• Lambda 表达式的深化及使用


    1.1  Lambda 表达式

    1.1.1 创建委托或表达式树

    “Lambda 表达式是一个匿名函数,它可以包含表达式和语句,并且可用于创建委托或表达式树类型。

        class Program

        {

            static void Main(string[] args)

            {

                del myDelegate = x => x * x;

                int j = myDelegate(5); //得到j = 25

     

                Console.WriteLine(j);

                Console.Read();

            }

        }

        delegate int del(int i);

        class Program

        {

            static void Main(string[] args)

            {

                Expression<del> myET = x => x * x;

                Console.WriteLine(myET);//得到x => ( x * x )

     

                Console.Read();

            }

        }

        delegate int del(int i);

        class Program

        {

            static void Main(string[] args)

            {

                del myET = new del((x, y) => x + y);

     

                Console.WriteLine(myET(7,8));//15

     

                Console.Read();

            }

        }

        delegate int del(int i,int y);

     

    1.2     带有标准查询运算符的 Lambda

    许多标准查询运算符都具有输入参数,其类型是泛型委托的 Func<T, TResult> 系列的其中之一。 Func<T, TResult> 委托使用类型参数定义输入参数的数目和类型,以及委托的返回类型。

                Func<int, bool> myFunc = x => x == 5;

                bool result = myFunc(6); //得到false

                Console.WriteLine(result);

                int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

                int oddNumbers = numbers.Count(n => n % 2 == 1);//得到5(2取余等于1的数字个数)

                Console.WriteLine(oddNumbers);

    1.3     Lambda 表达式创建表达式树

    在将 lambda 表达式分配给 Expression<TDelegate> 类型的变量时,编译器将发出代码以生成一个表示 lambda 表达式的表达式树。

                Expression<Func<int, bool>> lambda = num => num < 5;

                Console.WriteLine(lambda);//得到num =>( num < 5 )

     

    1.4     API 创建表达式树

                ParameterExpression numParam = Expression.Parameter(typeof(int), "num");

                ConstantExpression five = Expression.Constant(5, typeof(int));

                BinaryExpression numLessThanFive = Expression.LessThan(numParam, five);

                Expression<Func<int, bool>> lambda1 =

                    Expression.Lambda<Func<int, bool>>(

                        numLessThanFive,

                        new ParameterExpression[] { numParam });

     

                Console.WriteLine(numParam);//得到num

                Console.WriteLine(five);//5

                Console.WriteLine(numLessThanFive);//得到( num < 5 )

     

    1.5  分析表达式树

                Expression<Func<int, bool>> exprTree = num => num < 5;

     

                ParameterExpression param = (ParameterExpression)exprTree.Parameters[0];

                BinaryExpression operation = (BinaryExpression)exprTree.Body;

                ParameterExpression left = (ParameterExpression)operation.Left;

                ConstantExpression right = (ConstantExpression)operation.Right;

     

                Console.WriteLine("Decomposed expression: {0} => {1} {2} {3}",

                                  param.Name, left.Name, operation.NodeType, right.Value);

    //得到Decomposed expression:num => num LessThan 5

     

    1.6  编译表达式树

                Expression<Func<int, bool>> expr = num => num < 5;

                Func<int, bool> result = expr.Compile();

     

                Console.WriteLine(result(4));//True

                Console.WriteLine(expr.Compile()(5));//False

    1.7  执行表达式

                BinaryExpression be = Expression.Power(Expression.Constant(2D), Expression.Constant(3D));

                Expression<Func<double>> le = Expression.Lambda<Func<double>>(be);

                Func<double> compiledExpression = le.Compile();

                double result = compiledExpression();

                Console.WriteLine(result);//得到8

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  • 原文地址:https://www.cnblogs.com/yexinw/p/2193585.html
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