回顾C#各版本特性

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C# 6.0

Read-only auto-properties(只读自动属性)

以前版本，声明只读属性时，示例：

public string FirstName { get; private set; }
public string LastName { get; private set; }

6.0，不显示写setter即是只读属性，但属性也只能在构造函数中初始化了

示例：

public string FirstName { get; }
public string LastName { get;  }

Auto-Property Initializers(自动属性初始化)

以前版本，属性的初始化需要放在构造函数中进行，6.0可以在属性声明时初始化

示例：

public ICollection<double> Grades { get; } = new List<double>();
public Standing YearInSchool { get; set;} = Standing.Freshman;

Expression-bodied function members(表达式体函数成员)

6.0可以直接在函数体使用表达式，这简化一些很简单的函数的书写（仅支持使用只读的属性）

示例：

public override string ToString() => $"{LastName}, {FirstName}";
public string FullName => $"{FirstName} {LastName}";

using static

可以直接引用具体对象，然后在本地直接使用其静态函数。

以前：

using System;

namespace TestCsharp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("ok");
        }
    }
}

6.0版本：

using  static System.Console;

namespace TestCsharp
{
    class Program
    {
        static void Main(string[] args)
        {
            WriteLine("ok");
        }
    }
}

Null-conditional operators(空条件操作符)

更加简化的空对象判断，不为空可直接访问其属性

以前先判断对象不为空，再访问属性：

class Program
    {
        static void Main(string[] args)
        {
            Person person = new Person();

            var first = person == null ? "" : person.FirstName;

            Console.WriteLine(first);
        }
    }

    class Person
    {
        public string FirstName { get; set; }

        public Person()
        {
            FirstName = "init name";
        }
    }

6.0版本，直接替换属性访问 . 为 ?. 一步完成判断和访问：

    class Program
    {
        static void Main(string[] args)
        {
            Person person = new Person();

            var first = person?.FirstName;

            Console.WriteLine(first);
        }
    }

    class Person
    {
        public string FirstName { get; set; }

        public Person()
        {
            FirstName = "init name";
        }
    }

String interpolation(字符串插值)

以前，字符串拼接时，要使用string.Format，示例：

public string FullName
{
    get
    {
        return string.Format("Name is{0} {1}", FirstName, LastName);
    }
}

6.0可以直接使用$代替string.Format，示例：

        public string FullName
        {
            get
            {
                return $"Name is {FirstName}{LastName}";
            }
        }

　进一步，可以在其中使用 : 标示格式化风格，如下，F2标识两位小数，示例：

public string GetGradePointPercentage() =>
    $"Name: {LastName}, {FirstName}. G.P.A: {Grades.Average():F2}";

Exception Filters(异常过滤器)

增加过滤器，直接在异常捕获后就可以进行相应的判断

以前：

public static async Task<string> MakeRequest()
{ 
    var client = new System.Net.Http.HttpClient();
    var streamTask = client.GetStringAsync("https://localHost:10000");
    try {
        var responseText = await streamTask;
        return responseText;
    } catch (System.Net.Http.HttpRequestException e)
    {
        if (e.Message.Contains("301"))
            return "Site Moved";
        else
            throw;
    }
}

6.0版本，在外部使用when进行条件匹配：

public static async Task<string> MakeRequest()
{ 
    var client = new System.Net.Http.HttpClient();
    var streamTask = client.GetStringAsync("https://localHost:10000");
    try {
        var responseText = await streamTask;
        return responseText;
    } catch (System.Net.Http.HttpRequestException e) when (e.Message.Contains("301"))
    {
        return "Site Moved";
    }
}

nameof Expressions

主要场景是异常中提供名字：

if (IsNullOrWhiteSpace(lastName))
    throw new ArgumentException(message: "Cannot be blank", paramName: nameof(lastName));

