c# Expression 擴展

一、簡介

當查詢比較復雜時，需要很多判斷或者跨方法傳遞參數時使用

二、擴展類

/// <summary>
    ///     Expression表達式擴展操作類
    /// 調用方法：repository.GetAll().AsExpandable().Where(predicate)
    /// </summary>
    public static class ExpressionExtensions
    { 
        /// <summary>
        ///     以特定的條件運行組合兩個Expression表達式
        /// </summary>
        /// <typeparam name="T">表達式的主實體類型</typeparam>
        /// <param name="first">第一個Expression表達式</param>
        /// <param name="second">要組合的Expression表達式</param>
        /// <param name="merge">組合條件運算方式</param>
        /// <returns>組合后的表達式</returns>
        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second,
            Func<Expression, Expression, Expression> merge)
        {
            var map =
                first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        /// <summary>
        ///     以 Expression.AndAlso 組合兩個Expression表達式
        /// </summary>
        /// <typeparam name="T">表達式的主實體類型</typeparam>
        /// <param name="first">第一個Expression表達式</param>
        /// <param name="second">要組合的Expression表達式</param>
        /// <returns>組合后的表達式</returns>
        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>
        ///     以 Expression.OrElse 組合兩個Expression表達式
        /// </summary>
        /// <typeparam name="T">表達式的主實體類型</typeparam>
        /// <param name="first">第一個Expression表達式</param>
        /// <param name="second">要組合的Expression表達式</param>
        /// <returns>組合后的表達式</returns>
        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);
        }

        private class ParameterRebinder : ExpressionVisitor
        {
            private readonly Dictionary<ParameterExpression, ParameterExpression> _map;

            private ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
            {
                _map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
            }

            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map,
                Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            }

            protected override Expression VisitParameter(ParameterExpression node)
            {
                ParameterExpression replacement;
                if (_map.TryGetValue(node, out replacement))
                    node = replacement;
                return base.VisitParameter(node);
            }
        }
    }

 

三、如何使用

1.關於引用

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

using LinqKit;

還需要引入擴展類的命名空間

2.使用示例

            Expression<Func<User, bool>> pre;
            pre = s => s.NickName.Contains("李");
            pre = pre.Or(s => s.NickName.Contains("陳"));
            pre = pre.And(s => s.CompanyId == "1");

            var data = _userRepository.GetAll().AsExpandable().Where(pre);