【笔记】【LINQ编程技术内幕】第三章定义扩展方法和分布方法

最新推荐文章于 2020-07-04 09:39:10 发布

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1. 扩展方法及其使用规则

使用扩展方法可以扩展密封类型，无需继承
利用扩展方法可以避免出现失控的深度继承层次体系
扩展方法是静态类中的静态方法
扩展方法第一个参数的类型之前必须使用this修饰符；this指带将要被扩展的类型
不支持扩展属性、事件和操作符
相对于实例方法而言，扩展方法的可视性要差一些，而且限制也会多一些
拓展方法是通过实例语法来调用的
拓展方法的优先级比常规方法要低；因此，如果某个类有用一个同名方法的话，那么被调用的将会是实例方法
扩展方法得到只能感知的支持
泛型扩展方法是得到支持的，事实上，System.Linq命名空间中许多LINQ关键字就是通过这个而实现的
扩展方法能在字面量上调用
拓展方法可以用在密封类和内部类上，因为扩展方法自动支持装箱和拆箱
拓展方法并不是真正的成员，因此只能在其中访问被扩展对象的公共成员
拓展方法隐式使用ExtensionAttribute

class Program
{
	static void Main(string[] args)
	{
		var song = new { Artist = "Jussi Bjorling", Song = "Aida" };
		song.Dump();
	}
}

// 静态类
public static class Dumper
{
	// 静态方法
	public static void Dump(this Object o)
	{
		PropertyInfo[] properties = o.GetType().GetProperties();
		foreach (PropertyInfo p in properties)
		{
			try
			{
				Debug.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name, p.GetValue(o, null)));
			}
			catch
			{
				Debug.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name, "unk."));
			}
		}
	}
}

2. 带返回值的扩展方法

class Program
{
	static void Main(string[] args)
	{
		var songs = new
		{
			Artist = "Green Day",
			Song = "Wake Me Up When September Ends"
		};

		Console.WriteLine(songs.Dump());
	}
}

public static class Dumper
{
	// 带返回值得扩展方法
	public static string Dump(this Object o)
	{
		PropertyInfo[] properties = o.GetType().GetProperties();
		StringBuilder builder = new StringBuilder();
		foreach (PropertyInfo p in properties)
		{
			try
			{
				builder.AppendFormat(string.Format("Name: {0}, Value: {1}", p.Name,p.GetValue(o, null)));
			}
			catch
			{
				builder.AppendFormat(string.Format("Name: {0}, Value: {1}", p.Name,"unk."));
			}
			builder.AppendLine();
		}
		return builder.ToString();
	}
}

3. 带参数得扩展方法

static void Main(string[] args)
{
	var song = new
	{
		Artist = "Avril Lavigne",
		Song = "My Happy Ending"
	};

	song.Dump(Console.Out);
}



public static class Dumper
{
	public static void Dump(this Object o, TextWriter writer)
	{
		PropertyInfo[] properties = o.GetType().GetProperties();
		foreach (PropertyInfo p in properties)
		{
			try
			{
				writer.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name, p.GetValue(o, null)));
			}
			catch
			{
				writer.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name, "unk."));
			}
		}
	}
}

4. 重载扩展方法

class Program
{
	static void Main(string[] args)
	{
		var songs = new[]
		{
		  new {Artist="Jussi Bjorling", Song="Aida"},
		  new {Artist="Sheryl Crow", Song="Steve McQueen"}
		};

		songs.Dump();
	}
}



public static class Dumper
{
	public static void Dump(this Object o)
	{
		PropertyInfo[] properties = o.GetType().GetProperties();
		foreach (PropertyInfo p in properties)
		{
			try
			{
				Debug.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name,p.GetValue(o, null)));
			}
			catch
			{
				Debug.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name,"unk."));
			}
		}
	}

	public static void Dump(this IList list)
	{
		foreach (object o in list)
			o.Dump();
	}
}

5. 泛型扩展方法

class Program
{
	static void Main(string[] args)
	{
		// 连接字符串
		string connectionString = "Data Source=CASPAR;Initial Catalog=Northwind;Integrated Security=True";
		
		// 存储结果集合
		List<Order> orders = new List<Order>();

		// 数据库连接
		using (SqlConnection connection = new SqlConnection(connectionString))
		{
			// 执行查询
			SqlCommand command = new SqlCommand("SELECT * FROM ORDERS", connection);
			command.CommandType = CommandType.Text;
			connection.Open();
			SqlDataReader reader = command.ExecuteReader();

			// 读取查询结果
			while (reader.Read())
			{
				Order order = new Order();
				
				// 根据读取得结果初始化Order实例
				order.Read(reader);
				// 存储结果
				orders.Add(order);
			}
		}

