How to use where in linq

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I want to use where in linq
the error says can't implicitly convert type string to xx.modules.Students

What I have tried:

Students student = new Students();
int userId = Convert.ToInt16(InputAPIVM.USERID);
student = db.Students.Where(x => x.UserId == userId).ToString();

Posted 30-Dec-18 22:52pm

NebroProg

Updated 31-Dec-18 5:00am

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OriginalGriff · Accepted Answer · 2018-12-30T23:03:00

Solution 1

That's not Where that causes the problem!
You have declared student as a Students variable:

Students student = new Students();

But your code converts the entire return collection to a string:

db.Students.Where(x => x.UserId == userId).ToString()

and tries to assign that string to the student!

The first thing you need to so is think about what you want to return - and Where returns a collection of items, not a single item. INstead of where, I'd suggest FirstOrDefault[^]:

student = db.Students.FirstOrDefault(x => x.UserId == userId);

Which returns a single value (or null if there are no matches)
This assumes that db.Students is a collection of Student items!

Posted 30-Dec-18 23:03pm

OriginalGriff

Comments

NebroProg 31-Dec-18 5:10am

Ok,
but this query update more than one column because UserId is foreinkey
so FirstOrDefault is returning one row

OriginalGriff 31-Dec-18 5:15am

That query doesn't update anything!
All it does it return matching rows; it doesn't change any of them.

NebroProg 31-Dec-18 5:20am

I know but I mean this code must return more than one column

OriginalGriff 31-Dec-18 5:52am

That code doesn't return "columns" - it returns a single class instance from a collection. If the class contains the "columns" you are interested in, then fine. Otherwise ...

NebroProg 31-Dec-18 6:02am

ok,
if the code be like this
student = db.Students.FirstOrDefault(x => x.UserId == userId).Select(x =>x.UserId);
what is the error

OriginalGriff 31-Dec-18 6:23am

:sigh:
First or Default returns a single value, not a collection. Where returns a collection, not a single value.

If you want to assign a value to a variable - as you show in your example code - then you want to use FirstOrDefault.
If you want multiple values then you use Where.

Only collections can be processed with Linq Methods, so if you return a single item you cannot then try to Select it's ID.
And - to be honest - your ID value is very unlikely to be a Student anyway; a Student will probably contain an ID value, but it won't be one! And why would you try to select an ID value when you already know it because you have only fetched items that have a specific value?
Try

MyIDType studentID = db.Students.FirstOrDefault(x => x.UserId == userId).UserId;

It may do what you want. But ... it's the same as saying:

MyIDType studentID = userId;

I'm getting the definite feeling that you are coding by "guess and hope" here: that won't work. You need to stop guessing and start thinking about what you are trying to do!

Maciej Los 2-Jan-19 5:39am

5ed!

NebroProg · Accepted Answer · 2018-12-31T05:01:00

Solution 2

int userId = Convert.ToInt16(InputAPIVM.USERID);

           student = db.Students.Where(x => x.UserId == userId).ToList();

           if (student != null)
           {
               foreach (Students s in student)
               {
                   s.lat_long = InputAPIVM.LAT_LONG;
                  }

Posted 31-Dec-18 5:01am

NebroProg

Comments

Dave Kreskowiak 31-Dec-18 11:08am

student will never be null. If there are no records to return, the call to .ToList() will create an empty collection, not null. You don't need the check for null at all.