解释函数

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();
}
}

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

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. :)

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 (i.e. "string" in this example) is the functions return type (i.e. 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