Linq in practice
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Copy: https://docs.google.com/presentation/d/1JKNJDU6KTnzaebh1U2qWRJ6t_E9GNTXY9w-ywDpWRYQ/edit?usp=sharing
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Linq in practice
- 1. LINQ in practice ”In theory there is no difference between theory and practice. But in practice there is”. Except in mathematics…
- 2. Func<TIn1, TIn2, TOut> public int PlusFive(int i){ return i+5; } Func<int, int> PlusFive = delegate(int i) { return i + 5; }; Func<int, int> PlusFive = i => i + 5;
- 4. ” Infiniteloop” publicstaticIEnumerable<int> NaturalNumbers() { inti=0; while(true){ yieldreturn i++; } }
- 5. Filtered publicstaticIEnumerable<int> Odds() { returnNaturalNumbers().Where(i=>i%2==1); }
- 6. It works! static void Main(string[] args) { var nums = Odds().Take(5); //var executed = nums.ToList(); foreach(var i in nums) Console.WriteLine(i); }
- 7. What is inside LINQ? public static class FetchExtension{ public static IEnumerable<T> MyWhere<T>( this IEnumerable<T> list, Func<T, bool> cond){ foreach(var i in list){ if (cond(i)) yield return i; } } }
- 8. Imperative vs. declarative var result = new List<int>(); foreach(var i in list){ if(i % 2 == 0){ result.Add(i); } } var result = from i in list where i % 2 == 0 select i;
Notes de l'éditeur
- Func<…> is “function as type”These code blocks are equals.First type parameters are input parameters. The last type parameter is the return valueIn type declaration we already know the function type…
- Instead of List<T> use the base classIEnumerable<T>- It is the base class for the lists (and LINQ) It doesn’t contain any kind of .RemoveAt(…) methods, so no-one will mess with your list
- Using System.LinqFilter the ones that are not interesting
- When you debug you notice what happens! Don’t do if(list.Count() == 0) but if(!list.Any()) so you don’t evaluate the whole list
- Declarative is better: Higher level of abstaction Define what computer does, not howSQL-like syntax is not the point, but the functionality!SQL-like syntax is just one way to express .Where(i=>i%2==0);