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Convert SQL query to LINQ or lambda expression in C# and use in EF Core

I have 3 table
Tbl_City , Tbl_GroupCities , Tbl_CtrCar .
I want to convert this SQL query to LINQ or lambda expression in C#
declare #fk_group uniqueidentifier
SELECT #fk_group= FK_Group
FROM dbo.Tbl_User
WHERE UserName='meysam'
SELECT dbo.Tbl_City.ID_City, dbo.Tbl_City.Name_City,COUNT( dbo.Tbl_CtrCar.Cur_year)
FROM dbo.Tbl_City
INNER JOIN dbo.Tbl_CtrCar ON dbo.Tbl_City.ID_City = dbo.Tbl_CtrCar.FK_City
WHERE ID_City IN (SELECT FK_City
FROM dbo.Tbl_GroupCities
WHERE Active=1 AND ID_Group=#fk_group)
GROUP BY ID_City , Name_City
I try it but it's not work
var model = _TblUser.FirstOrDefault(x => x.UserName == "sampleUserName");
var q = _TblGroupCities.Where(x => x.IdGroup == model.FkGroup && x.Active == true);
var sample2 =
(from x in _TblCity
join a in _TblGroupCities on x.IdCity equals a.FkCity
where a.Active == true && a.IdGroup == model.FkGroup
select new
{
x.IdCity,
x.NameCity
}).ToList();

Please take a look here the features you have in your query are not yet implemented. GroupBy and i think also subselects will do an
SELECT * FROM TableName
And in memory it will do the group by or even for each row a new SQL query.
Better to use the RawSql method for this purpose.
But if you realy want to learn LINQ and convert your SQL take a look at LINQPad

This issue is done. I found my problem, I don't Understand use two joins and use group by in Linq
I use this linq for the solution and run
var model = _TblUser.SingleOrDefault(x => x.UserName == type.UserName);
var q = _TblGroupCities.Where(x => x.IdGroup == model.FkGroup && x.Active == true);
tblCityViewModel = new List<MohasebKhodro.ViewModels.TblCityViewModel>();
var sample2 =
(from x in _TblCity
join a in _TblGroupCities on x.IdCity equals a.FkCity
where a.Active == true && a.IdGroup == model.FkGroup
select new
{
x.IdCity,
x.NameCity
}).ToList();
foreach (var item in sample2)
{
var er = _TblCtrCar.Where(x => x.FkCity == item.IdCity).Max(x => x.CurYear);
tblCityViewModel.Add(new MohasebKhodro.ViewModels.TblCityViewModel
{
IdCity = item.IdCity,
NameCity = item.NameCity,
MaxCurrentYear = Convert.ToString(er)
});
}

LINQ - using result from one in another

I'm completely new to LINQ, i want to rewrite some of mine SQL querys into LINQ (just to learn) and i'v already stuck at the beginning. Probably solution is very simple but as i'v said I'm completely new and i didn't find solution to this.
I have one query :
string typMoneta = textBox1.Text;
var moneta = from x in db.grupyTowarowes
where x.typ == typMoneta
select new
{
x.grupa
};
Which works ok and when i set
dataGridView1.DataSource = moneta;
Then i got output
And i want to use this output in my second query :
var query = from c in dbContext.Picking
where c.Number == 1000 && c.Group == moneta
select new
{
c.id
};
Problem is with c.Group == moneta. I don't know the correct syntax. Could someone help me?

I think you meant to use moneta.Contains(c.Group). In first query, make sure you use ToList() to load data into memory.
IList<string> moneta = (from x in db.grupyTowarowes
where x.typ == typMoneta
select x.grupa).ToList();
var query = (from c in dbContext.Picking
where c.Number == 1000 && moneta.Contains(c.Group)
select c.id).ToList();

The moneta is an IEnumerable<T> where T in your case is the type of grupa
That being said you should write your query like below:
var query = from c in dbContext.Picking
where c.Number == 1000
&& moneta.Contais(c.Group)
select new
{
c.id
};
or in fluent syntax like below:
var query = dbContext.Picking
.Where(pick => pick.Number == 1000
&& moneta.Contains(pick.Group))
.Select(pick => pick.id);

Note that moneta is not a collection of strings. It's a collection of objects that have a string property named "grupa".
Does this work for you?
var query =
from c in dbContext.Picking
where c.Number == 1000
&& moneta.Any(m => m.grupa == c.Group)
select new { c.id };
You could also do this:
// Get list of strings
var groups = moneta.Select(m => m.grupa).ToList();
// Get items where "Group" value is one of the strings in groups list, above.
var query =
from c in dbContext.Picking
where c.Number == 1000
&& groups.Contains(c.Group)
select new { c.id };

Where clause in NHibernate

How do I express this in NHibernate?
DECLARE #EntityId INT = 800;
SELECT *
FROM UserAlert
WHERE UserAlertId =
(
SELECT MAX(UserAlertId)
FROM UserAlert
WHERE EntityId = #EntityId
)
This is what I'm trying to do.
var senderUA = session.CreateCriteria<UserAlert>()
.Add(Restrictions.Eq("EntityId", id))
.SetProjection( Projections.Max("Id") )
. UniqueResult();
And I keep getting an error that can convert object to UserAlert type, i.e. it's not even compiling.
Thanks for helping

