What is Func, how and when is it used

问题:

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

回答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;

回答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.

回答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.

回答4:

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.

回答5:

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 Array[1, 2, 3];
        var printer = new PrintListToConsole<int>();
        printer.SetRenderFunc((o) => "Number:" + o);
        printer.Print();
        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.

回答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.

回答7:

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.