In search page I have some options based on them search query must be different . I have wrote this :
int userId = Convert.ToInt32(HttpContext.User.Identity.GetUserId());
var followings = (from f in _context.Followers
where f.FollowersFollowerId == userId && f.FollowersIsAccept == true
select f.FollowersUserId).ToList();
int value;
if (spto.Page == 0)
{
var post = _context.Posts.AsNoTracking().Where(p => (followings.Contains(p.PostsUserId) || p.PostsUser.UserIsPublic == true || p.PostsUserId == userId) && p.PostIsAccept == true).Select(p => p).AsEnumerable();
if(spto.MinCost != null)
{
post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost).Select(p => p);
}
if (spto.MaxCost != null)
{
post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost).Select(p => p);
}
if (spto.TypeId != null)
{
post = post.Where(p => p.PostTypeId == spto.TypeId).Select(p => p);
}
if (spto.CityId != null)
{
post = post.Where(p => p.PostCityId == spto.CityId).Select(p => p);
}
if (spto.IsImmidiate != null)
{
post = post.Where(p => p.PostIsImmediate == true).Select(p => p);
}
var posts = post.Select(p => new
{
p.Id,
Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
p.PostCity.CityName,
p.PostType.TypeName
}).AsEnumerable().Take(15).Select(p => p).ToList();
if (posts.Count != 0)
return Ok(posts);
return NotFound();
In this case I have 6 Query that take time and the performance is low and code is too long . Is there any better way for writing better code ?
Short answer: if you don't do the ToList and AsEnumerable until the end, then you will only execute one query on your dbContext.
So keep everything IQueryable<...>, until you create List<...> posts:
var posts = post.Select(p => new
{
p.Id,
Image = p.PostsImages
.Select(i => i.PostImagesImage.ImageAddress)
.FirstOrDefault(),
p.PostCity.CityName,
p.PostType.TypeName,
})
.Take(15)
.ToList();
IQueryable and IEnumerable
For the reason why skipping all the ToList / AsEnumerable would help to improve performance, you need to be aware about the difference between an IEnumerable<...> and an IQueryable<...>.
IEnumerable
A object of a class that implements IEnumerable<...> represents the potential to enumerate over a sequence that the object can produce.
The object holds everything to produce the sequence. Once you ask for the sequence, it is your local process that will execute the code to produce the sequence.
At low level, you produce the sequence by using GetEnumerator and repeatedly call MoveNext. As long as MoveNext returns true, there is a next element in the sequence. You can access this next element using property Current.
Enumerating the sequence is done like this:
IEnumerable<Customer> customers = ...
using (IEnumarator<Customer> customerEnumerator = customers.GetEnumerator())
{
while (customerEnumerator.MoveNext())
{
// there is still a Customer in the sequence, fetch it and process it
Customer customer = customerEnumerator.Current;
ProcessCustomer(customer);
}
}
Well this is a lot of code, so the creators of C# invented foreach, which will do most of the code:
foreach (Customer customer in customers)
ProcessCustomer(customer);
Now that you know what code is behind the foreach, you might understand what happens in the first line of the foreach.
It is important to remember, that an IEnumerable<...> is meant to be processed by your local process. The IEnumerable<...> can call every method that your local process can call.
IQueryable
An object of a class that implements IQueryable<...> seems very much like an IEnumerable<...>, it also represents the potential to produce an enumerable sequence of similar object. The difference however, is that another process is supposed to provide the data.
For this, the IQueryable<...> object holds an Expression and a Provider. The Expression represents a formula to what data must be fetched in some generic format; the Provider knows who must provide the data (usually a database management system), and what language is used to communicate with this DBMS (usually SQL).
As long as you concatenate LINQ methods, or your own methods that only return IQueryable<...>, only the Expression is changed. No query is executed, the database is not contacted. Concatenating such statements is a fast method.
Only when you start enumerating, either at its lowest level using GetEnumerator / MoveNext / Current, or higher level using foreach, the Expression is sent to the Provider, who will translate it to SQL and fetch the data from the database. The returned data is represented as an enumerable sequence to the caller.
