ExPRession表達式樹

表達式樹表示樹狀數據結構的代碼，樹狀結構中的每個節點都是一個表達式，例如一個方法調用或類似 x < y 的二元運算

1.利用 Lambda 表達式創建表達式樹

Expression<Func<int, int, int, int>> expr = (x, y, z) => (x + y) / z;

2.編譯表達式樹，該方法將表達式樹表示的代碼編譯成一個可執行委托

expr.Compile()(1, 2, 3)

3.IQueryable<T>的擴展方法，WhereIn的實現

 var d = list.AsQueryable().WhereIn(o => o.Id1, new int[] { 1, 2 });

完整代碼：

using MongoDB.Bson;using MongoDB.Driver;using MongoDB.Driver.Builders;using System;using System.Collections.Generic;using System.Linq;using System.Linq.Expressions;using System.Text;using System.Threading.Tasks;namespace MongoDBTest{    class Program    {        static void Main(string[] args)        {            //使用LambdaExpression構建表達式樹            Expression<Func<int, int, int, int>> expr = (x, y, z) => (x + y) / z;            Console.WriteLine(expr.Compile()(1, 2, 3));            //使用LambdaExpression構建可執行的代碼            Func<int, int, int, int> fun = (x, y, z) => (x + y) / z;            Console.WriteLine(fun(1, 2, 3));            //動態構建表達式樹            ParameterExpression pe1 = Expression.Parameter(typeof(int), "x");            ParameterExpression pe2 = Expression.Parameter(typeof(int), "y");            ParameterExpression pe3 = Expression.Parameter(typeof(int), "z");            var body = Expression.Divide(Expression.Add(pe1, pe2), pe3);            var w = Expression.Lambda<Func<int, int, int, int>>(body, new ParameterExpression[] { pe1, pe2, pe3 });            Console.WriteLine(w.Compile()(1, 2, 3));            List<Entity> list = new List<Entity> { new Entity { Id1 = 1 }, new Entity { Id1 = 2 }, new Entity { Id1 = 3 } };            var d = list.AsQueryable().WhereIn(o => o.Id1, new int[] { 1, 2 });            d.ToList().ForEach(o =>            {                Console.WriteLine(o.Id1);            });            Console.ReadKey();        }    }    public class Entity    {        public ObjectId Id;        public int Id1;        public string Name { get; set; }    }    public static class cc    {        public static IQueryable<T> WhereIn<T, TValue>(this IQueryable<T> query, Expression<Func<T, TValue>> obj, IEnumerable<TValue> values)        {            return query.Where(BuildContainsExpression(obj, values));        }        private static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)        {            if (null == valueSelector)            {                throw new ArgumentNullException("valueSelector");            }            if (null == values)            {                throw new ArgumentNullException("values");            }            var p = valueSelector.Parameters.Single();            if (!values.Any()) return e => false;            var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue))));            var body = equals.Aggregate(Expression.Or);            return Expression.Lambda<Func<TElement, bool>>(body, p);        }    }}

參考博客：

1. C#中的表達式樹
2. 表達式樹基礎
3. C#高級程序設計（九）——表達式樹
4. 表達式樹MSDN