What are some clever uses of LINQ?

Question

What are some clever uses for LINQ outside of LINQ to SQL?

Have you found any problems that LINQ made a whole lot easier to solve? Please post examples.

Answer 1

• Almost anything to do with collections is easier with LINQ to Objects. I find that much more useful than LINQ to SQL.
• I've become rather fond of a framework that Marc Gravell and I developed called Push LINQ (currently part of MiscUtil, but probably moving to MoreLINQ eventually). This is great for calculating aggregates over large streaming data sets, e.g. huge log files
• LINQ to XML is the nicest XML API I've used, and it integrates really nicely with LINQ to Objects.
• Parallel LINQ is a very nice part of the Parallel Extensions framework - it certainly makes it easier to parallelize tasks where it's appropriate (although it can go hideously wrong if you're not careful)

Answer 2

Robert Shelton of Microsoft was cool enough to list a few LINQ implementations for us:

As of month 7, 2008:

• LINQ to Amazon
• LINQ to Active Directory
• LINQ to Bindable Sources (SyncLINQ)
• LINQ over C- project
• LINQ to CRM
• LINQ To Geo—Language Integrated Query for Geospatial Data
• LINQ to Excel
• LINQ to Expressions (MetaLinq)
• LINQ Extender (Toolkit for building LINQ Providers)
• LINQ to Flickr
• LINQ to Google
• LINQ to Indexes (LINQ and i40)
• LINQ to IQueryable (Matt Warren on Providers)
• LINQ to JSON
• LINQ to NHibernate
• LINQ to JavaScript
• LINQ to LDAP
• LINQ to LLBLGen Pro
• LINQ to Lucene
• LINQ to Metaweb(freebase)
• LINQ to MySQL, Oracle and PostgreSql (DbLinq)
• LINQ to NCover
• LINQ to Opf3
• LINQ to Parallel (PLINQ)
• LINQ to RDF Files
• LINQ to Sharepoint
• LINQ to SimpleDB
• LINQ to Streams
• LINQ to WebQueries
• LINQ to WMI
• LINQ to XtraGrid

Answer 3

I used LINQ to solve some of Project Euler in single C# statements. (Note that statements aren't the same as lines)

Beware: Evil nasty tricks.

//Euler 1
//Add all the natural numbers below one thousand that are multiples of 3 or 5.
Enumerable.Range(0, 1000).Where(i => i % 5 == 0 || i % 3 == 0).Sum()

//Euler 2
//Find the sum of all the even-valued terms in the sequence which do not exceed four million
//Enumerable.Repeat(new List<long>(1024){ 1, 1 }, 1).First(fib => Enumerable.Range(0, int.MaxValue).TakeWhile(i => fib.Last() <= 4000000)
    .Aggregate(true, (u1, u2) => { fib.Add(fib.Last() + fib[fib.Count - 2]); return true; })).Where(n => n % 2 == 0).Sum()

//Euler 3 (>32bit)
//What is the largest prime factor of the number 600851475143?
Enumerable.Range(2, Int32.MaxValue - 2).Where(n => 600851475143 % n == 0 && Enumerable.Range(2, n / 2 - 1).All(f => n % f > 0)).Max()

//Euler 4
//Find the largest palindrome made from the product of two 3-digit numbers.
Enumerable.Range(100, 900).SelectMany(x => Enumerable.Range(100, 900).Select(y => x * y))
                          .Where(n => { var s = n.ToString(); return s.SequenceEqual(s.Reverse()); }).Max()

//Euler 5 (>32bit)
//What is the smallest number divisible by each of the numbers 1 to 20?
Enumerable.Range(20, Int32.MaxValue - 21).Where(n => Enumerable.Range(1, 20).All(i => n % i == 0)).First()

//Euler 6
//Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.
Math.Pow(Enumerable.Range(1, 100).Sum(), 2) - Enumerable.Range(1, 100).Select(i => i * i).Sum()

//Euler 7
//Find the 10001st prime.
Enumerable.Range(2, Int32.MaxValue - 1).Where(n => Enumerable.Range(2, n / 2 - 1).All(f => n % f > 0)).Skip(10000).First()

//Euler 8
//Discover the largest product of five consecutive digits in the 1000-digit number.
Enumerable.Range(0, 995).Select(i => "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
    .Substring(i,5).Select(c => c - '0').Aggregate(1, (x, y) => x * y)).Max()


//Euler 10
//Find the sum of all the primes below two million.
Enumerable.Range(2, 2000000).Where(n => Enumerable.Range(2, n / 2 - 1).All(f => n % f > 0)).Select(x => (long)x).Sum()

Enumerable.Range(0, 168).Aggregate(Enumerable.Range(2, 2000000).Select(x => (long)x).ToList(), (result, index) => { result.RemoveAll(i => i > result[index] && i % result[index] == 0); return result; }).Sum()

