有人可以解释一下 C#“Func<T,T> “ 做？

Question

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!

Answer 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));

Answer 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;
}

Answer 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;
}

Answer 4

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

Answer 5

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 .

Answer 6

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. :-)

Answer 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 );
        }

Answer 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);