玩匿名类型

Question

来自Jon Skeet的精彩书籍C#In Depth,First Edition:

class Film
{
    public string Name { get; set; }
    public int Year { get; set; }

    public override string ToString()
    {
        return string.Format("Name={0}, Year={1}", Name, Year);
    }
}

var films = new List<Film>
{
    new Film {Name="Jaws", Year=1975},
    new Film {Name="Singing in the Rain", Year=1952},
    new Film {Name="Some Like It Hot", Year=1959},
    new Film {Name="The Wizard of Oz", Year=1939},
    new Film {Name="It's a Wonderful Life", Year=1946},
    new Film {Name="American Beauty", Year=1999},
    new Film {Name="High Fidelity", Year=2000},
    new Film {Name="The Usual Suspects", Year=1995}
};

Action<Film> print = film => { Console.WriteLine(film); };
films.ForEach(print);
films.FindAll(film => film.Year < 1960)
.ForEach(print);
films.Sort((f1, f2) => f1.Name.CompareTo(f2.Name));
films.ForEach(print);

一段落在上面列出的代码片段之后.

清单9.4的前半部分仅涉及设置数据.我会使用匿名类型,但从匿名类型实例的集合创建通用列表相对棘手.(您可以通过创建一个接受数组并将其转换为相同类型的列表的泛型方法,然后将隐式类型数组传递给该方法来实现.在.NET 3.5中,名为ToList的扩展方法也提供此功能,但是因为我们还没有看过扩展方法,所以会作弊!)

上面提供的代码片段是该段引用的书的9.4.

我的问题: 我正在尝试用上面段落中概述的技术(看一下斜体文本),但我不太明白他的意思.

我试过这样的事情,但我认为这不是他的意思,因为它不起作用(我没想到):

using System;
using System.Collections.Generic;

namespace ScratchPad
{

class Film
{
    public string Name { get; set; }
    public int Year { get; set; }

    public override string ToString()
    {
        return string.Format("Name = {0}\tYear = {1}", 
            Name, Year);
    }
}

class Program
{
    static void Main(string[] args)
    {
        ToList<Film>( new[]
        {
            new { Name = "North By Northwest", Year = 1959 },
            new { Name = "The Green Mile", Year = 1999},
            new { Name = "The Pursuit of Happyness", Year = 2006}
        }).ForEach( f => {Console.WriteLine(f);} );

        Console.ReadKey();
    }

    static List<T> ToList<T>(
        System.Collections.IEnumerable list)
    {
        var newList = new List<T>();

        foreach (var thing in list)
            if (thing is T)
                newList.Add((T)thing);

        return newList;

    }
}

}

注意:我知道IEnumerable.ToList()扩展方法并多次使用它.我只想亲自尝试段落中概述的技术.

此外,我对Linq之外使用匿名类型的场景很感兴趣,因为语法方便,下面给出了其中一种场景.我总是可以dynamic在C#4中使用并接受匿名类型作为参数,并且知道我的期望.我希望我能用C#3做到这一点.如下所示:

using System;
using Microsoft.CSharp.RuntimeBinder;

namespace PlayWithAnonType
{
    class Program
    {
        static void Main(string[] args)
        {
            PrintThingy(new { Name = "The Secret", 
Genre = "Documentary", Year = 2006 });
            Console.ReadKey();
        }

    static void PrintWhatever(dynamic whatever)
    {
        // the anonymous type's ToString() will print
        Console.WriteLine(whatever);
    }

    static void PrintThingy(dynamic thingy)
    {
        try
        {
            // I know what the thingy is
            Console.WriteLine("Name = {0}\tGenre = {1}\tYear = {2}",
                thingy.Name, thingy.Genre, thingy.Year);
        }
        catch(RuntimeBinderException ex)
        {
#pragma warning disable 0168
            Console.WriteLine("By thingy, I really meant film. 
Sorry, I should've clarified.");
#pragma warning restore 0168
        }
    }
}

}

编辑 他们应该有一个名为jon-skeet的标签.

Answer 1

关键在于,如果我们知道ToList我们有一种创建列表的方法,而Film根本没有我们自己的类型.并不是说我们能够将匿名类型与Film类型混合在一起.换句话说,我们可以这样做:

// The type of list will be List<T> where T is the anonymous type
var list = new[]
{
    new { Name = "North By Northwest", Year = 1959 },
    new { Name = "The Green Mile", Year = 1999},
    new { Name = "The Pursuit of Happyness", Year = 2006}
}.ToList();

list.ForEach(x => Console.WriteLine("{0} ({1})", x.Name, x.Year));

很高兴你正在享受第一版,顺便说一下 - 希望在第二版出来之前不会太久:)

Answer 2

要真正做到这一点:

public void Main (string[] args)
{
    var films = ToList(new [] {
        new {Name = "Jaws", Year = 1975},
        new {Name = "Singing in the Rain", Year = 1952},
        new {Name = "Some Like It Hot", Year = 1959},
        new {Name = "The Wizard of Oz", Year = 1939},
        new {Name = "It's a Wonderful Life", Year = 1946},
        new {Name = "American Beauty", Year = 1999},
        new {Name = "High Fidelity", Year = 2000},
        new {Name = "The Usual Suspects", Year = 1995}
    }
    );


    films.ForEach(f => Console.Write(f.Name + " - " + f.Year));

}

public List<T> ToList<T> (IEnumerable<T> list)
{
    return new List<T>(list);
}

正如其他人所提到的,我不确定这是多么有用.你写的时候会得到intellisense和所有这些,所以至少可以节省一些打字费用吗？:)