动态表达式帮助类

以前有一段时间被EF LINQ中的动态表达式实现困扰，因为在实际项目中有很多的可变条件搜索，比方说，我要查询一个人，那么搜索条件就可能有按照用户名搜索、按照昵称搜索、按照年龄搜索，那么这些条件组合在一起就会有很多种，但是我们不肯能为每一种单独写一段程序，所以应该把这些可能产生条件抽离出来。

网上找了很多，都不是很理想，不知道哪里找到的代码（具体哪里忘了，也就懒得再找一次了），贴在这，希望对其他人也有帮助

具体代码：

/* ===============================================================
 * 创 建 者：wms
 * 创建日期：2016/12/5 13:11:22
 * 功能描述：动态表达式帮助类
 * ===============================================================*/

using System;
using System.Linq;
using System.Linq.Expressions;

namespace Framework.Core.Linq
{
    /// <summary>
    /// 动态表达式帮助类
    /// </summary>
    public static class ExpressionHelper
    {
        /// <summary>
        /// Creates a predicate that evaluates to true.
        /// </summary>
        public static Expression<Func<T, bool>> True<T>() { return param => true; }

        /// <summary>
        /// Creates a predicate that evaluates to false.
        /// </summary>
        public static Expression<Func<T, bool>> False<T>() { return param => false; }

        /// <summary>
        /// Creates a predicate expression from the specified lambda expression.
        /// </summary>
        public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; }

        /// <summary>
        /// Combines the first predicate with the second using the logical "and".
        /// </summary>
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.AndAlso);
        }

        /// <summary>
        /// Combines the first predicate with the second using the logical "or".
        /// </summary>
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.OrElse);
        }

        /// <summary>
        /// Negates the predicate.
        /// </summary>
        public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)
        {
            var negated = Expression.Not(expression.Body);
            return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);
        }

        /// <summary>
        /// Combines the first expression with the second using the specified merge function.
        /// </summary>
        static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // zip parameters (map from parameters of second to parameters of first)
            var map = first.Parameters
                .Select((f, i) => new { f, s = second.Parameters[i] })
                .ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with the parameters in the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // create a merged lambda expression with parameters from the first expression
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }
    }
}

using System.Collections.Generic;
using System.Linq.Expressions;

namespace Framework.Core.Linq
{
    /// <summary>
    /// ParameterRebinder
    /// </summary>
    public class ParameterRebinder : ExpressionVisitor
    {
        /// <summary>
        /// The ParameterExpression map
        /// </summary>
        readonly Dictionary<ParameterExpression, ParameterExpression> map;

        /// <summary>
        /// Initializes a new instance of the <see cref="ParameterRebinder"/> class.
        /// </summary>
        /// <param name="map">The map.</param>
        ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
        {
            this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
        }

        /// <summary>
        /// Replaces the parameters.
        /// </summary>
        /// <param name="map">The map.</param>
        /// <param name="exp">The exp.</param>
        /// <returns>Expression</returns>
        public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
        {
            return new ParameterRebinder(map).Visit(exp);
        }

        /// <summary>
        /// Visits the parameter.
        /// </summary>
        /// <param name="p">The p.</param>
        /// <returns>Expression</returns>
        protected override Expression VisitParameter(ParameterExpression p)
        {
            ParameterExpression replacement;

            if (map.TryGetValue(p, out replacement))
            {
                p = replacement;
            }

            return base.VisitParameter(p);
        }
    }
}

我们看一下具体使用方法

 假如要查询用户信息，查询条件有用户名、年龄、昵称

Action代码：

        public ActionResult Index(TestUser user)
        {
            Expression<Func<TestUser, bool>> predicate = Framework.Core.Linq.ExpressionHelper.True<TestUser>();
            if (!string.IsNullOrEmpty(user.UserName))
            {
                predicate = predicate.And(m => m.UserName.Contains(user.UserName));
            }
            if (user.Age != null)
            {
                predicate = predicate.And(m => m.Age == user.Age);
            }
            if (!string.IsNullOrEmpty(user.NickName))
            {
                predicate = predicate.And(m => m.NickName.Contains(user.NickName));
            }
            TestContext dbContext = new TestContext();

            var list = dbContext.TestUser.Where(predicate).ToList();

            return View(list);
        }

这样就简单干净多了，爽