表达式树表示树状数据结构的代码，树状结构中的每个节点都是一个表达式，例如一个方法调用或类似 x < y 的二元运算

1.利用 Lambda 表达式创建表达式树

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

2.编译表达式树，该方法将表达式树表示的代码编译成一个可执行委托

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

3.IQueryable<T>的扩展方法，WhereIn的实现

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

 完整代码：

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构建表达式树            Expression
     
      
       > expr = (x, y, z) => (x + y) / z;            Console.WriteLine(expr.Compile()(1, 2, 3));            //使用LambdaExpression构建可执行的代码            Func
       
         fun = (x, y, z) => (x + y) / z;            Console.WriteLine(fun(1, 2, 3));            //动态构建表达式树            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
        
         
          >(body, new ParameterExpression[] { pe1, pe2, pe3 });            Console.WriteLine(w.Compile()(1, 2, 3));            List
          
            list = new List
           
             { 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
            
              WhereIn
             
              (this IQueryable
              
                query, Expression
               
                
                 > obj, IEnumerable
                 
                   values) { return query.Where(BuildContainsExpression(obj, values)); } private static Expression
                  
                   
                    > BuildContainsExpression
                    
                     (Expression
                     
                      
                       > valueSelector, IEnumerable
                       
                         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
                        
                         
                          >(body, p); } }}
                         
                        
                       
                      
                     
                    
                   
                  
                 
                
               
              
             
            
           
          
         
        
       
      
     

 

 参考博客：