Can someone explain what the C# “Func<T,T>” does?

问题

I'm reading the Pro MVC 2 book, and there is an example of creating an extension method for the HtmlHelper class.

Here the code example:

public static MvcHtmlString PageLinks(this HtmlHelper html, PagingInfo pagingInfo, Func<int,string> pageUrl)
{
    //Magic here.
}

And here is an example usage:

[Test]
public void Can_Generate_Links_To_Other_Pages()
{
    //Arrange: We're going to extend the Html helper class.
    //It doesn't matter if the variable we use is null            
    HtmlHelper html = null;

    PagingInfo pagingInfo = PagingInfo(){
        CurrentPage = 2,
        TotalItems = 28,
        ItemsPerPage = 10
    };

    Func<int, String> pageUrl = i => "Page" + i;

    //Act: Here's how it should format the links.
    MvcHtmlString result = html.PageLinks(pagingInfo, pageUrl);

    //Assert:
    result.ToString().ShouldEqual(@"<a href=""Page1"">1</a><a href=""Page2"">2</a><a href=""Page3"">3</a>")           

}

Edit: Removed part that confused the point of this question.

The question is: Why is the example using Func? When should I use it? What is Func?

Thanks!

回答1:

A Func<int, string> like

Func<int, String> pageUrl = i => "Page" + i;

is a delegate accepting int as its sole parameter and returning a string. In this example, it accepts an int parameter with name i and returns the string "Page" + i which just concatenates a standard string representation of i to the string "Page".

In general, Func<TSource, TResult> accepts one parameter that is of type TSource and returns a parameter of type TResult. For example,

Func<string, string> toUpper = s => s.ToUpper();

then you can say

string upper = toUpper("hello, world!");

or

Func<DateTime, int> month = d => d.Month;

so you can say

int m = month(new DateTime(3, 15, 2011));

回答2:

Func<int, String> means a callback method that takes an int parameter and returns a String as the result.

The following expression, which is known as a lambda expression:

Func<int, String> pageUrl = i => "Page" + i;

expands to something like this:

Func<int, String> pageUrl = delegate(int i)
{
    return "Page" + i;
}

回答3:

The Func<int, string> line that you are inquiring about is known as a lambda expression.

Func<int, String> pageUrl = i => "Page" + i;

This line can be described as a function that takes an int parameter (i) and returns a string "Page" + i;

It can be re-written as:

delegate(int i)
{
    return "Page" + i;
}

回答4:

Because the PageLinks method is an Extension Method.

In extension method, the first parameter starts with this keyword to indicate that it is an Extension method on the type represented by the first parameter.

The Func<T1, T2> is a delegate which represents a transformation from type T1 to type T2. So basically, your PageLinks method will apply that transformation to int to produce a string.

回答5:

Func<T, TResult>: Encapsulates a method that has one parameter and returns a value of the type specified by the TResult parameter. See this page for more details and examples. :-)

回答6:

Have a blog post on this. Using Func you can resolve some of functional discrepancy. Read here.

回答7:

I have implemented a where() extension method using Func please have a look...

public static IEnumerable<Tsource> Where<Tsource> ( this IEnumerable<Tsource> a , Func<Tsource , bool> Method )
{

    foreach ( var data in a )
    {
        //If the lambda Expression(delegate) returns "true" Then return the Data. (use 'yield' for deferred return)
        if ( Method.Invoke ( data ) )
        {
            yield return data;
        }
    }
}

You can use it like,

        foreach ( var item in Emps.Where ( e => e.Name == "Shiv" ).Select ( e1 => e1.Name ) )
        {
            Console.WriteLine ( item );
        }

回答8:

Create your own

Func<int,string> myfunc; 

then right click Func to view definition. You will see it is a delegate underneith

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

来源：https://stackoverflow.com/questions/5315500/can-someone-explain-what-the-c-sharp-funct-t-does