windows C#-析构元组和其他类型(下)

用户定义类型的扩展方法

如果没有创建类、结构或接口，仍可通过实现一个或多个 Deconstruct 扩展方法来析构该类型的对象，以返回所需值。

以下示例为 System.Reflection.PropertyInfo 类定义了两个 Deconstruct 扩展方法。第一个方法返回一组值，指示属性的特征，包括其类型、是静态还是实例、是否为只读，以及是否已编制索引。第二个方法指示属性的可访问性。因为 get 和 set 访问器的可访问性可能不同，所以布尔值指示属性是否具有单独的 get 和 set 访问器，如果是，则指示它们是否具有相同的可访问性。如果只有一个访问器，或者 get 和 set 访问器具有相同的可访问性，则 access 变量指示整个属性的可访问性。否则，get 和 set 访问器的可访问性由 getAccess 和 setAccess 变量指示。

using System;
using System.Collections.Generic;
using System.Reflection;

public static class ReflectionExtensions
{
    public static void Deconstruct(this PropertyInfo p, out bool isStatic,
                                   out bool isReadOnly, out bool isIndexed,
                                   out Type propertyType)
    {
        var getter = p.GetMethod;

        // Is the property read-only?
        isReadOnly = ! p.CanWrite;

        // Is the property instance or static?
        isStatic = getter.IsStatic;

        // Is the property indexed?
        isIndexed = p.GetIndexParameters().Length > 0;

        // Get the property type.
        propertyType = p.PropertyType;
    }

    public static void Deconstruct(this PropertyInfo p, out bool hasGetAndSet,
                                   out bool sameAccess, out string access,
                                   out string getAccess, out string setAccess)
    {
        hasGetAndSet = sameAccess = false;
        string getAccessTemp = null;
        string setAccessTemp = null;

        MethodInfo getter = null;
        if (p.CanRead)
            getter = p.GetMethod;

        MethodInfo setter = null;
        if (p.CanWrite)
            setter = p.SetMethod;

        if (setter != null && getter != null)
            hasGetAndSet = true;

        if (getter != null)
        {
            if (getter.IsPublic)
                getAccessTemp = "public";
            else if (getter.IsPrivate)
                getAccessTemp = "private";
            else if (getter.IsAssembly)
                getAccessTemp = "internal";
            else if (getter.IsFamily)
                getAccessTemp = "protected";
            else if (getter.IsFamilyOrAssembly)
                getAccessTemp = "protected internal";
        }

        if (setter != null)
        {
            if (setter.IsPublic)
                setAccessTemp = "public";
            else if (setter.IsPrivate)
                setAccessTemp = "private";
            else if (setter.IsAssembly)
                setAccessTemp = "internal";
            else if (setter.IsFamily)
                setAccessTemp = "protected";
            else if (setter.IsFamilyOrAssembly)
                setAccessTemp = "protected internal";
        }

        // Are the accessibility of the getter and setter the same?
        if (setAccessTemp == getAccessTemp)
        {
            sameAccess = true;
            access = getAccessTemp;
            getAccess = setAccess = String.Empty;
        }
        else
        {
            access = null;
            getAccess = getAccessTemp;
            setAccess = setAccessTemp;
        }
    }
}

public class ExampleExtension
{
    public static void Main()
    {
        Type dateType = typeof(DateTime);
        PropertyInfo prop = dateType.GetProperty("Now");
        var (isStatic, isRO, isIndexed, propType) = prop;
        Console.WriteLine($"\nThe {dateType.FullName}.{prop.Name} property:");
        Console.WriteLine($"   PropertyType: {propType.Name}");
        Console.WriteLine($"   Static:       {isStatic}");
        Console.WriteLine($"   Read-only:    {isRO}");
        Console.WriteLine($"   Indexed:      {isIndexed}");

        Type listType = typeof(List<>);
        prop = listType.GetProperty("Item",
                                    BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static);
        var (hasGetAndSet, sameAccess, accessibility, getAccessibility, setAccessibility) = prop;
        Console.Write($"\nAccessibility of the {listType.FullName}.{prop.Name} property: ");

        if (!hasGetAndSet | sameAccess)
        {
            Console.WriteLine(accessibility);
        }
        else
        {
            Console.WriteLine($"\n   The get accessor: {getAccessibility}");
            Console.WriteLine($"   The set accessor: {setAccessibility}");
        }
    }
}
// The example displays the following output:
//       The System.DateTime.Now property:
//          PropertyType: DateTime
//          Static:       True
//          Read-only:    True
//          Indexed:      False
//
//       Accessibility of the System.Collections.Generic.List`1.Item property: public

系统类型的扩展方法

为了方便起见，某些系统类型提供 Deconstruct 方法。例如，System.Collections.Generic.KeyValuePair<TKey,TValue> 类型提供此功能。循环访问 System.Collections.Generic.Dictionary<TKey,TValue> 时，每个元素都是 KeyValuePair<TKey, TValue>，并且可以析构。请考虑以下示例：

Dictionary<string, int> snapshotCommitMap = new(StringComparer.OrdinalIgnoreCase)
{
    ["https://github.com/dotnet/docs"] = 16_465,
    ["https://github.com/dotnet/runtime"] = 114_223,
    ["https://github.com/dotnet/installer"] = 22_436,
    ["https://github.com/dotnet/roslyn"] = 79_484,
    ["https://github.com/dotnet/aspnetcore"] = 48_386
};

foreach (var (repo, commitCount) in snapshotCommitMap)
{
    Console.WriteLine(
        $"The {repo} repository had {commitCount:N0} commits as of November 10th, 2021.");
}

向要没有 Deconstruct 方法的系统类型添加此方法。请考虑以下扩展方法：

public static class NullableExtensions
{
    public static void Deconstruct<T>(
        this T? nullable,
        out bool hasValue,
        out T value) where T : struct
    {
        hasValue = nullable.HasValue;
        value = nullable.GetValueOrDefault();
    }
}

通过此扩展方法，可将所有 Nullable<T> 类型析构为 (bool hasValue, T value) 的元组。下面的示例演示了使用此扩展方法的代码：

DateTime? questionableDateTime = default;
var (hasValue, value) = questionableDateTime;
Console.WriteLine(
    $"{
   
   { HasValue = {hasValue}, Value = {value} }}");

questionableDateTime = DateTime.Now;
(hasValue, value) = questionableDateTime;
Console.WriteLine(
    $"{
   
   { HasValue = {hasValue}, Value = {value} }}");

// Example outputs:
// { HasValue = False, Value = 1/1/0001 12:00:00 AM }
// { HasValue = True, Value = 11/10/2021 6:11:45 PM }

record 类型

使用两个或多个位置参数声明记录类型时，编译器将为 record 声明中的每个位置参数创建一个带有 out 参数的 Deconstruct 方法。