🚪 👨‍👨‍👧 🖖 C＃中的魔术方法签名 ✍🏼 🧖🏽 👨‍🔧

我要向你的翻译C＃中的妙用方法通过CEZARYPIĄTEK。

C＃中有一组特定的方法签名，这些签名具有语言支持。具有此类签名的方法可让您使用自定义语法，并具有所有优点。例如，它们可以用于简化我们的代码或创建DSL，以便以更美观的方式表示问题的解决方案。我到处都遇到这样的方法，因此我决定写一篇文章，总结关于该主题的所有发现，即：

集合初始化语法
字典初始化语法
解构函数
自定义等待类型
查询表达模式

集合初始化语法

集合初始化语法相当古老，因为它自C＃3.0（2007年末发布）以来就已经存在。让我提醒您，集合初始化语法允许您创建一个在一个块中包含元素的列表：

var list = new List<int> { 1, 2, 3 };

此代码等效于以下代码：

var temp = new List<int>();
temp.Add(1);
temp.Add(2);
temp.Add(3);
var list = temp;

BCL. , :

IEnumerable
void Add(T item)

public class CustomList<T>: IEnumerable
{
    public IEnumerator GetEnumerator() => throw new NotImplementedException();
    public void Add(T item) => throw new NotImplementedException();
}

, Add :

public static class ExistingTypeExtensions
{
    public static void Add<T>(this ExistingType @this, T item) => throw new NotImplementedException();
}

- :

class CustomType
{
    public List<string> CollectionField { get; private set; }  = new List<string>();
}

class Program
{
    static void Main(string[] args)
    {
        var obj = new CustomType
        {
            CollectionField =
            {
                "item1",
                "item2"
            }
        };
    }
}

. ? :

var obj = new CustomType
{
    CollectionField =
    {
        { existingItems }
    }
};

, :

IEnumerable
void Add(IEnumerable<T> items)

public class CustomList<T>: IEnumerable
{
    public IEnumerator GetEnumerator() => throw new NotImplementedException();
    public void Add(IEnumerable<T> items) => throw new NotImplementedException();
}

, BCL void Add(IEnumerable<T> items), , :

public static class ListExtensions
{
    public static void Add<T>(this List<T> @this, IEnumerable<T> items) => @this.AddRange(items);
}

var obj = new CustomType
{
    CollectionField =
    {
        { existingItems.Where(x => /*Filter items*/).Select(x => /*Map items*/) }
    }
};

(IEnumerable):

var obj = new CustomType
{
    CollectionField =
    {
        individualElement1,
        individualElement2,
        { list1.Where(x => /*Filter items*/).Select(x => /*Map items*/) },
        { list2.Where(x => /*Filter items*/).Select(x => /*Map items*/) },
    }
};

, -, protobuf. , protobuf: grpctools .NET proto, - :

[DebuggerNonUserCode]
public RepeatableField<ItemType> SomeCollectionField
{
    get
    {
        return this.someCollectionField_;
    }
}

, - , RepeatableField void Add(IEnumerable items), - :

/// <summary>
/// Adds all of the specified values into this collection. This method is present to
/// allow repeated fields to be constructed from queries within collection initializers.
/// Within non-collection-initializer code, consider using the equivalent <see cref="AddRange"/>
/// method instead for clarity.
/// </summary>
/// <param name="values">The values to add to this collection.</param>
public void Add(IEnumerable<T> values)
{
    AddRange(values);
}

C# 6.0 — , . :

var errorCodes = new Dictionary<int, string>
{
    [404] = "Page not Found",
    [302] = "Page moved, but left a forwarding address.",
    [500] = "The web server can't come out to play today."
};

var errorCodes = new Dictionary<int, string>();
errorCodes[404] = "Page not Found";
errorCodes[302] = "Page moved, but left a forwarding address.";
errorCodes[500] = "The web server can't come out to play today.";

, .

— , Dictionary<T> , :

class HttpHeaders
{
    public string this[string key]
    {
        get => throw new NotImplementedException();
        set => throw new NotImplementedException();
    }
}

class Program
{
    static void Main(string[] args)
    {
        var headers = new HttpHeaders
        {
            ["access-control-allow-origin"] = "*",
            ["cache-control"] = "max-age=315360000, public, immutable"
        };
    }
}

C# 7.0 . :

var point = (5, 7);
// decomposing tuple into separated variables
var (x, y) = point;

ValueTuple<int, int> point = new ValueTuple<int, int>(1, 4);
int x = point.Item1;
int y = point.Item2;

int x = 5, y = 7;
//switch
(x, y) = (y,x);

class Point
{
    public int X { get; }
    public int Y { get; }

    public Point(int x, int y)  => (X, Y) = (x, y);
}

, . , :

