// returns the first item of the collection
var value = selectedSiteType.FirstOrDefault();
// returns the first item of the collection or null if none exists
var value = selectedSiteType.Single();
// returns the only one item of the collection, exception is thrown if more then one exists
var value = selectedSiteType.SingleOrDefault();
// returns the only item from the collection or null, if none exists. If the collection contains more than one item, an exception is thrown.
If your return type is a single:
var selectedSiteType = (from sites in siteTypeList
where sites.SiteTypeID == temp
select sites).SingleOrDefault();
If a list (potentially more than one item):
var selectedSiteType = (from sites in siteTypeList
where sites.SiteTypeID == temp
select sites).ToList();
It's the SingleOrDefault / ToList that you're missing from your query.
Shane,
I'm not going to improve on the previous answers. They were both correct. I am going to try and explain a little bit to you, so that you understand it in the future a bit better.
What happens, when you write a piece of code like:
var selectedSiteType = from sites in siteTypeList
where sites.SiteTypeID == temp
select sites;
you don't put the answer into the var (selectedSiteType), instead, you are creating an expression tree, that is evaluated ONLY when you actually use it (in a foreach, or by calling one of the methods (like .First(), .ToList(), SingleOrDefault(), etc).
The default return type of a from statement, is an IEnumerable<>, but if you call the .First() or .SingleOrDefault() (etc), you will dive into that IEnumerable<> and get a specific item.
I hope this helps you better understand what is going on.
Lemme know if I can add anything or if I got anything wrong.
Cheers,
Max
I've searched around and haven't really found a clear answer as to when you'd want to use .First and when you'd want to use .FirstOrDefault with LINQ.
When would you want to use .First? Only when you'd want to catch the exception if no results where returned?
var result = List.Where(x => x == "foo").First();
And when would you want to use .FirstOrDefault? When you'd always want the default type if no result?
var result = List.Where(x => x == "foo").FirstOrDefault();
And for that matter, what about Take?
var result = List.Where(x => x == "foo").Take(1);
I would use First() when I know or expect the sequence to have at least one element. In other words, when it is an exceptional occurrence that the sequence is empty.
Use FirstOrDefault() when you know that you will need to check whether there was an element or not. In other words, when it is legal for the sequence to be empty. You should not rely on exception handling for the check. (It is bad practice and might hurt performance).
Finally, the difference between First() and Take(1) is that First() returns the element itself, while Take(1) returns a sequence of elements that contains exactly one element.
.First will throw an exception when there are no results. .FirstOrDefault won't, it will simply return either null (reference types) or the default value of the value type. (e.g like 0 for an int.) The question here is not when you want the default type, but more: Are you willing to handle an exception or handle a default value? Since exceptions should be exceptional, FirstOrDefault is preferred when you're not sure if you're going to get results out of your query. When logically the data should be there, exception handling can be considered.
Skip() and Take() are normally used when setting up paging in results. (Like showing the first 10 results, and the next 10 on the next page, etc.)
.First() will throw an exception if there's no row to be returned, while .FirstOrDefault() will return the default value (NULL for all reference types) instead.
So if you're prepared and willing to handle a possible exception, .First() is fine. If you prefer to check the return value for != null anyway, then .FirstOrDefault() is your better choice.
But I guess it's a bit of a personal preference, too. Use whichever makes more sense to you and fits your coding style better.
First()
Returns first element of a sequence.
It throw an error when There is no element in the result or source is null.
you should use it,If more than one element is expected and you want only first element.
FirstOrDefault()
Returns first element of a sequence, or a default value if no element is found.
It throws an error Only if the source is null.
you should use it, If more than one element is expected and you want only first element.
Also good if result is empty.
We have an UserInfos table, which have some records as shown below. On the basis of this table below I have created example...
How to use First()
var result = dc.UserInfos.First(x => x.ID == 1);
There is only one record where ID== 1. Should return this record
ID: 1 First Name: Manish Last Name: Dubey Email: xyz#xyz.com
var result = dc.UserInfos.First(x => x.FName == "Rahul");
There are multiple records where FName == "Rahul". First record should be return.
ID: 7 First Name: Rahul Last Name: Sharma Email: xyz1#xyz.com
var result = dc.UserInfos.First(x => x.ID ==13);
There is no record with ID== 13. An error should be occur.
InvalidOperationException: Sequence contains no elements
How to Use FirstOrDefault()
var result = dc.UserInfos.FirstOrDefault(x => x.ID == 1);
There is only one record where ID== 1. Should return this record
ID: 1 First Name: Manish Last Name: Dubey Email: xyz#xyz.com
var result = dc.UserInfos.FirstOrDefault(x => x.FName == "Rahul");
There are multiple records where FName == "Rahul". First record should be return.
