What is Func, how and when is it used

Question

What is Func<> and what is it used for?

Tremblay

It's just a shortcut for delegates with a specific signature. To fully understand the answers below you'll need to understand delegates ;-) – Subterranean

In the answer of @Oded it says 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. – Oui

Answer 1

Func<T> 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<string> f = GetMessage;

Gauge

But it can also represent a static one-argument function =) – Faxun

@Faxun no, that's not correct. The definition of Func<T> is delegate TResult Func<out TResult>(). No arguments. Func<T1, T2> would be a function that takes one argument. – Gauge

No, I'm correct. static int OneArgFunc(this string i) { return 42; } Func<int> f = "foo".OneArgFunc;. =) – Faxun

That's an extension method which is special. – Gauge

The only special thing about it is the Extension attribute which is only read by the C#/VB.Net compilers, not CLR. Basically, instance methods (unlike static functions) have a hidden 0th "this" parameter. So, 1-argument instance method is very similar to 2-argument static function. Then, we have delegates which store target object and function pointer. Delegates can store the first argument in target or not do that. – Faxun

Okay, let me put is this way then: A reference of the type Func<int> cannot point to public static int F(int i) { return 0; } which is a static one-argument function. You need Func<int, int> for that. – Gauge

The trick only works for reference types, so let's use string argument instead of int. Func<int> can point to public static int F(string s) { return 0; }. As I've said, there is nothing special about extension methods (at the IL level). The IL code that creates a Func<int> instance from the function is valid regardless of the function's attributes. (But I must say I didn't believe my own eyes when I saw the compiler doing that.) – Faxun

All in all, there are 2x2=4 different situations: (instance function or static function) x (using target or not using target). Common cases are: "instance function; using target" and "static function; not using target". But other two cases are possible too. "static function; using target" is the extension method example that I've shown. And "instance function; not using target" is called an open instance method delegate and is somewhat opposite to the concept of the extension method. – Faxun

Speaking of the 4th situation (open instance method delegates): it's possible to create a Func<MyClass, string> delegate form an instance method like class MyClass { public string MyMethod() { return ""; } }. – Faxun

Well, despite what you're saying public static int F(string s) { return 0; } and Func<int> f = F; will not compile. – Gauge

It doesn't work because you don't provide the string argument. But as I've said, yes, with C#/VB.Net compilers the static method must be marked as Extension for this to work. But on the IL level there is no difference between extension methods and static methods. You can write public static int StaticF(string s) { return 1; } public static int ExtensionF(this string s) { return 2; } Func<int> f = "a".ExtensionF;, compile it, then replace ExtensionF with StaticF in the IL and it will work just fine. – Faxun

The point here is that this is not an IL issue. The compiler cannot bind the reference because the types do not match. – Gauge

Yes. The C# compiler can only bind the delegate construction call to a method group when the signature of the method's free parameters matches the delegate signature. The C# compiler thus allows binding the extension method to Func<int> or Func<string, int> depending on the syntax used ("instance" call syntax or "static" call syntax) (which affects the number of free parameters and the free parameters signature). – Faxun

Good discussion. After 4 years what would both of you add to this? – Madge

And, for the sake of your team mates, never use this abstracted garbage. Just write out the code explicitly. you got two hands, hopefully. – Liberal

Answer 2

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<T, TResult> 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<Person> and I want just the name of every person in the list. I can do this:

var names = people.Select(p => p.Name);

Or say I want the age of every person:

var ages = people.Select(p => 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 => p.Name and p => 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.

Aura

Answer 3

Func<T1, T2, ..., Tn, Tr> 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<double, double> f1 = sqr;
Func<double, double> f2 = x => 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.

Maricela

Excelent answer, but for compilling keyword static is neeed – Terracotta

Answer 4

I find Func<T> very useful when I create a component that needs to be personalized "on the fly".

Take this very simple example: a PrintListToConsole<T> 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<T> {
  String Render(T input);
}

Then you have to create the class PrintListToConsole<T> that takes the previously created interface and uses it over each element of the list.

class PrintListToConsole<T> {

    private PrintListConsoleRender<T> _renderer;

    public void SetRenderer(PrintListConsoleRender<T> r) {
        // this is the point where I can personalize the render mechanism
        _renderer = r;
    }

    public void PrintToConsole(List<T> list) {
        foreach (var item in list) {
            Console.Write(_renderer.Render(item));
        }
    }   
}

The developer that needs to use your component has to:

1. implement the interface

2. pass the real class to the PrintListToConsole

   class MyRenderer : PrintListConsoleRender<int> {
       public String Render(int input) {
           return "Number: " + input;
       }
   }
   
   class Program {
       static void Main(string[] args) {
           var list = new List<int> { 1, 2, 3 };
           var printer = new PrintListToConsole<int>();
           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<T,String> 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<T> {

    private Func<T, String> _renderFunc;

    public void SetRenderFunc(Func<T, String> r) {
        // this is the point where I can set the render mechanism
        _renderFunc = r;
    }

    public void Print(List<T> 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<T, String> type, that is a function that creates the output for the console.

class Program {
    static void Main(string[] args) {
        var list = new List<int> { 1, 2, 3 }; // should be a list as the method signature expects
        var printer = new PrintListToConsole<int>();
        printer.SetRenderFunc((o) => "Number:" + o);
        printer.Print(list); 
        string result = Console.ReadLine();
    }
}

Func<T> 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.

Baroscope

Thanks for posting this Marco. It has really helped me. I have been trying to understand func for a while and also actively use it in my programming. This example will clear the path. I had to add the StampaFunc method as that was left out in the original code which prevented it from displaying. – Neurath

I think there is a line missed in Func sample, Where is the call for print function or StampaFunc ? – Unattached

Answer 5

Func<T1,R> and the other predefined generic Func delegates (Func<T1,T2,R>, Func<T1,T2,T3,R> 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.

Laforge

Answer 6

It is just a predefined generic delegate. Using it you don't need to declare every delegate. There is another predefined delegate, Action<T, T2...>, which is the same but returns void.

Zither

Answer 7

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<int, int, int> randInt = (n1, n2) => new Random().Next(n1, n2); 

So, Func is a built-in delegate which brings some functional programming features and helps reduce code verbosity.

Lorrinelorry

Answer 8

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 & how2use:

Func<input_1, input_2, ..., input1_6, output> funcDelegate = someMethod;

Definition:

public delegate TResult Func<in T, out TResult>(T arg);

Where it is used:

It is used in lambda expressions and anonymous methods.

Severson

Answer 9

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<int, int, int> Sum = (x, y) => x + y;

Morale