　　还有就是MVVM中INotifyPropertyChanged实现：

public string LastName
{
    get { return lastName; }
    set
    {
        if (value != lastName)
        {
            lastName = value;
            PropertyChanged?.Invoke(this, 
                new PropertyChangedEventArgs(nameof(LastName)));
        }
    }
}
private string lastName;

Await in Catch and Finally blocks

版本5中对await的使用做了一些限制，6中去除了这些限制。

示例：

public static async Task<string> MakeRequestAndLogFailures()
{ 
    await logMethodEntrance();
    var client = new System.Net.Http.HttpClient();
    var streamTask = client.GetStringAsync("https://localHost:10000");
    try {
        var responseText = await streamTask;
        return responseText;
    } catch (System.Net.Http.HttpRequestException e) when (e.Message.Contains("301"))
    {
        await logError("Recovered from redirect", e);
        return "Site Moved";
    }
    finally
    {
        await logMethodExit();
        client.Dispose();
    }
}

Index Initializers

之前版本示例：

private List<string> messages = new List<string> 
{
    "Page not Found",
    "Page moved, but left a forwarding address.",
    "The web server can't come out to play today."
};

6.0版本：

private Dictionary<int, string> webErrors = 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."
};

Extension Add methods in collection inializers

假定有以下一个类，Enroll添加对象到集合中：

public class Enrollment : IEnumerable<Student>
{
    private List<Student> allStudents = new List<Student>();

    public void Enroll(Student s)
    {
        allStudents.Add(s);
    }

    public IEnumerator<Student> GetEnumerator()
    {
        return ((IEnumerable<Student>)allStudents).GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return ((IEnumerable<Student>)allStudents).GetEnumerator();
    }
}

以前版本，你只能过显示调用Enroll方法，一个个添加对象。

var classList = new Enrollment();
 classList.Enroll(new Student("Lessie", "Crosby"));

6.0增加了Add方法，允许通过扩展方法的形式调用Enroll，进行可以用构造函数直接添加对象。

示例：

public static class StudentExtensions
{
    public static void Add(this Enrollment e, Student s) => e.Enroll(s);
}

　　然后，就可以通过构造函数来直接绑定对象了。如下：

var classList = new Enrollment()
        {
            new Student("Lessie", "Crosby"),
            new Student("Vicki", "Petty"),
            new Student("Ofelia", "Hobbs"),
            new Student("Leah", "Kinney"),
            new Student("Alton", "Stoker"),
            new Student("Luella", "Ferrell"),
            new Student("Marcy", "Riggs"),
            new Student("Ida", "Bean"),
            new Student("Ollie", "Cottle"),
            new Student("Tommy", "Broadnax"),
            new Student("Jody", "Yates"),
            new Student("Marguerite", "Dawson"),
            new Student("Francisca", "Barnett"),
            new Student("Arlene", "Velasquez"),
            new Student("Jodi", "Green"),
            new Student("Fran", "Mosley"),
            new Student("Taylor", "Nesmith"),
            new Student("Ernesto", "Greathouse"),
            new Student("Margret", "Albert"),
            new Student("Pansy", "House"),
            new Student("Sharon", "Byrd"),
            new Student("Keith", "Roldan"),
            new Student("Martha", "Miranda"),
            new Student("Kari", "Campos"),
            new Student("Muriel", "Middleton"),
            new Student("Georgette", "Jarvis"),
            new Student("Pam", "Boyle"),
            new Student("Deena", "Travis"),
            new Student("Cary", "Totten"),
            new Student("Althea", "Goodwin")
        };

Improved overload resolution

针对Task.Run()进行了优化，之前版本不能正确的区分重载Task.Run(Action)和Task.Run(Func<Task>())

当以下场景时：

static Task DoThings() 
{
     return Task.FromResult(0); 
}

　　之前版本，只能写成：

Task.Run(() => DoThings());

　　6.0就可以正确的写成：

Task.Run(DoThings);

参考：https://docs.microsoft.com/zh-cn/dotnet/articles/csharp/whats-new/csharp-6

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