Be aware, that there are LINQ methods, that don't return IQueryable<TResult>, but a List<TResult>, TResult, a bool, or int, etc: ToList / FirstOrDefault / Any / Count / etc. Those methods will deep inside call GetEnumerator / MoveNext / Current`; so those are the methods that will fetch data from the database.
Back to your question
Database management systems are extremely optimized for handling data: fetching, ordering, filtering, etc. One of the slower parts of a database query is the transfer of the fetched data to your local process.
Hence it is wise to let the DBMS do as much database handling as possible, and only transfer the data to your local process that you actually plan to use.
So, try to avoid ToList, if your local process doesn't use the fetched data. In your case: you transfer the followings to your local process, only to transfer it back to the database in the IQueryable.Contains method.
Furthermore, (it depends a bit on the framework you are using), the AsEnumerable transfers data to your local process, so your local process has to do the filtering with the Where and the Contains.
Alas you forgot to give us a description of your requirements ("From all Posts, give me only those Posts that ..."), and it is a bit too much for me to analyze all your queries, but you gain most efficiency if you try to keep everything IQueryable<...> as long as possible.
There might be some problems with the Int.TryParse(...). Your provider probably will not know how to translate this into SQL. There are several solutions possible:
Apparently PostCost represents a number. Consider to store it as a number. If it is an amount (price or something, something with a limited number of decimals), consider to store it as a decimal.
If you really can't convince your project leaders that numbers should be stored as decimals, either search for a job where they make proper databases, or consider to create a stored procedure that converts the string that is in PostCost to a decimal / int.
if you will only use fifteen elements, use the IQueryable.Take(15), not the IEnumerable.Take(15).
Further optimizations:
int userId =
var followerUserIds = _context.Followers
.Where(follower => follower.FollowersFollowerId == userId
&& follower.FollowersIsAccept)
.Select(follower => follower.FollowersUserId);
In words: make the following IQueryable, but don't execute it yet: "From all Followers, keep only those Followers that are Accepted and have a FollowersFollowerId equal to userId. From the remaining Followers, take the FollowersUserId".
It seems that you only plan to use it if page is zero. Why create this query also if page not zero?
By the way, never use statements like where a == true, or even worse: if (a == true) then b == true else b == false, this gives readers the impression that you have difficulty to grasp the idea of Booleans, just use: where a and b = a.
Next you decide to create a query that zero or more Posts, and thought it would be a good idea to give it a singular noun as identifier: post.
var post = _context.Posts
.Where(post => (followings.Contains(post.PostsUserId)
|| post.PostsUser.UserIsPublic
|| post.PostsUserId == userId)
&& post.PostIsAccept);
Contains will cause a Join with the Followers table. It will probably be more efficient if you only join Accepted posts with the followers table. So first check on PostIsAccept and the other predicates before you decide to join:
.Where(post => post.PostIsAccept
&& (post.PostsUser.UserIsPublic || post.PostsUserId == userId
|| followings.Contains(post.PostsUserId));
All non-accepted Posts won't have to be joined with the Followings; depending on whether your Provider is smart enough: it won't join all public users, or the one with userId, because it knows that it will already pass the filter.
Consider to use a Contains, instead of Any
It seems to me that you want the following:
I have a UserId; Give me all Accepted Posts, that are either from this user, or that are from a public user, or that have an accepted follower
var posts = dbContext.Posts
.Were(post => post.IsAccepted
&& (post.PostsUser.UserIsPublic || post.PostsUserId == userId
|| dbContext.Followers
.Where(followers => ... // filter the followers as above)
.Any());
Be aware: I still haven't executed the query, I only have changed the Expression!
AFter this first definition of posts, you filter the posts further, depending on various values of spto. You could consider to make this one big query, but I think that won't speed up the process. It will only make it more unreadable.
Finally: why use:
.Select(post => post)
This doesn't do anything to your sequence, it will only make it slower.
Some observations:
.AsEnumerable()
This is meant to hide where operators if you use a custom collection. It should not be needed in this case.