Enumerable.Repeat(Enumerable.Range(2, 2000000).Select(x => (long)x).ToList(), 1).SelectMany(list => Enumerable.Range(0, Int32.MaxValue).Select(i => new { List = list, Index = i }))
    .TakeWhile((g, i) => g.List[g.Index] * g.List[g.Index] <= 2000000 || i.Dump("Rounds") != i).Aggregate((List<long>) null, (result, g) => { g.List.RemoveAll(i => i > g.List[g.Index] && i % g.List[g.Index] == 0); return g.List; }).Sum()

Enumerable.Repeat(Enumerable.Range(2, 2000000).Select(x => (long)x).ToList(), 1).First(list => Enumerable.Range(0, Int32.MaxValue)
    .TakeWhile(i => list[i] * list[i] <= 2000000 || i.Dump("Rounds")!=i).Aggregate(0, (count, i) => count + list.RemoveAll(j => j > list[i] && j % list[i] == 0)) != null).Sum()

//Euler 14
Enumerable.Range(1, 1000000).Select(s => Enumerable.Repeat(new List<long>(32) { s }, 1).First(list => Enumerable.Range(0, Int32.MaxValue).TakeWhile(i => list.Last() > 1)
    .Aggregate(0, (index, unused) => { list.Add(list.Last() % 2 == 0 ? list.Last() / 2 : 3 * list.Last() + 1); return 1; }) == 1 || true))
    .Aggregate(new List<long>(), (list, result) => list.Count <= result.Count ? result : list)

//Euler 19
//How many Sundays fell on the first of the month during the twentieth century?
Enumerable.Range(1901,100).SelectMany(y => Enumerable.Range(1,12).Select(m => new DateTime(y, m, 1))).Where(d => d.DayOfWeek == DayOfWeek.Sunday)

//Euler 21
//Evaluate the sum of all the amicable numbers under 10000.
Enumerable.Repeat(new Func<int, int>(n => Enumerable.Range(1, n / 2).Where(d => n % d == 0).Sum()), 1)
          .Select(D => Enumerable.Range(1, 10000).Where(a => a == D(D(a)) && a != D(a)).Sum())

//Euler 34
//Find the sum of all numbers which are equal to the sum of the factorial of their digits.
Enumerable.Range(3, 40600).Where(n => n == n.ToString().Select(d => Enumerable.Range(1, d - '0').Aggregate(1, (r, i) => r * i)).Sum()).Sum()

//Euler 40  
Enumerable.Repeat(new StringBuilder(), 1)
    .Where(result => Enumerable.Range(0, Int32.MaxValue)
                               .TakeWhile(i => result.Length <= 1000000)
                               .Aggregate(result, (unused, index) => result.Append(index)) != null)
    .Select(result => Enumerable.Range(1, 6).Select(i => result[(int)Math.Pow(10, i)] - '0')).First().Aggregate(1, (x, y) => x * y)

Other LINQ one-liners:

//Primes (Ineffecient)
Enumerable.Range(2, 1000000).Where(n => Enumerable.Range(2, n / 2 - 1).All(f => n % f > 0)).Count()

//Sieve of Eratosthenes
Enumerable.Range(0, 168)
          .Aggregate(Enumerable.Range(2, 1000000).ToList(), (result, index) => { 
               result.RemoveAll(i => i > result[index] && i % result[index] == 0); 
                return result; 
            }).Count
//Prime Factors
Enumerable.Range(2,13195 / 2)
          .Where(n => 13195 % n == 0
                   && Enumerable.Range(2, n / 2 - 1).All(f => n % f > 0))

//Fibonacci
Enumerable.Repeat(new List<long>(32){ 1, 1 }, 1)
    .First(fib => Enumerable.Range(0, 32).Aggregate(true, (u1, u2) => { 
        fib.Add(fib.Last() + fib[fib.Count - 2]); 
        return true; 
    }))

Answer 4

You should also check out Bindable LINQ, from the CodePlex site:

"Bindable LINQ is a set of extensions to LINQ that add data binding and change propagation capabilities to standard LINQ queries.

As well as propogating change, Bindable LINQ can analyse your queries at runtime and detect any dependencies your query has. If these dependencies provide events to subscribe to, Bindable LINQ will automatically monitor them for change. "

Here's one of the examples from the site:

Take this query for example:

   contactsListBox.ItemsSource = from c in customers
                                 where c.Name.StartsWith(textBox1.Text)
                                 select c;

Bindable LINQ will detect that the query relies on the Text property of the TextBox object, textBox1. Since the TextBox is a WPF control, Bindable LINQ knows to subscribe to the TextChanged event on the control.

The end result is that as the user types, the items in the query are re-evaluated and the changes appear on screen. No additional code is needed to handle events.