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 }