What is Func how and when is it used

Question

What is Func lt  gt  and what is it used for

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Both C  and Java don t have plain functions only member functions  aka methods   And the methods are not first-class citizens  First-class functions allow us to create beautiful and powerful code  as seen in F  or Clojure languages   For instance  first-class functions can be passed as parameters and can return functions    Java and C  ameliorate this somewhat with interfaces delegates  Func lt int  int  int gt  randInt    n1  n2    gt  new Random   Next n1  n2     So  Func is a built-in delegate which brings some functional programming features and helps reduce code verbosity

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Maybe it is not too late to add some info   Sum   The Func is a custom delegate defined in System namespace that allows you to point to a method with the same signature  as delegates do   using 0 to 16 input parameters and that must return something   Nomenclature  amp  how2use   Func lt input 1  input 2       input1 6  output gt  funcDelegate   someMethod    Definition   public delegate TResult Func lt in T  out TResult gt  T arg     Where it is used   It is  used in lambda expressions and anonymous methods

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Func lt T1  T2       Tn  Tr gt   represents a function  that takes  T1  T2       Tn  arguments and returns Tr   For example  if you have a function   double sqr double x    return x   x      You could save it as some kind of a function-variable   Func lt double  double gt  f1   sqr  Func lt double  double gt  f2   x   gt  x   x    And then use exactly as you would use sqr   f1 2   Console WriteLine f2 f1 4       etc   Remember though  that it s a delegate  for more advanced info refer to documentation

User · Answer

Think of it as a placeholder  It can be quite useful when you have code that follows a certain pattern but need not be tied to any particular functionality   For example  consider the Enumerable Select extension method    The pattern is  for every item in a sequence  select some value from that item  e g   a property  and create a new sequence consisting of these values  The placeholder is  some selector function that actually gets the values for the sequence described above    This method takes a Func lt T  TResult gt  instead of any concrete function  This allows it to be used in any context where the above pattern applies   So for example  say I have a List lt Person gt  and I want just the name of every person in the list  I can do this   var names   people Select p   gt  p Name     Or say I want the age of every person   var ages   people Select p   gt  p Age     Right away  you can see how I was able to leverage the same code representing a pattern  with Select  with two different functions  p   gt  p Name and p   gt  p Age    The alternative would be to write a different version of Select every time you wanted to scan a sequence for a different kind of value  So to achieve the same effect as above  I would need      Presumably  the code inside these two methods would look almost identical     the only difference would be the part that actually selects a value    based on a Person  var names   GetPersonNames people   var ages   GetPersonAges people     With a delegate acting as placeholder  I free myself from having to write out the same pattern over and over in cases like this

User · Answer

Aforementioned answers are great  just putting few points I see might be helpful   Func is built-in delegate type  Func delegate type must return a value  Use Action delegate if no return type needed   Func delegate type can have zero to 16 input parameters   Func delegate does not allow ref and out parameters   Func delegate type can be used with an anonymous method or lambda expression  Func lt int  int  int gt   Sum     x  y    gt  x   y

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Func lt T gt  is a predefined delegate type for a method that returns some value of the type T   In other words  you can use this type to reference a method that returns some value of T  E g   public static string GetMessage     return  Hello world       may be referenced like this  Func lt string gt  f   GetMessage

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Func lt T1 R gt  and the other predefined generic Func delegates  Func lt T1 T2 R gt   Func lt T1 T2 T3 R gt  and others  are generic delegates that return the type of the last generic parameter   If you have a function that needs to return different types  depending on the parameters  you can use a Func delegate  specifying the return type

User · Answer

I find Func lt T gt  very useful when I create a component that needs to be personalized  on the fly    Take this very simple example  a PrintListToConsole lt T gt  component   A very simple object that prints this list of objects to the console  You want to let the developer that uses it personalize the output   For example  you want to let him define a particular type of number format and so on   Without Func   First  you have to create an interface for a class that takes the input and produces the string to print to the console   interface PrintListConsoleRender lt T gt      String Render T input       Then you have to create the class PrintListToConsole lt T gt  that takes the previously created interface and uses it over each element of the list   class PrintListToConsole lt T gt         private PrintListConsoleRender lt T gt   renderer       public void SetRenderer PrintListConsoleRender lt T gt  r               this is the point where I can personalize the render mechanism          renderer   r             public void PrintToConsole List lt T gt  list            foreach  var item in list                Console Write  renderer Render item                           The developer that needs to use your component has to    implement the interface pass the real class to the PrintListToConsole  class MyRenderer   PrintListConsoleRender lt int gt        public String Render int input            return  Number      input           class Program       static void Main string   args            var list   new List lt int gt    1  2  3            var printer   new PrintListToConsole lt int gt             printer SetRenderer new MyRenderer             printer PrintToConsole list           string result   Console ReadLine                     Using Func it s much simpler  Inside the component you define a parameter of type Func lt T String gt  that represents an interface of a function that takes an input parameter of type T and returns a string  the output for the console   class PrintListToConsole lt T gt         private Func lt T  String gt   renderFunc       public void SetRenderFunc Func lt T  String gt  r               this is the point where I can set the render mechanism          renderFunc   r             public void Print List lt T gt  list            foreach  var item in list                Console Write  renderFunc item                        When the developer uses your component he simply passes to the component the implementation of the Func lt T  String gt  type  that is a function that creates the output for the console   class Program       static void Main string   args            var list   new List lt int gt    1  2  3       should be a list as the method signature expects         var printer   new PrintListToConsole lt int gt             printer SetRenderFunc  o    gt   Number     o           printer Print list            string result   Console ReadLine              Func lt T gt  lets you define a generic method interface on the fly  You define what type the input is and what type the output is  Simple and concise

User · Answer

It is just a predefined generic delegate  Using it you don t need to declare every delegate  There is another predefined delegate  Action lt T  T2    gt   which is the same but returns void

[c#] What is Func, how and when is it used

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