Repository 在做查询的时候,如果查询条件多的话,linq
查询表达式会写的很复杂,比如:
public IQueryable<Student> Get(int id, string name, string address, Status? status, DateTime createTime) { var query = _entities; if(id != 0) { query = query.where(x => x.Id == id); } if(!string.IsNullOrWhiteSpace(name)) { query = query.where(x => x.Name.Contains(name)); } if(!string.IsNullOrWhiteSpace(address)) { query = query.where(x => x.Address.Contains(address)); } if(status.HasValue) { query = query.where(x => x.Status == status.Value); } if(createTime != null) { query = query.where(x => x.CreateTime == createTime); } // ... return query; }
可以看到,查询条件多的话,我们会写很多的if
判断,代码看起来很不美观,解决方式使用Expression<Func<T, bool>>
,示例代码:
using System.Linq.Expressions; public IQueryable<Student> Get(int id, string name, string address, Status? status, DateTime createTime) { Expression<Func<Student, bool>> studentFunc = x => (id == 0 || x.Id == id) && (string.IsNullOrWhiteSpace(name) || x.Name.Contains(name)) && (string.IsNullOrWhiteSpace(address) || x.Address.Contains(address)) && (!status.HasValue || x.Status == status.Value) && (createTime == null || x.CreateTime <= createTime); return _entities.Where(studentFunc); }
生成示例sql
代码:
SELECT `x`.`id`, `x`.`name`, `x`.`address`, `x`.`status`, `x`.`create_time` FROM `students` AS `x` WHERE (`x`.`id` = 2) AND (`x`.`status` = 0) AND (`x`.`create_time` == ‘2017-04-25T16:24:29.769+08:00‘))
原文地址:http://linyuheng.blog.51cto.com/13002621/1939873