Deconstruct
void
out

Point :

class Point
{
    public int X { get; }
    public int Y { get; }

    public Point(int x, int y) => (X, Y) = (x, y);

    public void Deconstruct(out int x, out int y) => (x, y) = (X, Y);
}

var point = new Point(2, 4);
var (x, y) = point;

" " :

int x;
int y;
new Point(2, 4).Deconstruct(out x, out y);

public static class PointExtensions
{
     public static void Deconstruct(this Point @this, out int x, out int y) => (x, y) = (@this.X, @this.Y);
}

— KeyValuePair<TKey, TValue>, :

foreach (var (key, value) in new Dictionary<int, string> { [1] = "val1", [2] = "val2" })
{
    //TODO: Do something
}

KeyValuePair<TKey, TValue>.Deconstruct(TKey, TValue) netstandard2.1. netstandard .

awaitable

C# 5.0 ( Visual Studio 2012) async/await, . , :

void DoSomething()
{
    DoSomethingAsync().ContinueWith((task1) => {
        if (task1.IsCompletedSuccessfully)
        {
            DoSomethingElse1Async(task1.Result).ContinueWith((task2) => {
                if (task2.IsCompletedSuccessfully)
                {
                    DoSomethingElse2Async(task2.Result).ContinueWith((task3) => {
                        //TODO: Do something
                    });
                }
            });
        }
    });
}

private Task<int> DoSomethingAsync() => throw new NotImplementedException();
private Task<int> DoSomethingElse1Async(int i) => throw new NotImplementedException();
private Task<int> DoSomethingElse2Async(int i) => throw new NotImplementedException();

async/await:

async Task DoSomething()
{
    var res1 = await DoSomethingAsync();
    var res2 = await DoSomethingElse1Async(res1);
    await DoSomethingElse2Async(res2);
}

, await Task. , GetAwaiter, :

System.Runtime.CompilerServices.INotifyCompletion void OnCompleted(Action continuation)
IsCompleted
GetResult

await GetAwaiter, TaskAwaiter<TResult> , :

class CustomAwaitable
{
    public CustomAwaiter GetAwaiter() => throw new NotImplementedException();
}

class CustomAwaiter: INotifyCompletion
{
    public void OnCompleted(Action continuation) => throw new NotImplementedException();

    public bool IsCompleted => throw new NotImplementedException();

    public void GetResult() => throw new NotImplementedException();
}

: " await awaitable ?". , Stephen Toub "await anything", .

query expression

C# 3.0 — Language-Integrated Query, LINQ, SQL- . LINQ : SQL- . , . . , . LINQ , SQL- , . . C#, CLR. LINQ IEnumerable, IEnumerable<T> IQuerable<T>, , , query expression. , LINQ, :

class C
{
    public C<T> Cast<T>();
}

class C<T> : C
{
    public C<T> Where(Func<T,bool> predicate);

    public C<U> Select<U>(Func<T,U> selector);

    public C<V> SelectMany<U,V>(Func<T,C<U>> selector, Func<T,U,V> resultSelector);

    public C<V> Join<U,K,V>(C<U> inner, Func<T,K> outerKeySelector, Func<U,K> innerKeySelector, Func<T,U,V> resultSelector);

    public C<V> GroupJoin<U,K,V>(C<U> inner, Func<T,K> outerKeySelector, Func<U,K> innerKeySelector, Func<T,C<U>,V> resultSelector);

    public O<T> OrderBy<K>(Func<T,K> keySelector);

    public O<T> OrderByDescending<K>(Func<T,K> keySelector);

    public C<G<K,T>> GroupBy<K>(Func<T,K> keySelector);

    public C<G<K,E>> GroupBy<K,E>(Func<T,K> keySelector, Func<T,E> elementSelector);
}

class O<T> : C<T>
{
    public O<T> ThenBy<K>(Func<T,K> keySelector);

    public O<T> ThenByDescending<K>(Func<T,K> keySelector);
}

class G<K,T> : C<T>
{
    public K Key { get; }
}

, , LINQ . LINQ . , , Understand monads with LINQ Miłosz Piechocki.

本文的目的并不是要说服您滥用这些语法技巧，而是要使它们更清楚。另一方面，不能总是避免它们。它们被设计为可以使用，有时它们可以使您的代码更好。如果您担心产生的代码对您的同事来说是难以理解的，那么您需要找到一种与他们共享知识的方法（或者至少是本文的链接）。我不确定这是否是此类“魔术方法”的完整集合，因此，如果您知道更多信息，请在评论中分享。

C＃中的魔术方法签名

集合初始化语法

awaitable

query expression

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