Ordering by UserAlertId descending and selecting top 1 would be simpler.
var senderUA = session.CreateCriteria<UserAlert>()
.Add(Restrictions.Eq("EntityId", id))
.AddOrder(Order.Desc("UserAlertId"))
.SetMaxResults(1)
.UniqueResult();
Additionally you can
var senderUA = session
.Query<UserAlert>()
.Where(x=>x.EntityId==id &&
x.UserAlertId==session.Query<UserAlert>()
.Where(x=>x.EntiryId==id).Max(x=>x.UserAlertId)
).FirstOrDefault();

Here's a solution using QueryOver.
var maxUserAlertId = QueryOver.Of<UserAlert>
.Where(ua => ua.EntityId == id)
.Select(
Projections.Max(
Projections.Property<UserAlert>
(u => u.UserAlertId)
)
);
var maxUserQuery = session
.QueryOver<UserAlert>()
.WithSubquery
.WhereProperty(u => u.EntityId)
.Eq(maxUserAlertId);
// Dealing with the situation that the maximum value is shared
// by more than one row. If you're certain that it can only
// be one, call SingleOrDefault instead of List
IList<UserAlert> results = maxUserQuery.List();

Nested query in entity framework

I am getting the following exception:
The nested query is not supported. Operation1='Case' Operation2='Collect'
with this query
var Games = context.Games.Select(a => new GameModel
{
Members = (a.Type == 1 ? (a.UsersInGames.Where(b => b.GameID == a.ID && b.StatusID == 1).Select(c => new Member
{
ID = c.UserID,
email = c.UserInfo.EmailAddress,
screenName = c.UserInfo.ScreenName
})) :
(a.Teams.Where(b => b.GameID == a.ID).SelectMany(b => b.UsersInTeams.Where(c => c.StatusID == 1)).Select(d => new Member
{
ID = d.UserID,
email = d.UserInfo.EmailAddress,
screenName = d.UserInfo.ScreenName
)))
})
when I don't include the condition in selecting Members, the query works fine. Is there a way I can do the conditional inside the query?

You're overestimating the power of LINQ translation to SQL. Not everything is translatable and there is no compiler warning for that due to the way LINQ works.
Nested collections are usually either a) not supported or b) end up in horrible SELECT N+1 queries. What you ask EF to do is to return an object tree. SQL does not support tree like results so you run into the object-relational impedance mismatch and it hurts.
I advise you to fetch the nested collection data as a second, completely separate query. That allows you more control and is guaranteed to work.
As a non-essential side-note, you will probably not be able to convince EF to use the ?: operator over sequences. That is very hard to translate. Think how you would write this as SQL - very hard and convoluted.

It looks like Linq to EF doesn't support the following
context.Games.Select(g => new
{
Field = g.IsX? queryable1 : queryable2
});
But, here's a hack you can use to get it to work:
context.Games.Select(g => new
{
Field = queryable1.Where(q => g.IsX)
.Concat(queryable2.Where(q => !g.IsX))
});

I faced the same problem. The solution was to load both results and determine what to use after the query (I know it has performance downside), but at least you can do it temporarily if deadline attacks you:
At the LINQ side
var Games = context.Games.Select(a => new GameModel
{
// carries type1 results
Members = a.UsersInGames.Where(b => b.GameID == a.ID && b.StatusID == 1).Select(c => new Member
{
ID = c.UserID,
email = c.UserInfo.EmailAddress,
screenName = c.UserInfo.ScreenName
})),
//You need to create this temporary carrier to carry type 2 results
MembersOfType2 = a.Teams.Where(b => b.GameID == a.ID).SelectMany(b => b.UsersInTeams.Where(c => c.StatusID == 1)).Select(d => new Member
{
ID = d.UserID,
email = d.UserInfo.EmailAddress,
screenName = d.UserInfo.ScreenName
})))
})
}
After that you may loop Gamesand make the assignment Members = MembersOfType2 if Type == 1 for a certain game.

I had this error too. I had code like this:
var Games = context.Games.Select(a => new GameModel
{
Members = (!filters.GetDatailedDataToo ? null : new List<MemberModel>())
};
This error occurs when null is used in ? : operation.
This is not that case, written up here, but I've wasted lot of time, I think anyone uses this case, who searches this error text..

Linq - Retrieve a single value in a String

I use Asp.net 3.5 and EF 4.
I need find a specific row in my DataBase and display on a label a single value as string.
At the moment I use this code, it is working, so I find a single Object and read its properties.
var myAuthor = (from at in context.CmsAuthors
where at.AuthorId == myRow.AuthorId
select at).Single();
myAuthorNameLabel.Text = myAuthor.LastName;
I would like to know:
If there is another syntax in Linq to achieve the same result.
How to do it using Lamba?
Which approach would you suggest me?

Here's the method syntax (using lambdas)
myAuthorNameLabel.Text = context.CmsAuthors
.Where(at => at.AuthorId == myRow.AuthorId)
.Select(at => at.LastName)
.SingleOrDefault() ?? string.Empty;

You can use:
var myAuthorName =
(from at in context.CmsAuthors where at.AuthorId == myRow.AuthorId select at).Single().Select(a => a.LastName);
actually this would be even better:
var myAuthorName =
(from at in context.CmsAuthors where at.AuthorId == myRow.AuthorId select at).Select(a => a.LastName).Single();
Update
An example of how to use with Anonymous type:
var myAuthorNames =
(from at in context.CmsAuthors where at.AuthorId == myRow.AuthorId select at).Select( a => new {a.LastName, a.FirstName}).Single();