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:

   /// 

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

    /// 

    public static class PredicateBuilder

    {

        /// 

        /// Creates a predicate that evaluates to true.

        /// 

        public static Expressionbool>> True() { return param => true; }

        /// 

        /// Creates a predicate that evaluates to false.

        /// 

        public static Expressionbool>> False() { return param => false; }

        /// 

        /// Creates a predicate expression from the specified lambda expression.

        /// 

        public static Expressionbool>> Create(Expressionbool>> predicate) { return predicate; }

        /// 

        /// Combines the first predicate with the second using the logical "and".

        /// 

        public static Expressionbool>> And(this Expressionbool>> first, Expressionbool>> second)

        {

            return first.Compose(second, Expression.AndAlso);

        }

        /// 

        /// Combines the first predicate with the second using the logical "or".

        /// 

        public static Expressionbool>> Or(this Expressionbool>> first, Expressionbool>> second)

        {

            return first.Compose(second, Expression.OrElse);

        }

        /// 

        /// Negates the predicate.

        /// 

        public static Expressionbool>> Not(this Expressionbool>> expression)

        {

            var negated = Expression.Not(expression.Body);

            return Expression.Lambdabool>>(negated, expression.Parameters);

        }

        /// 

        /// Combines the first expression with the second using the specified merge function.

        /// 

        static Expression Compose(this Expression first, Expression second, Func 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(merge(first.Body, secondBody), first.Parameters);

        }

        /// 

        /// ParameterRebinder

        /// 

        class ParameterRebinder : ExpressionVisitor

        {

            /// 

            /// The ParameterExpression map

            /// 

            readonly Dictionary map;

            /// 

            /// Initializes a new instance of the  class.

            /// 

            /// The map.

            ParameterRebinder(Dictionary map)

            {

                this.map = map ?? new Dictionary();

            }

            /// 

            /// Replaces the parameters.

            /// 

            /// The map.

            /// The exp.

            /// Expression

            public static Expression ReplaceParameters(Dictionary map, Expression exp)

            {

                return new ParameterRebinder(map).Visit(exp);

            }

            /// 

            /// Visits the parameter.

            /// 

            /// The p.

            /// Expression

            protected override Expression VisitParameter(ParameterExpression p)

            {

                ParameterExpression replacement;

                if (map.TryGetValue(p, out replacement))

                {

                    p = replacement;

                }

                return base.VisitParameter(p);

            }

        }

    }

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);

UnitTest使用到 生成查询条件 的 私有方法:

         /// 

         /// Builds the find by all query.

         /// 

         private static Expressionbool>> BuildFindByAllQuery(string productName,DateTime? beignUpdateDate, DateTime? endUpdateDate)

         {

             var list = new Listbool>>>();

             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

             Expressionbool>> productQueryTotal = null;

             foreach (var expression in list)

             {

                 productQueryTotal = expression.And(expression);

             }

             return productQueryTotal;

         }

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.