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LINQ - Using FirstOrDefault or DefaultIfEmpty?

I'm getting confused about how to use FirstOrDefault or DefaultIfEmpty.
The snippet below may be empty, but if it's not, I definitely want the first one.
var vThr = _context.PostThrs.FirstOrDefault(m =>
m.ThrZero == zero
&& m.ThrText.Substring(0,8) == "SERVICE-");
If it is empty, I would like the result to be "Empty". How would I do that?
I've taken some stabs at it, but I'm not sure that it's helpful to share.
EDIT: After posting, I realized that the question doesn't really work as you cannot insert a single string into the result.

Summarize between:
.FirstOrDefault()
.DefaultIfEmpty()
Query with the result of the first item that fulfills.
Query with the result of IEnumerable. Use to initialize a default item if the sequence is empty.
- If there is item(s) fulfilled, return the first T item.
If there is item(s) fulfilled, return at least one or more T items as IEnumerable<T>.
- If not, returns default.
- If not, the defaultValue parameter is used to initialize into IEnumerable<T>. Returns an IEnumerable<T> with a single item (Count = 1).
So, based on your requirement, you are looking for .FirstOrDefault() to check the returned result is null and perform the following implementation.
Didn't cover the part that you want to assign an "Empty" string to the variable when null and you found out that it is not feasible to do as the variable is T which will conflict with the type.
References
.FirstOrDefault()
.DefaultIfEmpty()

Use FirstOrDefault() it will find first matched by condition element but if not will return just null

Access database with LINQ, datatype conflict

I am trying to access data from my database with LINQ but I am running into a data type error.
Here is my code:
public static String GetCheckIfCsIsRunning()
{
using (Entities db = new Entities())
{
Stringl status = (from stat in db.Messenger_Settings
where stat.Id == 1
select stat.SettingValue);
return status;
}
}
I am currently getting an error at
where stat.Id == 1
saying
Cannot implicitly convert type 'int' to 'bool'
I am trying to select from my table the ID of the row but the line of code is saying the Id is type bool? Perhaps the 'status' variable of the code creates 'stat' into a bool type. But how would I select the row according to Id?
my table structure is
ID(int) | Name(varchar) | SettingValue(varchar)
EDIT
I forgot to add an extra '=' ('=' --> '==')
EDIT-2
replaced int's with bool's new error occurs
- data I want to access is varchar (select stat.SettingValue)
Cannot implicitly convert type 'System.LINQ.IQueryable,string.' to 'string'

You're mistakenly using an assignment operator (=) in place of an equality operator (==).
Your line
where stat.Id = 1
actually assigns 1 to stat.ID, not what you want at all; then, the assignment returns the value assigned, in this case 1. Since C# won't implicitly convert an int to bool, you get the error you see. What you want to do is instead use ==, the equality operator, which will do what you expect: check if the value of stat.Id is 1 and return true or false.
Also, for future reference, you can avoid accidental assignment errors like this by using a different programming idiom: put the constant first, i.e.
1 = stat.Id
While you'd still get an error either way here, in other cases where you do an accidental assignment, or in languages that will implicitly convert int to bool (which would be a runtime error that's hard to track down), you'll instead get an error, since you can't assign a value to a numeric literal like 1.
Per your edit, LINQ always returns a "lazy-loaded" query. As soon as you try to access any elements in that query, it enumerates it. An IQueryable is the object representation of that query. Since we know that the query represents a sequence of strings (0 or more), we can call one of a few methods against IQueryable<string> to get at the results:
.First() will return the first element in the sequence, and throw if the sequence has no elements.
.FirstOrDefault() will return the first element in the sequence, or a default value if the sequence contains no elements.
.Single() will return the element if there is only one element in the sequence, and throw otherwise (if there are zero or more than one element).
.SingleOrDefault() will return the element if there is only one element in the sequence, a default value if there are no elements, or throw if there are more than one.
Which one to choose depends on your database schema as well as your application's desired behavior. If you know there will only ever be one and exactly one value for a setting, use .Single(), so that you can error out and detect if you get several results. If there might be a value for a setting (or it might not be defined), use .SingleOrDefault(). And if a setting could have several values in the table, use .First() or .FirstOrDefault().
In any case, what you'll get is either a string or a runtime exception in the case of unexpected results.

= is an assignment operator.
You want to compare two values using the == comparison operator.

where stat.Id = 1 needs to be where stat.Id == 1
You're attempting to set the value rather than doing a comparison.

I can see two issues here.
First of all you are trying to pass a string (varchar) to an int.
Second the LINQ you have will produce something like IEnumerable or IQueryable which won't be able to go to an int.
You should probably consider getting First, FirstOrDefault, Sinle, SingleOrDefault

How to use returned linq variable?

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

What do I use in place of "FirstOrDefault()" if I want to find an exact search result in my database?

I am currently using the following code within my controller:
Instructor instructor = db.Instructors.FirstOrDefault(
o => o.UserName == User.Identity.Name);
to select someone by username. My understanding is that I will have trouble using "FirstOrDefault()" if I have user's with similar user names (i.e. searching for "MrUser" when I have users named "MrUserOne" "MrUserTwo" and "MrUser" may yield "MrUserOne" because it was the 'first' search result to show up), If I am correct in my understanding of future difficulties with "FirstOrDefault," what should I use in it's place?
Or am I wrong in my understanding of how FirstOrDefault will work?

It appears you don't quite understand how the == operator works in this scenario. Assuming the UserName and Name values are both string then == will do an exact ordinal match. The name "MrUser" won't match "MrUserOne" at all. It will only match "MrUser".
That being said this code will return the first user whose Name value exactly matches o.UserName or null if none do.

JaredPar is quite correct in what he's saying. But you can also use SingleOrDefault if what you're expecting is to be exactly one result. You can read more about that in this Stackoverflow question: LINQ: When to use SingleOrDefault vs. FirstOrDefault() with filtering criteria

Your query will just return the first user whose UserName exactly matches User.Identity.Name (it will not match users that start with User.Identity.Name, as you seem to be saying).
So if there are 2 users with identical names, the return value will just be the first one. And the "first" one might be different, depending on how the database feels on returning the results to you (since you are not specifying an order).
As a side note, you should get familiar with these 4 similar methods, they are worth knowing:
>1 result 0 results
First() return 1st throw
FirstOrDefault() return 1st return null
Single() throw throw
SingleOrDefault() throw return null

When you use ==, you're already doing an exact search result. The expression
o => o.UserName == User.Identity.Name
is translated to
WHERE UserName = [Username]
on the database side. This is not to be confused with a statement like
o => o.UserName.Contains(User.Identity.Name)
which would translate to
WHERE UserName like '%[Username]%'
See http://msdn.microsoft.com/en-us/library/bb738681.aspx for how string functions are translated for Linq to Entities.
That being said, your FirstOrDefault will give you the first name that exactly matches the username you provided. So your example would match the first user with the name "MrUser".

When to use .First and when to use .FirstOrDefault with LINQ?

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.