Func vs Action vs Predicate

Question

With real examples and their use  can someone please help me understand   When do we need a Func lt T     gt  delegate  When do we need an Action lt T gt  delegate  When do we need a Predicate lt T gt  delegate

User · Answer

Func - When you want a delegate for a function that may or may not take parameters and returns a value  The most common example would be Select from LINQ   var result   someCollection Select  x   gt  new   x Name  x Address       Action - When you want a delegate for a function that may or may not take parameters and does not return a value  I use these often for anonymous event handlers   button1 Click     sender  e    gt       Do Some Work        Predicate -  When you want a specialized version of a Func that evaluates a value against a set of criteria and returns a boolean result  true for a match  false otherwise   Again  these are used in LINQ quite frequently for things like Where   var filteredResults        someCollection Where x   gt  x someCriteriaHolder    someCriteria       I just double checked and it turns out that LINQ doesn t use Predicates  Not sure why they made that decision   but theoretically it is still a situation where a Predicate would fit

User · Answer

Action is a delegate  pointer  to a method  that takes zero  one or more input parameters  but does not return anything   Func is a delegate  pointer  to a method  that takes zero  one or more input parameters  and returns a value  or reference    Predicate is a special kind of Func often used for comparisons   Though widely used with Linq  Action and Func are concepts logically independent of Linq  C   already contained the basic concept in form of typed function pointers   Here is a small example for Action and Func without using Linq   class Program       static void Main string   args                Action lt int gt  myAction   new Action lt int gt  DoSomething           myAction 123                Prints out  123                                      can be also called as myAction Invoke 123            Func lt int  double gt  myFunc   new Func lt int  double gt  CalculateSomething           Console WriteLine myFunc 5         Prints out  2 5             static void DoSomething int i                Console WriteLine i              static double CalculateSomething int i                return  double i 2

User · Answer

The difference between Func and Action is simply whether you want the delegate to return a value  use Func  or not  use Action    Func is probably most commonly used in LINQ - for example in projections    list Select x   gt  x SomeProperty    or filtering    list Where x   gt  x SomeValue    someOtherValue    or key selection    list Join otherList  x   gt  x FirstKey  y   gt  y SecondKey         Action is more commonly used for things like List lt T gt  ForEach  execute the given action for each item in the list  I use this less often than Func  although I do sometimes use the parameterless version for things like Control BeginInvoke and Dispatcher BeginInvoke   Predicate is just a special cased Func lt T  bool gt  really  introduced before all of the Func and most of the Action delegates came along  I suspect that if we d already had Func and Action in their various guises  Predicate wouldn t have been introduced    although it does impart a certain meaning to the use of the delegate  whereas Func and Action are used for widely disparate purposes   Predicate is mostly used in List lt T gt  for methods like FindAll and RemoveAll

[c#] Func vs. Action vs. Predicate

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