.Select(p => p)
I fail to see any purpose for this, remove it.
int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost
Parsing can be expensive, so you want to do as little as possible, and this does it twice, of four times if you have both min and max. replace with direct compares with value, i.e. int.TryParse(p.PostCost, out value) && value >= spo.MinCost. I would also suggest have an explicit case when there is both a min and max cost to avoid parsing twice.
followings.Contains(p.PostsUserId)
Followings is a list, so it will search thru all items. Use a HashSet to speed up performance. I.e. Replace .ToList() with ToHashSet() when creating the followings list. HashSet uses a hash table to make Contains() a constant time operation rather than a linear operation.
Query order
You would want to order the checks to eliminate as many items as early as possible, and make simple, fast, checks before slower checks.
Merge where operators
A single where operator is in general faster than multiple calls.
Use plain loop
If you really need as high performance as possible it might be better to use regular loops. Linq is great for writing compact code, but performance is usually better with plain loops.
Profile
Whenever you talk about performance it is important to point out the importance of profiling. The comments above are reasonable places to start, but there might be some completely different things that takes time. The only way to know is to profile. That should also give a good indication about the improvements.
I have solved my problem with ternary operator :
var post = _context.Posts.AsNoTracking().Where(p =>
(followings.Contains(p.PostsUserId) || p.PostsUser.UserIsPublic == true || p.PostsUserId == userId) && p.PostIsAccept == true
&& (spto.MinCost != null ? int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost : 1 == 1)
&& (spto.MaxCost != null ? int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost : 1 == 1)
&& (spto.TypeId != null ? p.PostTypeId == spto.TypeId : 1 == 1)
&& (spto.CityId != null ? p.PostCityId == spto.CityId : 1 == 1)
&& (spto.IsImmidiate != null && spto.IsImmidiate == true ? p.PostIsImmediate == true : 1 == 1)).Select(p => new
{
p.Id,
Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
p.PostCity.CityName,
p.PostType.TypeName
}).Skip(spto.Page * 15).Take(15).ToList();
EDIT (Better Code) :
Thanx to #ZoharPeled , #HaraldCoppoolse , #JonasH I have Changed the Code like this :
int value;
var post = _context.Posts.AsNoTracking().Where(p =>
(followings.Contains(p.PostsUserId) || p.PostsUser.UserIsPublic == true || p.PostsUserId == userId) && p.PostIsAccept == true
&& (spto.MinCost == null || (int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost))
&& (spto.MaxCost == null || (int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost))
&& (spto.TypeId == null || p.PostTypeId == spto.TypeId)
&& (spto.CityId == null || p.PostCityId == spto.CityId)
&& (spto.IsImmidiate == null || p.PostIsImmediate == true)).Select(p => new
{
p.Id,
Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
p.PostCity.CityName,
p.PostType.TypeName
}).Skip(spto.Page * 15).Take(15).ToList();
Edit (Best Code) :
int userId = Convert.ToInt32(HttpContext.User.Identity.GetUserId());
var followings = _context.Followers
.Where(follower => follower.FollowersFollowerId == userId
&& follower.FollowersIsAccept)
.Select(follower => follower.FollowersUserId);
int value;
var post = _context.Posts.AsNoTracking().Where(p => p.PostIsAccept
&& (p.PostsUser.UserIsPublic || p.PostsUserId == userId
|| _context.Followers.Where(f => f.FollowersFollowerId == userId
&& f.FollowersIsAccept).Select(f => f.FollowersUserId).Any()));
if (spto.MinCost != null)
post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) >= spto.MinCost);
if (spto.MaxCost != null)
post = post.Where(p => int.TryParse(p.PostCost, out value) && Convert.ToInt32(p.PostCost) <= spto.MaxCost);
if (spto.TypeId != null)
post = post.Where(p => p.PostTypeId == spto.TypeId);
if (spto.CityId != null)
post = post.Where(p => p.PostCityId == spto.CityId);
if (spto.IsImmidiate != null)
post = post.Where(p => p.PostIsImmediate == true);
var posts = post.Select(p => new
{
p.Id,
Image = p.PostsImages.Select(i => i.PostImagesImage.ImageAddress).FirstOrDefault(),
p.PostCity.CityName,
p.PostType.TypeName
}).Skip(spto.Page).Take(15).ToList();
if (posts.Count != 0)
return Ok(posts);
So I'm having issues with this LINQ query I had been using for some time, and now it seems to not be working as expected.
