Explanation of Func

Question

我想知道是否有人可以解释一下Func<int, string>是什么以及它是如何与一些明显的例子一起使用的。

Answer 1

Are you familiar with delegates in general? I have a page about delegates and events which may help if not, although it's more geared towards explaining the differences between the two.

Func<T, TResult> is just a generic delegate - work out what it means in any particular situation by replacing the type parameters ( T and TResult ) with the corresponding type arguments ( int and string ) in the declaration. I've also renamed it to avoid confusion:

string ExpandedFunc(int x)

In other words, Func<int, string> is a delegate which represents a function taking an int argument and returning a string .

Func<T, TResult> is often used in LINQ, both for projections and predicates (in the latter case, TResult is always bool ). For example, you could use a Func<int, string> to project a sequence of integers into a sequence of strings. Lambda expressions are usually used in LINQ to create the relevant delegates:

Func<int, string> projection = x => "Value=" + x;
int[] values = { 3, 7, 10 };
var strings = values.Select(projection);

foreach (string s in strings)
{
    Console.WriteLine(s);
}

Result:

Value=3
Value=7
Value=10

Answer 2

A Func<int, string> eats ints and returns strings. So, what eats ints and returns strings? How about this ...

public string IntAsString( int i )
{
  return i.ToString();
}

There, I just made up a function that eats ints and returns strings. How would I use it?

var lst = new List<int>() { 1, 2, 3, 4, 5 };
string str = String.Empty;

foreach( int i in lst )
{
  str += IntAsString(i);
}

// str will be "12345"

Not very sexy, I know, but that's the simple idea that a lot of tricks are based upon. Now, let's use a Func instead.

Func<int, string> fnc = IntAsString;

foreach (int i in lst)
{
  str += fnc(i);
}

// str will be "1234512345" assuming we have same str as before

Instead of calling IntAsString on each member, I created a reference to it called fnc (these references to methods are called delegates ) and used that instead. (Remember fnc eats ints and returns strings).

This example is not very sexy, but a ton of the clever stuff you will see is based on the simple idea of functions, delegates and extension methods .

One of the best primers on this stuff I've seen is here . He's got a lot more real examples. :)

Answer 3

It is a delegate that takes one int as a parameter and returns a value of type string .

Here is an example of its usage:

using System;

class Program
{
    static void Main()
    {
        Func<Int32, String> func = bar;

        // now I have a delegate which 
        // I can invoke or pass to other
        // methods.
        func(1);
    }

    static String bar(Int32 value)
    {
        return value.ToString();
    }
}

Answer 4

Func<int, string> accepts an int value parameter and returns a string value. Here's an example where an additional supporting method is unnecessary.

Func<int, string> GetDogMessage = dogAge =>
        {
            if (dogAge < 3) return "You have a puppy!";
            if (dogAge < 7) return "Strong adult dog!";

            return "Age is catching up with the dog!";
        };

string youngDogMessage = GetDogMessage(2);

NOTE: The last object type in Func (ie "string" in this example) is the functions return type (ie not limited to primitives, but any object). Therefore, Func<int, bool, float> accepts int and bool value parameters, and returns a float value.

Func<int, bool, float> WorthlessFunc = (intValue, boolValue) =>
        {
            if(intValue > 100 && boolValue) return 100;

            return 1;
        };
float willReturn1 = WorthlessFunc(21, false);
float willReturn100 = WorthlessFunc(1000, true);

HTH