ID: 7 First Name: Rahul Last Name: Sharma Email: xyz1#xyz.com
var result = dc.UserInfos.FirstOrDefault(x => x.ID ==13);
There is no record with ID== 13. The return value is null
Hope it will help you to understand when to use First() or FirstOrDefault().
First of all, Take is a completely different method. It returns an IEnumerable<T> and not a single T, so that's out.
Between First and FirstOrDefault, you should use First when you're sure that an element exists and if it doesn't, then there's an error.
By the way, if your sequence contains default(T) elements (e.g. null) and you need to distinguish between being empty and the first element being null, you can't use FirstOrDefault.
First:
Returns the first element of a sequence
Throws exception: There are no elements in the result
Use when: When more than 1 element is expected and you want only the first
FirstOrDefault:
Returns the first element of a sequence, or a default value if no element is found
Throws exception: Only if the source is null
Use when: When more than 1 element is expected and you want only the first. Also it is ok for the result to be empty
From: http://www.technicaloverload.com/linq-single-vs-singleordefault-vs-first-vs-firstordefault/
Another difference to note is that if you're debugging an application in a Production environment you might not have access to line numbers, so identifying which particular .First() statement in a method threw the exception may be difficult.
The exception message will also not include any Lambda expressions you might have used which would make any problem even are harder to debug.
That's why I always use FirstOrDefault() even though I know a null entry would constitute an exceptional situation.
var customer = context.Customers.FirstOrDefault(i => i.Id == customerId);
if (customer == null)
{
throw new Exception(string.Format("Can't find customer {0}.", customerId));
}
First()
When you know that result contain more than 1 element expected and you should only the first element of sequence.
FirstOrDefault()
FirstOrDefault() is just like First() except that, if no element match the specified condition than it returns default value of underlying type of generic collection. It does not throw InvalidOperationException if no element found. But collection of element or a sequence is null than it throws an exception.
This type of the function belongs to element operators. Some useful element operators are defined below.
First/FirstOrDefault
Last/LastOrDefault
Single/SingleOrDefault
We use element operators when we need to select a single element from a sequence based on a certain condition. Here is an example.
List<int> items = new List<int>() { 8, 5, 2, 4, 2, 6, 9, 2, 10 };
First() operator returns the first element of a sequence after satisfied the condition. If no element is found then it will throw an exception.
int result = items.Where(item => item == 2).First();
FirstOrDefault() operator returns the first element of a sequence after satisfied the condition. If no element is found then it will return default value of that type.
int result1 = items.Where(item => item == 2).FirstOrDefault();
I found a website that apperars to explain the need for FirstOrDefault
http://thepursuitofalife.com/the-linq-firstordefault-method-and-null-resultsets/
If there are no results to a query, and you want to to call First() or Single() to get a single row... You will get an “Sequence contains no elements” exception.
Disclaimer: I have never used LINQ, so my apologies if this is way off the mark.
Others have very well described the difference between First() and FirstOrDefault(). I want to take a further step in interpreting the semantics of these methods. In my opinion FirstOrDefault is being overused a lot. In the majority of the cases when you’re filtering data you would either expect to get back a collection of elements matching the logical condition or a single unique element by its unique identifier – such as a user, book, post etc... That’s why we can even get as far as saying that FirstOrDefault() is a code smell not because there is something wrong with it but because it’s being used way too often. This blog post explores the topic in details. IMO most of the times SingleOrDefault() is a much better alternative so watch out for this mistake and make sure you use the most appropriate method that clearly represents your contract and expectations.
someList.First(); // exception if collection is empty.
someList.FirstOrDefault(); // first item or default(Type)
Which one to use?
It should be decided by the business logic, and not the fear of exception/programm failure.
For instance,
If business logic says that we can not have zero transactions on any working day (Just assume). Then you should not try to handle this scenario with some smart programming.
I will always use First() over such collection, and let the program fail if something else screwed up the business logic.
Code:
var transactionsOnWorkingDay = GetTransactionOnLatestWorkingDay();
var justNeedOneToProcess = transactionsOnWorkingDay.First(): //Not FirstOrDefault()
I would like to see others comments over this.
Ok let me give my two cents.
First / Firstordefault are for when you use the second constructor. I won't explain what it is, but it's when you would potentially always use one because you don't want to cause an exception.
person = tmp.FirstOrDefault(new Func<Person, bool>((p) =>
{
return string.IsNullOrEmpty(p.Relationship);
}));
linq many ways to implement single simple query on collections, just we write joins in sql, a filter can be applied first or last depending on the need and necessity.
Here is an example where we can find an element with a id in a collection.
To add more on this, methods First, FirstOrDefault, would ideally return same when a collection has at least one record. If, however, a collection is okay to be empty. then First will return an exception but FirstOrDefault will return null or default. For instance, int will return 0. Thus usage of such is although said to be personal preference, but its better to use FirstOrDefault to avoid exception handling.