Develop Reference

Set several class values using LINQ expression

I have the following two LINQ statements which set different values in the same item in a list
List<MyClass> myList = GetList();
myList.Where(x => x.Name == "someName").Select(x => x.MyArray = someList.ToArray()).ToList();
myList.Where(x => x.Name == "someName").Select( x => x.AnotherValue = GetValue()).ToList();
Is it possible to combine this so both are set in the one expression?

myList
.Where(x => x.Name == "someName")
.ToList()
.ForEach(x => {
x.MyArray = someList.ToArray();
x.AnotherValue = GetValue();
});
Why are you calling ToList() at the end of each of those expressions and discarding the result?
Also, Jon Skeet is right that this is an abuse of LINQ, and especially so in your original form: It's explicit that LINQ expressions aren't even necessarily expected to be fully enumerated. The fact that you needed those ToList() calls to make anything happen should have given you a grave and queasy sense that you were misusing a language feature. When you have to do something weird to use your chosen construct instead of the usual way of doing it, finish getting it to work (because weird is cool), and then go back and redo it the boring, lame way before you check it in.
What advantage do you see in the LINQ + ForEach() version above, compared to this version?
foreach (var x in myList.Where(x => x.Name == "someName"))
{
x.MyArray = someList.ToArray();
x.AnotherValue = GetValue();
}
The old-style loop version is shorter, instantly understandable because it's the default idiom, and IMO cleaner. You don't have to do everything with LINQ.
N.B., ForEach() isn't LINQ; it's a member of List<T>. That's why you have to call ToList() to use it.

Just use the lambda operator to pass an entire lambda expression defined inside a
{...} block:
myList.Where(x => x.Name == "someName").Select(x => { x.MyArray = someList.ToArray(); x.AnotherValue = GetValue(); return x;}).ToList();

Using LINQ in Parallel.Foreach

I have following normal foreach using LINQ query, How can i transform it using Parallel.Foreach
foreach (var i in media.Where(x => x is Video)
{
this.Update(i);
}
How can i do it like
Parallel.ForEach(media,i =>
{
//LINQ
});

First of all, Where(x => x is Video) can be replaced by OfType<Video>().
Second, for fluent syntax it's better to use ParallelEnumerable.ForAll extension method:
media.OfType<Video>()
.AsParallel()
.ForAll(this.Update)

The first argument to Parallel.ForEach is an enumerable, so the obvious way would be:
Parallel.ForEach(media.Where(x => x is Video).OrderBy(x => x.Contains("a")), i =>
{
//this.Update(i);
// commented out because you'll probably want to Invoke it
// depending on what it does exactly.
});

You can do either
Parallel.ForEach(media.Where(x => x is Video), this.Update);
or
media.AsParallel().Where(x => x is Video).ForAll(this.Update);
Adding an order in a parallel process makes no sense here.

Try this :
Parallel.ForEach<Video>(media.Where(x => x is Video), i =>
{
this.Update(i);
};

Well ... basically like you've said ... but ... keep in mind that if you need to order the results and you're acting on that, it's not really parallel ...
The linq parallel foreach will partition your collection and work on it at the same time, which you also need to take into account making sure your this.Update can work with multiple users without messing up .
So, what's the question really ?

Replacing a foreach with a linq statement

I am trying to convert this code to linq:
foreach (var printer in printers)
{
if (printer.Installed)
installedPrinters.Add(printer);
}
I am new to Linq and would appreciate pointers on how it works when iterating through a collection.

printers.Where(printer => printer.Installed)
.ToList()
.ForEach(printer => installedPrinters.Add(printer));
Note the need to call ToList() before ForEach (see Lambda Expression using Foreach Clause).
Also note that while this works, your original code is probably easier to read... LINQ is cool but don't feel obligated to use it for everything :)

If you are just trying to create a new list, you could always just do:
var installedPrinters = printers.Where(p => p.Installed).ToList();
If you are adding to a list that may already have items in it, then you could try:
installedPrinters.AddRange(printers.Where(p => p.Installed));
Assuming your installedPrinters is actually a collection that supports AddRange such as List.