messagesWithoutConditional =
await
MobileServiceInstance.GetSyncTable<Messages>()
.OrderByDescending(key => key.SentDate)
.Take(50)
.Where(
p =>
(p.Uuid == myUuid && p.RecipientUuid == otherUuid) ||
(p.Uuid == otherUuid && p.RecipientUuid == myUuid))
.ToListAsync();
So lets say I have this query that simply returns the last 50 messages sent between 2 parties. Now if I want to add an additional condition that the 50 messages should also be before a certain date I would expect to do something like this
messagesWithConditional =
await
MobileServiceInstance.GetSyncTable<Messages>()
.OrderByDescending(key => key.SentDate)
.Take(50)
.Where(
p =>
((p.Uuid == myUuid && p.otherUuid == recipientUuid) ||
(p.Uuid == otherUuid && p.RecipientUuid == myUuid))
&& p.SentDate < 'some date')
.ToListAsync();
Lets suppose I should expect this to return 40 messages, but it returns 0. However, if I alternate the query into this
messagesWithConditional = messagesWithoutConditional.Where(p => p.SentDate < 'some date').ToList();
then I will receive the 40 expected messages by querying the result of my original expression.
How is the second approach any different from the first? Ideally I would like to use the && operator to add a new conditional expression rather than break off into a second where clause
Edit
I should also note that the times are in UTC, and the SentDate attribute is of type DateTimeOffset
As of right now it appears to be a bug, unless some other insight is made. Please refer to the bug report for more information
.Where should be put before the .OrderByDescending and .Take(50)
I am struggling to figure out how to get a LINQ statement to produce a specific WHERE clause in SQL in a single statement.
I am after it producing something like this:
SELECT ColA, ColB, ColC, ColN...
FROM Orders
WHERE Client = #ClientId
AND (#CompanyId IS NULL OR #CompanyId = CompanyId)
My (failing) LINQ statement looks like this:
var includeAllCompanies = company == null;
var data = context.Orders.Where(o => o.Client.Id == clientId
&& (includeAllCompanies
|| (c.Company != null && c.Company.Id == company.Id)).ToList();
However, it always throws an exception when the variable company is NULL (it works fine when it has been initialised). The exception being:
Non-static method requires a target.
My current fix is to split my LINQ statement into two. One using an Expression<Func<>> (to be transformed to a SQL statement with partial filtering). Then another that uses Func<> to perform the remaining filters on the returned list.
Expression<Func<>> to let SQL do some of the work (excluding nullable objects)
var data = context.Orders.Where(o => o.Client.Id == clientId).ToList();
Func<> to then filter out the nullable objects
data = data.Where(c => (territory == null
|| (c.Territory != null && c.Territory.Id == territory.Id))).ToList();
This works, however, I want SQL to be performing this query.
The problem is that, company is server-side variable. Regardles includeAllCompanies value, EF has to translate whole LINQ query to SQL - and in this case SQL doesn't know what is company.Id - so EF has to always get company.Id value in order to put into SQL query. Even if company is null (so that is why you get exception). I hope you see my point, if not - I'll try to give some sample.
In order get rid of exception you can do the following:
var companyId = company == null ? null : (int?)company.Id;
var data = context.Orders.Where(o => o.Client.Id == clientId
&& (companyId == null
|| (c.Company != null && c.Company.Id == companyId)).ToList();
I stumbled across a null ref exception today in one of my Linq-To-Entitites queries and wondered how that could be possible. It seems an conditional OR used as an OR-gate has no effect in Linq-To-Entities. A simplified example of my query is this:
from a in db.Articles
where a.Author == "John Doe"
&& (tag == null || a.Tags.Any(t => t.TagName == tag.TagName))
select a;
Now when tag is NULL, the right side of the where query still gets executed and a NULL reference exception occurs on tag.TagName. Maybe this is because Linq-To-Entities always translates the complete statement into SQL?
Anyhow ... how to get around this issue?
