c#3.0新特性

Posted 2020-07-26 浩魔

1.自动属性

   public int ID { get; set; }
    
    // 上面的ID属性（自动属性）等同于下面的ID属性

    // private int _id;
    // public int ID
    // {
    //     get { return _id; }
    //     set { _id = value; }
    // }

 

2.对象初始化器

/// <summary>
/// ObjectInitializers（对象初始化器）的摘要说明
/// </summary>
public class ObjectInitializers
{
    public int ID { get; set; }
    public string Name { get; set; }

    public void ObjectInitializersTest()
    {
        ObjectInitializers oi = new ObjectInitializers { ID = 1, Name = "webabcd" };

        // 上面的oi对象（对象初始化器）等同于下面的oi对象

        // ObjectInitializers oi = new ObjectInitializers();
        // oi.ID = 1;
        // oi.Name = "webabcd";
    }
}

 3.集合初始化器

/// <summary>
/// CollectionInitializers（集合初始化器）的摘要说明
/// </summary>
public class CollectionInitializers
{
    public int ID { get; set; }
    public string Name { get; set; }

    public void CollectionInitializersTest()
    {
        List<CollectionInitializers> list = new List<CollectionInitializers>
        {
            new CollectionInitializers { ID = 1, Name = "webabcd" },
            new CollectionInitializers { ID = 2, Name = "webabcdefg" },
            new CollectionInitializers { ID = 3, Name = "webabcdefghijklmn" }
        };

        // 上面的list集合（集合初始化器）等同于下面的list集合

        // List<CollectionInitializers> list = new List<CollectionInitializers>();
        // list.Add(new CollectionInitializers { ID = 1, Name = "webabcd" });
        // list.Add(new CollectionInitializers { ID = 2, Name = "webabcdefg" });
        // list.Add(new CollectionInitializers { ID = 3, Name = "webabcdefghijklmn" });
    }
}

4.扩展方法

/// <summary>
/// 扩展方法（类和方法均为static）
/// 使用的时候要引用该类的命名空间
/// </summary>
public static class MyExtensionMethods
{
    // this代表扩展方法应用于string类型上
    // ToInt32()是将string类型转换为int类型的扩展方法
    public static int ToInt32(this string s)
    {
        int i;
        Int32.TryParse(s, out i);

        return i;
    }

    // this代表扩展方法应用于object类型上
    // 该扩展方法需要一个类型为System.Collections.IEnumerable的参数
    // In()是判断一个object是否存在于一个System.Collections.IEnumerable中的扩展方法
    public static bool In(this object o, System.Collections.IEnumerable e)
    {
        foreach (object i in e)
        {
            if (i.Equals(o))
            {
                return true;
            }
        }

        return false;
    }
}

        string s = "123";
        // 使用string的ToInt32()扩展方法
        int i = s.ToInt32();
        // i == 123

        string[] ary = new string[] { "a", "b", "c" };
        // 使用object的In()扩展方法
        bool b = "b".In(ary);
        // b == true    

5.Lambda表达式

/// <summary>
/// LambdaExpressions（Lambda表达式）的摘要说明
/// </summary>
public class LambdaExpressions
{
    public int ID { get; set; }
    public string Name { get; set; }

    public void LambdaExpressionsTest()
    {
        List<LambdaExpressions> list = new List<LambdaExpressions>
        {
            new LambdaExpressions { ID = 1, Name = "webabcd" },
            new LambdaExpressions { ID = 2, Name = "webabcdefg" },
            new LambdaExpressions { ID = 3, Name = "webabcdefghijklmn" }
        };

        IEnumerable<LambdaExpressions> l = list.Where(le => le.Name == "webabcd");

        // 上面的（Lambda表达式）等同于下面的（匿名方法）

        // IEnumerable<LambdaExpressions> l2 = list.Where(delegate(LambdaExpressions le) { return le.Name == "webabcd"; });



        // 相关委托
        // public delegate TResult Func<T, TResult>(T arg);
     
        // 相关Where扩展方法
        // Func<TSource, bool>：接受一个类型为TSource的参数
        // Func<TSource, bool>：某个需要满足的条件，返回bool值
        // public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
        // {
        //     foreach (TSource item in source)
        //     {
        //         if (predicate(item))
        //         {
        //             yield return item;
        //         }
        //     }
        // }
        
    }
}

6.查询语法

/// <summary>
/// QuerySyntax（查询语法）的摘要说明
/// </summary>
public class QuerySyntax
{
    public int ID { get; set; }
    public string Name { get; set; }

    public void QuerySyntaxTest()
    {
        List<QuerySyntax> list = new List<QuerySyntax>
        {
            new QuerySyntax { ID = 1, Name = "webabcd" },
            new QuerySyntax { ID = 2, Name = "webabcde" },
            new QuerySyntax { ID = 2, Name = "webabcdef" },
            new QuerySyntax { ID = 2, Name = "webabcdefg" },
            new QuerySyntax { ID = 2, Name = "webabcdefgh" },
            new QuerySyntax { ID = 2, Name = "webabcdefghi" },
            new QuerySyntax { ID = 2, Name = "webabcdefghij" },
            new QuerySyntax { ID = 2, Name = "webabcdefghijk" },
            new QuerySyntax { ID = 2, Name = "webabcdefghijkl" },
            new QuerySyntax { ID = 2, Name = "webabcdefghijklm" },
            new QuerySyntax { ID = 3, Name = "webabcdefghijklmn" }
        };

        IEnumerable<QuerySyntax> l = from o in list
                                     where o.Name.Length > 10
                                     orderby o.Name.Length descending
                                     select o;

        // 上面的（查询语法）等同于下面的（LINQ扩展方法和Lambda表达式）
        // 查询语法相对更容易理解

        // IEnumerable<QuerySyntax> l = list.Where(o => o.Name.Length > 10).OrderByDescending(o => o.Name.Length);



        // Projection（映射）
        // 可以返回一个新的类型
        IEnumerable<Projection> l2 = from o in list
                                      where o.Name.Length > 10
                                      orderby o.Name.Length descending
                                      select new Projection { Name = o.Name };

        var l3 = from o in list
                 where o.Name.Length > 10
                 orderby o.Name.Length descending
                 select new { Name=o.Name};
    }
}

/// <summary>
/// 为了演示Projection（映射）而写的实体类
/// </summary>
public class Projection
{
    public string Name { get; set; }
}

7.匿名对象

/// <summary>
/// AnonymousTypes（匿名类型）的摘要说明
/// </summary>
public class AnonymousTypes
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }

    public void AnonymousTypesTest()
    {
        List<AnonymousTypes> list = new List<AnonymousTypes>
        {
            new AnonymousTypes { ID = 1, Name = "webabcd", Age = 10 },
            new AnonymousTypes { ID = 2, Name = "webabcdefg", Age = 20 },
            new AnonymousTypes { ID = 3, Name = "webabcdefghijklmn", Age = 30 }
        };

        // listAnonymousTypes - 匿名类型
        var listAnonymousTypes = from l in list
                                 where l.Name == "webabcd"
                                 select new { Name = l.Name, Age = l.Age };

        foreach (var v in listAnonymousTypes)
        {
            // v - 匿名类型，可以在Visual Studio中得到编译时检查和完整的intellisense
            string name = v.Name;
            int age = v.Age;
        }

        // 声明匿名类型：将new关键词后面的类型名称省略掉
        var person = new { Name = "webabcd", Age = 27 };
        // person - 匿名类型，可以在Visual Studio中得到编译时检查和完整的intellisense
        string myName = person.Name;
        int myAge = person.Age;
    }
}