封装表达式信息，EF动态构造条件

前言

分享一个动态封装表达式的一个方法，对应的 类库方法以及如何使用都会在下文中进行一个体现

代码展示

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

namespace NFine.Code
{
    public static partial class ExtLinq
    {
        public static Expression Property(this Expression expression, string propertyName)
        {
            return Expression.Property(expression, propertyName);
        }
        public static Expression AndAlso(this Expression left, Expression right)
        {
            return Expression.AndAlso(left, right);
        }
        public static Expression Call(this Expression instance, string methodName, params Expression[] arguments)
        {
            return Expression.Call(instance, instance.Type.GetMethod(methodName), arguments);
        }
        public static Expression GreaterThan(this Expression left, Expression right)
        {
            return Expression.GreaterThan(left, right);
        }
        public static Expression<T> ToLambda<T>(this Expression body, params  ParameterExpression[] parameters)
        {
            return Expression.Lambda<T>(body, parameters);
        }
        public static Expression<Func<T, bool>> True<T>() { return param => true; }
        public static Expression<Func<T, bool>> False<T>() { return param => false; }
        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);
        }
        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);
        }
        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);
        }
        private class ParameterRebinder : ExpressionVisitor
        {
            readonly Dictionary<ParameterExpression, ParameterExpression> map;
            /// 

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

            /// Replaces the parameters.
            /// 
            /// The map.
            /// The exp.
            /// Expression
            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            }
            protected override Expression VisitParameter(ParameterExpression p)
            {
                ParameterExpression replacement;

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

}

使用展示

		//获取用户列表信息
   		public List<TestEntity> GetList(Pagination pagination, string keyword)
        {
            var expression = ExtLinq.True<UserEntity>();
            if (!string.IsNullOrEmpty(keyword))
            {
                expression = expression.And(t => t.F_UserName.Contains(keyword));
                expression = expression.Or(t => t.F_RealName.Contains(keyword));
                expression = expression.Or(t => t.F_MobilePhone.Contains(keyword));
            }
            expression = expression.And(t => t.F_UserName != "admin");


			//直接使用EF查询数据获取信息，expression 表达式当作筛选条件进行处理
            return service.FindList(expression, pagination);
            
			//最终到数据库层展示的效果是这样
			//dbcontext.Set().Where(expression);
        }

尾语

简单的一个表达式封装可以节省我们很多无用的代码操作，虽然先查询集合之后在通过集合筛选也可以是实现，但是感觉不是特别的好 ，还是想通过一步到位的操作进行处理