So first use a Where to filter the Installed==true, then run over them with ForEach:
printers.Where(p => p.Installed).ForEach(p => installedPrinters.Add(p));

foreach (var printer in printers.Where (p => p.Installed) { installedPrinters.Add(printer); }

Try this
printer.Where(x => x.Installed).ToList()
.ForEach(
p=>
{
installedPrinters.Add(p)
}
);

extract one column from a var using linq

i have something like this
var emp = db.Employees.toList();
in my employees table i have emp name, emp id and salary, using linq with lambda expressions, how do i access emp id in some another variable.
I have tried looking up for it, couldn't find a solution which does it using linq with lambda expressions
var employeeLeaves = db.Leaves
.Include("Employee")
.Include("Employee.Manager")
.Where(l => l.Status.Id == 1)
.GroupBy(l => l.Employee.Manager.Id)
.Select(l => l.GroupBy(k=>k.Employee.Id).Select(j=>j.GroupBy(‌​p=>p.GroupId)))
.ToList();
this is the actual query i have,dont ask me how i wrote it..:P now i want to get id column from employeeLeaves and store it in some another variable

It's not entirely clear what you're trying to do, which is why you've got so many answers with different approaches. If you're trying to turn the "collection of employees" into a "collection of IDs" then you want something like this:
var ids = emp.Select(x => x.Id);
Or more directly:
var ids = db.Employees.Select(x => x.Id);
(Optionally with ToList at the end of each of these.)
I would strongly advise you to learn LINQ (and the somewhat-related languages features such as var) thoroughly, from scratch, with the help of a good book or tutorial. Learning bits piecemeal by just finding samples which do something a bit like what you want is not a good approach, IMO.

var empId = db.Employees.Single(x => x.id == 5).Id;

You can use the select. Maybe something like this:
var result=db.Employees
.Select(s=>s.emp_id);

Try this syntax:
var empIds = db.Employees.Select(e=>e.EmpID)

Database.Tablename.Single(x => x.id == anyId)
This should select single row

Can I use linq to achieve the same thing this foreach loop does?

Here's the c# code that I have:
private double get806Fees (Loan loan)
{
Loan.Fee.Items class806;
foreach (Loan.Fee.Item currentFee in loan.Item.Fees)
{
if (currentFee.Classification == 806) class806.Add(currentFee);
}
// then down here I will return the sum of all items in class806
}
Can I do this using linq? If so, how? I have never used linq and i've read in several places that using linq instead of a foreach loop is faster... is this true?

Similar to some existing answers, but doing the projection in the query, to make the Sum call a lot simpler:
var sum = (from fee in loan.Items.Fees
where fee.Classification == 806
select fee.SomeValueToSum).Sum();

loan.Item.Fees.
Where(x => x.Classification == 806).
Sum(x => x.SomeValueProperty)
Whether it is faster or not is debatable. IMO, both complexities are the same, the non-LINQ version may be faster.

var q =
from currentFee in loan.Item.Fees
where currentFee.Classification == 806
select currentFee;
var sum = q.Sum(currentFee => currentFee.Fee);

private double get806Fees(Loan loan)
{
return load.Item.Fees.
Where(f => f.Classification == 806).
Sum(f => f.ValueToCalculateSum);
}
I'm assuming here that ValueToCalculateSum is also a double. If it's not then you have to convert it before it is returned.

All of the answers so far are assuming that you're summing up loan.Fees. But the code you actually posted calls Items.Add() to add each Item in loan.Fees.Items to an Items object, and it's that Items object (and not loan.Fees, which is also an Items object) that you say you want to sum up.
Now, if Items is just a simple collection class, then there's no need to do anything other than what people are suggesting here. But if there's some side-effect of the Add method that we don't know about (or, worse, that you don't know about), simply summing up a filtered list of Item objects might not give you the results you're looking for.
You could still use Linq:
foreach (Loan.Fee.Item currentFee in loan.Item.Fees.Where(x => x.Classification == 806)
{
class806.Add(currentFee);
}
return class806.Sum(x => x.Fee)
I'll confess that I'm a little perplexed by the class hierarchy implied here, though, in which the Loan.Item.Fees property is a collection of Loan.Fee.Item objects. I don't know if what I'm seeing is a namespace hierarchy that conflicts with a class hierarchy, or if you're using nested classes, or what. I know I don't like it.