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
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 a many-to-many relationship between tables of Games and Genres. During an analysis, i need to get items from Games that match specific criteria.
The problem is, to check for this criteria, i need to analyse genres of this specific game. And linq won't let me do it.
My request now looks like this:
var result = GDB.Games.Where((g)=>
g.GamesToGenres.Select((gtg)=>
(weights.ContainsKey(gtg.Genre.Name) ? weights[gtg.Genre.Name]:0.0)
).Sum() > Threshhold
).ToArray();
When I execute it, I receive SQL exception
Only one expression can be specified in the select list when the
subquery is not introduced with EXISTS.
Is there a workaround? How can i perform such Select inside of Where?
EDIT: weights is a Dictionary<string, double>.
EDIT: I was playing with lambdas, and found out a strange thing in their behaviour:
this code won't work, throwing nvarchar to float conversion exception:
Func<string, double> getW = (name) => 1;
var t = GDB.Games.Where((g)=>
g.GamesToGenres.Select((gtg)=>
getW(gtg.Genre.Name)
).Sum() > Threshhold
).ToArray();
but this one will work nicely:
var t = GDB.Games.Where((g)=>
g.GamesToGenres.Select((gtg)=>
1
).Sum() > Threshhold
).ToArray();
This leads me to conclusion that linq lambdas are not usual lambdas. What's wrong with them, then? What are their limitations? What i can and what i can't do inside of them? Why is it ok for me to place a .select call inside of lambda, but not my own call of getW?
RESOLVED. See the answer below. Long story short, C# can't into clojures unless explicitly told so. If anyone knows better answer, i am still confused.
Your problem is you're trying to select something form the dictionary weights that exists in your application and not in your DB. If it was the result of a query to your DB, use the query.Single(...) in its place
Well, i am confused beyond imagination. The following code works perfectly:
Func<Game, bool> predicate = (g) =>
g.GamesToGenres.Select((gtg) =>
(weights.ContainsKey(gtg.Genre.Name) ? weights[gtg.Genre.Name] : 0.0)
).Sum() > Threshhold;
var t = GDB.Games.Where(predicate).ToArray();
careful reader might want to say "Hey! Isn't that the very same code you wrote in the question? You just explicitly assigned it to a variable!", and he will be right. Right now it seems like C# lambda processor is a piece of something, and it creates clojure only when you explicitly declare a lambda. If someone can describe this phenomena to me, i will be gratefull, for right now i am more confused than a newborn baby.
LINQ allows you to combine SQL data with local data like (Dictionary, etc.) with one restriction. You need to select data from SQL first. This means your code will work if you replace GDB.Games.Where with GDB.Games.ToList().Where. You can ask about performance, but you able to select a slice of data like GameId, Genre Name, etc. Then filter out games. Then return end list of full game's info by game ID list.
I am trying to wrap my head around LINQ from Objects, and specifically in dictionaries, and failing abysmally.
I have seen this syntax in a couple of threads on SO:
var items = (from pair in m_taskDictionary
from s in pair.Value
s).Distinct().ToList();
But when I try to compile this, I get a syntax error:
An expression of type 'OperationsPlannerData.OperationsTaskLabeledData' is not allowed in a subsequent from clause in a query expression with source type 'System.Collections.Generic.Dictionary'. Type inference failed in the call to 'SelectMany'.
What is wrong? Is my Visual Studio too old? (boy would THAT not surprise me.) I'm using VS 2008.
Here's a link to one of the threads that recommends this usage:
Dictionary Manipulation Using LINQ in C#
In a nutshell, here's the problem I want to resolve: I want to find the first item in the dictionary for which a particular field in the Value object matches a given string. I have the dictionary already, keyed by integers, for one purpose in my application, so I'd rather just use that dictionary. Various threads in SO suggest using a List<KeyValuePair<string, OperationsTaskLabeledData>> object for this. I could create that object easily enough, but I don't know how to search that structure either.
You are missing a select in your OP, but this answer is for your last paragraph.
m_taskDictionary.First(kvp => kvp.Value.WhateverFieldYouNeed == someString);
If you want it to return the default value (null for reference types) if it does not find a matching element, use FirstOrDefault.
Not sure if you have a typo on your question but you are missing select s part
var items = (from pair in m_taskDictionary
from s in pair.Value
select s).Distinct().ToList(); // <-- added select here
Try this
var items = (from value in m_taskDictionary.Values
where value.field == "a given string"
select value).Distinct().ToList();
where your dictionary is of type
Dictionary<int, CustomObject> m_taskDictionary = new Dictionary<int, CustomObject>();
and the CustomObject class has a string property called field which you're doing your search on.
I believe you are missing the select:
var items = (from pair in m_taskDictionary
from s in pair.Value
select s).Distinct().ToList();
http://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b
I am passing a populated SelectList to my View and I want to default the selected value to the last record in the list. So far I've got:
IQueryable<BuildNumber> projBuildNos = tcRepository.GetProjectBuildNos();
BuildList = new SelectList(projBuildNos, "ID", "value", projBuildNos.Last().ID);
I get an exception saying "the query operator 'Last' is not supported but I can't seem to get at the last record in my generic list and can't find any examples.
This code is from a 'form view model' passing select lists from my repository back to the view and I think I should be performing this here. However I'm new to generics/mvc/linq so am happy for alternative suggestions.
Thanks in advance for any help. Please let me know if you want any more info.
Are you already going to suck all the results from your query into memory? If so, I suggest you execute your query and get the result into a list:
List<BuildNumber> projBuildNos = tcRepository.GetProjectBuildNos().ToList();
BuildList = new SelectList(projBuildNos, "ID", "value", projBuildNos.Last().ID);
Otherwise you could easily end up with a workaround which executes the query twice - possibly giving inconsistent results.
I found this on MSDN:
Queryable.LastOrDefault -- Returns the last element in a sequence, or a default value if the sequence contains no elements.
Maybe this?
projBuildNos.Reverse().First().ID