Thanks very much :)
var query = db.Articles.Where(x => x.Author == "John Doe");
query = tag == null
? query
: query.Where(x => x.TagName == tag.TagName);
or:
var query = from a in db.Articles
where a.Author == "John Doe"
select a;
query = tag == null
? query
: from a in query
where a.Tags.Any(t => t.TagName == tag.TagName)
select a;
Personally, I find the first one cleaner
Think about sql. Linq converts your code as a whole into sql query passing 'outside' objects as parameters and evaluating them. That's why it fails on evaluation of your null object.
It's a good practice to construct a linq query bit by bit based on conditions to reduce number of unnecessary code in resulted query, so it's better to split your query:
var query = db.Articles.Where(x => x.Author == "John Doe");
if( tag != null)
query = query.Where(x => x.TagName == tag.TagName);
Because your query will be evaluated and executed on selection you are welcome to add more conditions without worrying about multiple requests.
I have these working in sql for large data set working great. However I'm having hard time converting to linq-I'm new to linq
Select * from table1 t1, table2 t2 where (t1.RoleId+t1.UserId)!=(t2.RoleId+t2.UserId).
On a side note, when the two variables are separated, I get undesired results.
Meaning the following: where (t1.RoleId != t2.RoleId && t1.UserId != t2.UserId)
In c# I have two anonymous lists. The last linq statement works great till nulls come into the picture. Nothing returns. I even thought of using a left join with no success.
So how would you tackle the above query with anonymous type lists?
Linq statments I have so far
var roleUserList =
(
from rls in roleResouceList
join user in userResourceList
on rls.FullResource.ToUpper() equals user.FullResource.ToUpper()
orderby rls.RoleID, user.UserID, rls.Res1, rls.Res2, rls.Res3
select new
{
RoleID = rls.RoleID,
UserID = user.UserID,
ServerId = rls.ServerID,
FullResource = rls.FullResource,
RlsRes1 = rls.Res1,
RlsRes2 = rls.Res2,
RlsRes3 = rls.Res3
}).Distinct().ToList();
var missingRoleUserList =
(
from rls in rlsCount
join usr in usrCount
on rls.Res1 equals usr.Res1
where rls.Total > usr.Total
select new
{
UserID = usr.UsrID,
RoleID = rls.RoleID
}).Distinct().ToList();
List<string> outputRoleUserList =
(
from rls in roleUserList
from mis in missingRoleUserList
where (rls.RoleID + rls.UserID) != (mis.RoleID +mis.UserID)
select rls.UserID + ",\"" + rls.RoleID
).DefaultIfEmpty().Distinct().ToList();
I'm not entirely certain that this is what you're looking for, but I'm going to give it a shot:
Try chaining your where clauses in Linq to SQL, and you may get a better result:
List<string> outputRoleUserList =
from rls in roleUserList
from mis in missingRoleUserList
where rls.RoleID != mis.RoleID
where rls.UserID != mis.UserID
select rls.UserID + ",\"" + rls.RoleID
This will actually generate SQL as follows:
rls.RoleId != mis.UserID AND rls.UserId != mis.UserID
However, you have already forced execution on roleUserList and missingRoleUserList, so what you're using in the third Linq statement is not really Linq to SQL but rather Linq to Objects, if I'm reading this correctly.
I'd be curious to see some additional information or clarification and then maybe I'll understand better what's going on!
EDIT: I realized another possibility, it's possible that the object.UserID or object.RoleID is throwing an internal NullPointerException and failing out because one of those values came back null. You could possibly solve this with the following:
List<string> outputRoleUserLIst2=roleUserList
.Where(x => x != null && x.UserID != null && x.RoleID != null && missingRoleUserList
.Where(y => y != null && y.UserID != null && y.RoleID != null && y.RoleID!=x.RoleID && y.UserID!=x.UserID)
.FirstOrDefault()!=null)
.Select(x => x.UserID + ",\"" + x.RoleID).Distinct().ToList();
This is not pretty, and this is the other Linq syntax (with which I am more comfortable) but hopefully you understand what I am going for here. I'd be curious to know what would happen if you dropped this into your program (If I've guessed all of your meanings correctly!). I'll look back in a bit to see if you have added any information!