基于Linq表达式做的一个简单的表达式生成器

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Linq.Expressions;

namespace Model
{
    [NotMapped]
    public class FilterInfo
    {
        /// <summary>
        /// 字段名称
        /// </summary>
        public string FieldName { get; set; }
        /// <summary>
        /// 字段值
        /// </summary>
        public object FieldValue { get; set; }
        /// <summary>
        /// 操作符
        /// </summary>
        public FilterOperator Operator { get; set; }
        /// <summary>
        /// 构建表达式
        /// </summary>
        /// <returns></returns>
        public Func<T, bool> BuildExpression<T>()
        {
            var constEexp = Expression.Constant(FieldValue, this.FieldValue.GetType());
            var paramExp = Expression.Parameter(typeof(T), "x");
            Expression expLeft = null;
            var tp = typeof(T);
            //基元类型直接去paramExp,自定义类型取PropExp
            if (!tp.IsPrimitive && tp != typeof(string))
            {
                expLeft = (Expression)Expression.Property(paramExp, FieldName);
            }
            else
            {
                expLeft = paramExp;
            }
            Expression bExp = null;
            switch (Operator)
            {
                case FilterOperator.GreaterThan://>
                    bExp = Expression.GreaterThan(expLeft, constEexp);
                    break;
                case FilterOperator.GreaterThanOrEqual://>=
                    bExp = Expression.GreaterThanOrEqual(expLeft, constEexp);
                    break;
                case FilterOperator.LessThan://<
                    bExp = Expression.LessThan(expLeft, constEexp);
                    break;
                case FilterOperator.LessThanOrEqul://<=
                    bExp = Expression.LessThanOrEqual(expLeft, constEexp);
                    break;
                case FilterOperator.OrElse://!=
                    bExp = Expression.NotEqual(expLeft, constEexp);
                    break;
                case FilterOperator.Like:
                    if (this.FieldValue.GetType() != typeof(string))
                    {
                        throw (new Exception("Like只能用于string类型的字段操作"));
                    }
                    bExp = Expression.Call(typeof(FilterInfo).GetMethod("Contains", new Type[] { typeof(string), typeof(string) }), expLeft, constEexp);
                    break;
                default://=
                    bExp = Expression.Equal(expLeft, constEexp);
                    break;
            }
            var exp = Expression.Lambda<Func<T, bool>>(bExp, paramExp);
            Console.WriteLine(exp);
            return exp.Compile();
        }
        /// <summary>
        /// 用该静态方法实现like
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <returns></returns>
        public static bool Contains(string a, string b)
        {
            if (string.IsNullOrEmpty(a) || string.IsNullOrEmpty(b))
            {
                return false;
            }
            return a.Contains(b);
        }
    }
    /// <summary>
    /// 操作符枚举
    /// </summary>
    public enum FilterOperator
    {
        /// <summary>
        /// >
        /// </summary>
        GreaterThan,
        /// <summary>
        /// =
        /// </summary>
        Equal,
        /// <summary>
        /// <
        /// </summary>
        LessThan,
        /// <summary>
        /// <=
        /// </summary>
        LessThanOrEqul,
        /// <summary>
        /// >=
        /// </summary>
        GreaterThanOrEqual,
        /// <summary>
        /// !=
        /// </summary>
        OrElse,
        /// <summary>
        /// contain
        /// </summary>
        Like
    }
}

使用该表达式生成器，可以生成简单的匿名查询函数来过滤查询结果。如果我们在项目中用了EF框架，就可以在业务层中通过这个类来构建查询条件然后传到DAL层来执行，使用方法如下：

 //整形数组查询
            int[] nums = { 5, 4, 2, 2, 9, 8, 6, 7, 2, 0 };
            FilterInfo filterInt = new FilterInfo() { FieldName = "x", FieldValue = 2, Operator = FilterOperator.LessThanOrEqul };
            var intFunc = filterInt.BuildExpression<int>();
            var r = nums.Where(intFunc);
            foreach (var i in r)
            {
                Console.WriteLine(i);
            }
            Console.WriteLine("int array search complete");
            //字符串数组查询
            string[] strArray = { "string1", "string11", "string2", "string2134", "stringOld" };
            FilterInfo f = new FilterInfo() { FieldName = "x", FieldValue = "1", Operator = FilterOperator.Like };
            var fc = f.BuildExpression<string>();
            strArray.Where(fc).ToList().ForEach(s => Console.WriteLine(s));
            //对象列表查询
            List<Org> orglist = new List<Org>();
            orglist.Add(new Org() { Guid = Guid.NewGuid().ToString(), Name = "spring" });
            for (int i = 0; i < 10; i++)
            {
                orglist.Add(new Org() { Guid = Guid.NewGuid().ToString(), Name = "orgName" + Guid.NewGuid().ToString() });
            }
            FilterInfo filter = new FilterInfo() { FieldName = "Name", FieldValue = "Name", Operator = FilterOperator.Like };
            var func = filter.BuildExpression<Org>();
            var orgSearchList = orglist.Where(func);
            foreach (var o in orgSearchList)
            {
                Console.WriteLine(o.Name);
            }
            Console.WriteLine("org list search Complete");