Chapter 3 - LINQ Query Operators

© Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelLINQ via C# 3.0Chapter 3 – LINQ Query Operators © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelQuery Operators•Why query operators?•Developing query operators for IEnumerable<T>•Language integration © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelWhat Are Query Operators?•Query operators are typically extension methods that operate on a sequence of elements•Some query operators return a sequence of elements, some return a single value © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelWhy Query Operators?•Query operators facilitate declarative programming instead of imperative programmingList<int> sortedPrimes = new List<int>();foreach (int i in numbers) { if (IsPrime(i)) { sortedPrimes.Add(i); }}sortedPrimes.Sort();sortedPrimes.ForEach( Console.WriteLine);numbers .Where(i => IsPrime(i)) .OrderBy(i => i) .ToList() .ForEach( Console.WriteLine);BecomesThe HowThe What © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelDeveloping Query Operators•Most query operators are very straightforward•Let’s develop some query operators that operate on IEnumerable<T>–Where (filtering a sequence)–Order (ordering a sequence)–Select (projecting a sequence to another sequence)–Count (aggregating a sequence)–First (first element) © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelWriting “Where” By Handstatic class EnumerableExtensions { public static IEnumerable<T> Where<T>( this IEnumerable<T> source, Predicate<T> filter) { return WhereEnumerable<T>(source, filter); }}class WhereEnumerable<T> : IEnumerable<T> { // . public IEnumerator<T> GetEnumerator() { return new WhereEnumerator<T>( .); }}class WhereEnumerator<T> : IEnumerator<T> { // .} © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelCompiler Magic To The Rescue•C# 2.0 includes compiler support for generating IEnumerable implementations•Just yield them!public static IEnumerable<int> Range( int min, int max) { for (; min < max; ++min) yield return min;}foreach (int num in Range(0, 100)) Console.WriteLine(num); © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelWhere’s The Magic?•The compiler generates code (edited):private sealed class <Range>d__0 : IEnumerable<int>, IEnumerable, IEnumerator<int>, IEnumerator, IDisposable { private int <>1__state; private int <>2__current; public int <>3__max; public int <>3__min; public int max; public int min; private bool MoveNext(); IEnumerator<int> IEnumerable<int>.GetEnumerator(); int IEnumerator<int>.Current { get; } //More code snipped} © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelBehind The Magicprivate bool MoveNext() { switch (this.<>1__state) { case 0: this.<>1__state = -1; while (this.min < this.max) { this.<>2__current = this.min; this.<>1__state = 1; return true; Label_003E: this.<>1__state = -1; this.min++; } break; case 1: goto Label_003E; } return false;} State Machine © Copyright SELA Software & Education Labs Ltd. 14-18 Baruch Hirsch St. Bnei Brak 51202 IsraelRevisiting “Where”static class EnumerableExtensions { public static IEnumerable<T> Where<T>( this IEnumerable<T> source, Predicate<T> filter) { foreach (T elem in source) if (filter(elem)) yield return elem; }}var numbers = Range(0, 100);var evens = numbers.Where(i => i%2==0);var here is IEnumerable<int> [...]... – We can throw the query away without executing it © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Review So Far • Query operators as declarative programming • Our hand-rolled query operators • Deferred execution © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Standard LINQ Query Operators • LINQ (in System.Core.dll)... Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Standard LINQ Query Operators • LINQ query operators are extension methods in the static Enumerable class • Can be combined with our own! – As long as there’s no name collision • Note: So far we have discussed LINQ to Objects query operators – Other LINQ provides do not work with IEnumerable © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch... Language Query Operators • Language query operators are language keywords that are mapped to query operators – Not all query operators are accessible via keywords • C# integrated query operators: – – – – – Projection: select Filtering: where Grouping: group by Ordering: orderby Miscellaneous: join, let © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Language Query. .. Brak 51202 Israel Syntactic Sugar • Language query operators translate directly to LINQ query operators • Query operators can be mixed: int cnt = (from i in Enumerable.Range(0, 100) where i % 2 == 0 select i).Count(i => i > 25); © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Advanced Example: Joins • Most SQL-like joins are unnecessary because objects are... OrderCount = orders.Count }; © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Chapter Summary • Why query operators? • Developing custom query operators • Language integrated query operators © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel ... Empty source } © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Our Operators In Action • This gives us some room for experimentation: var numbers = Range(0, 100); var query = numbers.Where(i => i % 2 == 0); Console.WriteLine("{0} numbers, first: {1}", query. Count(), query. First()); foreach (int i in query) Console.WriteLine(i); Employee[] employees = ; var sortedEmpIds... Language Query Operators var q2 = from i in Enumerable.Range(0, 100) select new { Id=i, Name=i.ToString() }; var q3 = from e in q2 where e.Name.StartsWith("1") orderby e.Id descending group e by e.Name.Substring(e.Name.Length-1) into g select g; var here is IEnumerable foreach (var group in q3) var here is IGrouping © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch... Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Advanced Example: Let • let is a helper keyword for complex queries: var query = from c in customers where c.Joined.AddYears(1) < DateTime.Now let orders = c.Orders.ToList() select new { Name = c.Name, OrderQuantity = orders.Sum(o=>o.Amount) OrderCount = orders.Count }; © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St... Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Examples Of LINQ Operators var q = Enumerable.Range(0, 100) Select(i => new { Id=i, Name=i.ToString() }) Where(e => e.Name.StartsWith("1")) OrderByDescending(e => e.Id) GroupBy(e => e.Name.Substring(e.Name.Length-1)) Select(grp => grp); foreach (var group in q) { Console.WriteLine("Key: " + group.Key); Console.WriteLine("Elements:... Labs Ltd 1 4-1 8 Baruch Hirsch St Bnei Brak 51202 Israel Deferred Execution • Statements such as var numbers = Range(0, 100); var evens = numbers.Where(i => i%2==0); provide deferred execution semantics • The enumeration is not executed yet! – foreach forces execution – Our Order operator forces execution – Our Count operator forces execution © Copyright SELA Software & Education Labs Ltd 1 4-1 8 Baruch . 1 4-1 8 Baruch Hirsch St. Bnei Brak 51202 IsraelLINQ via C# 3. 0Chapter 3 – LINQ Query Operators © Copyright SELA Software & Education Labs Ltd. 1 4-1 8. Ltd. 1 4-1 8 Baruch Hirsch St. Bnei Brak 51202 IsraelDeveloping Query Operators Most query operators are very straightforward•Let’s develop some query operators