C#之Expression表达式树实例

本文实例讲述了c#之expression表达式树，分享给大家供大家参考。具体实现方法如下：

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<func<int, int, int, int>> 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 });

完整代码如下：

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<func<int, int, int, int>> 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<int, int, int, int> 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<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);
        }
    }
}

希望本文所述对大家的c#程序设计有所帮助。