		// 显示结果
		orders.Dump(Console.Out);
	}
}

/// <summary>Order类得扩展方法，用于读取指定得字段。EntityClass拓展方法根据值是否为空返回不同得值</summary>
public static class ReaderHelper
{
	public static void Read(this Order order, IDataReader reader)
	{
		order.OrderID = order.SafeRead(reader, "OrderID", -1);
		order.CustomerID = order.SafeRead(reader, "CustomerID", "");
	}

	public static T SafeRead<T>(this EntityClass entity,IDataReader reader, string fieldName, T defaultValue)
	{
		try
		{
			object o = reader[fieldName];
			if (o == null || o == System.DBNull.Value)
				return defaultValue;

			return (T)Convert.ChangeType(o, defaultValue.GetType());
		}
		catch
		{
			return defaultValue;
		}
	}
}

/// <summary>封装扩展方法得静态类，该类中得拓展方法用于显示信息</summary>
public static class Dumper
{
	public static void Dump(this Object o, TextWriter writer)
	{
		PropertyInfo[] properties = o.GetType().GetProperties();
		foreach (PropertyInfo p in properties)
		{
			try
			{
				writer.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name,p.GetValue(o, null)));
			}
			catch
			{
				writer.WriteLine(string.Format("Name: {0}, Value: {1}", p.Name,"unk."));
			}
		}
	}

	public static void Dump<T>(this IList<T> list, TextWriter writer)
	{
		foreach (object o in list)
			o.Dump(writer);
	}
}


/// <summary>空得实体类，用于演示泛型拓展方法</summary>
public class EntityClass { }

/// <summary>该类继承自实体类</summary>
public class Order : EntityClass
{
	public Order(){}
	public int OrderID {get;set;}
	public string CustomerID {get;set;}
}

6. 拓展方法是如何支持LINQ得

LIQN是一种语法糖，在底层调用的则是对应的拓展方法。

LIQN查询

var numbers = new int[] { 1966, 1967, 1968, 1969, 1970 };

var evens = from num in numbers where num % 2 == 0 select num;

foreach (var result in evens)
	Console.WriteLine(result);

拓展方法

var numbers = new int[] { 1966, 1967, 1968, 1969, 1970 };

var evens = numbers.Where(item => item % 2 == 0).Select(item=>item);

foreach (var result in evens)
	Console.WriteLine(result);

7. 分部方法

分部方法要声明在分部类中
分部方法要使用partial修饰符
分部方法在声明时不能有方法体
分部方法之呢个返回void
分部方法可以时静态的，而且可以有参数和参数修饰符
分部方法是私有的，不过不能再字面上使用访问修饰符；也就是说，你不能显示使用private关键字
编译器忽略未使用的分部方法

class Program
{
	static void Main(string[] args)
	{
		// 复合类型初始化
		CustomerList customers = new CustomerList
			{
				new Customer{CustomerID=1, Name="Levi, Ray and Shoup"},
				new Customer{CustomerID=2, Name="General Dynamics Land Systems"},
				new Customer{CustomerID=3, Name="Microsoft"}
			};

		customers.Dump();
	}
}

// 分部类
public partial class CustomerList
{
	private string propertyName;
	private SortDirection direction;

	// 分部方法
	partial void SpecialSort(string propertyName, SortDirection direction)
	{
		this.propertyName = propertyName;
		this.direction = direction;

		Sort(Comparer);
	}

	/// <summary>
	/// Using an integer to change the direction of the sort was a suggestion made by 
	/// Chris Chartran, my good friend from Canada
	/// </summary>
	/// <param name="x"></param>
	/// <param name="y"></param>
	/// <returns></returns>
	private int Comparer(Customer x, Customer y)
	{
		try
		{
			PropertyInfo lhs = x.GetType().GetProperty(propertyName);
			PropertyInfo rhs = y.GetType().GetProperty(propertyName);

			int directionChanger = direction == SortDirection.Ascending ? 1 : -1;

			object o1 = lhs.GetValue(x, null);
			object o2 = rhs.GetValue(y, null);

			if (o1 is IComparable && o2 is IComparable)
			{
				return ((IComparable)o1).CompareTo(o2) * directionChanger;
			}

			// no sort
			return 0;
		}
		catch (Exception ex)
		{
			Debug.WriteLine(ex.Message);
			return 0;
		}
	}
}

// 分部类
public partial class CustomerList : List<Customer>
{
	// 声明分部方法
	partial void SpecialSort(string propertyName, SortDirection direction);

	public void Dump()
	{
		SpecialSort("CustomerID", SortDirection.Descending);

		foreach (var customer in this)
			Console.WriteLine(customer.ToString());
	}
}


public class Customer
{
	public int CustomerID { get; set; }
	public string Name { get; set; }

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