表达式树

            Console.WriteLine("***********************************************");

            {
                //1.0获取方法
                MethodInfo method = typeof(string).GetMethod("Substring", new[] { typeof(int), typeof(int) });

                //2.0构造表达式树的参数
                ParameterExpression target = Expression.Parameter(typeof(string), "s");
                ParameterExpression firstParam = Expression.Parameter(typeof(int), "a");
                ParameterExpression secondParam = Expression.Parameter(typeof(int), "b");

                //3.0构造方法的参数
                Expression[] methodArgs = new[] { firstParam, secondParam };
                Expression call = Expression.Call(target, method, methodArgs);
                Console.WriteLine(call);//  s.Substring(a,b);

                //4.0将表达式树的参数构造成 lambda表达式的参数
                var lambdaParam = new[] { target, firstParam, secondParam };
                foreach (var parameterExpression in lambdaParam)
                {
                    Console.WriteLine(parameterExpression);//s,a,b
                }

                //5.0构造 lambda表达式树
                var lambda = Expression.Lambda<Func<string, int, int, string>>(call, lambdaParam);
                Console.WriteLine(lambda);//    (s,a,b)=>s.Substring(a,b);

                //6.0将 lambda表达式树转换成 lambda表达式
                var compile = lambda.Compile();

                //7.0执行委托
                var result = compile("wjire", 0, 3);

                Console.WriteLine(result);
            }

            Console.WriteLine("***********************************************");

            {

                //1.0获取方法
                MethodInfo method = typeof(string).GetMethod("Substring", new[] { typeof(int), typeof(int) });

                //2.0构造表达式树的参数
                ParameterExpression target = Expression.Parameter(typeof(string), "s");

                //2.5设置表达式树的常量
                ConstantExpression firstParam = Expression.Constant(0);
                ConstantExpression secondParam = Expression.Constant(3);

                //3.0构造方法的参数
                Expression[] methodArgs = new[] { firstParam, secondParam };
                Expression call = Expression.Call(target, method, methodArgs);
                Console.WriteLine(call);//  s.Substring(0,3);

                //4.0将表达式树的参数构造成 lambda表达式的参数
                var lambdaParam = new[] { target };
                foreach (var parameterExpression in lambdaParam)
                {
                    Console.WriteLine(parameterExpression);//s
                }

                //5.0构造 lambda表达式树
                var lambda = Expression.Lambda<Func<string, string>>(call, lambdaParam);
                Console.WriteLine(lambda);//    s=>s.Substring(0,3);

                //6.0将 lambda表达式树转换成 lambda表达式
                var compile = lambda.Compile();

                //7.0执行委托
                var result = compile("wjire");

                Console.WriteLine(result);
            }

            Console.WriteLine("***********************************************");


            {
                PropertyInfo property = typeof(Person).GetProperty("Id");
                var target = Expression.Parameter(typeof(Person), "p");
                var pro = Expression.Property(target, property);
                Console.WriteLine(pro);//   p.Id

                var lambdaParam = new[] { target };

                var lambda = Expression.Lambda<Func<Person, int>>(pro, lambdaParam);
                Console.WriteLine(lambda);//    p=>p.Id    

                var compile = lambda.Compile();

                var result = compile(new Person { Id = 123 });

                Console.WriteLine(result);
            }