EntityFramework动态多条件查询与Lambda表达式树

          在常规的信息系统中, 我们有需要动态多条件查询的情况, 例如UI上有多个选择项可供用户选择多条件查询数据.
那么在.net平台Entity Framework下, 我们用Lambda表达式树如何实现, 这里我们需要一个PredicateBuilder的UML类图:

实现的代码是这样的:

   /// <summary>

    /// Enables the efficient, dynamic composition of query predicates.

    /// </summary>

    public static class PredicateBuilder

    {

        /// <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);

        }

        /// <summary>

        /// ParameterRebinder

        /// </summary>

        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);

            }

        }

    }

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UnitTest的片断代码, 一个产品查询的场景:

            var myProduct=pr.Repository.Find(

                  BuildFindByAllQuery(productName, beignUpdateDate, endUpdateDate) ,

                e => e.UpdatedTime,

                pageIndex,

                pageSize);

            Assert.IsTrue(myProduct.Count>0);

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UnitTest使用到 生成查询条件 的 私有方法:

         /// <summary>

         /// Builds the find by all query.

         /// </summary>

         private static Expression<Func<Product, bool>> BuildFindByAllQuery(string productName,DateTime? beignUpdateDate, DateTime? endUpdateDate)

         {

             var list = new List<Expression<Func<Product, bool>>>();

             if (!string.IsNullOrEmpty(productName)) list.Add(c => c.ProductName == productName);

             if (beignUpdateDate != null) list.Add(c => c.UpdatedTime >= beignUpdateDate);

             if (endUpdateDate != null) list.Add(c => c.UpdatedTime <= endUpdateDate);

             //Add more condition

             Expression<Func<Product, bool>> productQueryTotal = null;

             foreach (var expression in list)

             {

                 productQueryTotal = expression.And(expression);

             }

             return productQueryTotal;

         }

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上面的方法中由三个条件动态组成,  一个是匹配productName, 另两个是beginUpdateDate与endUpdateDate. 在判断它们是否为时, 构建最终查询条件集合.

最后把结果传给某个Repository类, 完成相应的数据访问.

是不是很简单, 希望对您的软件开发有帮助.

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作者：Petter Liu

出处：http://www.cnblogs.com/wintersun/

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