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所属分类：.NET技术
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：
1 public IQueryable<Student> Get(int id, string name, string address, Status? status, DateTime createTime) 2 { 3 var query = _entities; 4 if(id != 0) 5 { 6 query = query.where(x => x.Id == id); 7 } 8 if(!string.IsNullOrWhiteSpace(name)) 9 { 10 query = query.where(x => x.Name.Contains(name)); 11 } 12 if(!string.IsNullOrWhiteSpace(address)) 13 { 14 query = query.where(x => x.Address.Contains(address)); 15 } 16 if(status.HasValue) 17 { 18 query = query.where(x => x.Status == status.Value); 19 } 20 if(createTime != null) 21 { 22 query = query.where(x => x.CreateTime == createTime); 23 } 24 // ... 25 26 return query; 27 }
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：
1 SELECT `x`.`id`, `x`.`name`, `x`.`address`, `x`.`status`, `x`.`create_time` 2 FROM `students` AS `x` 3 WHERE (`x`.`id` = 2) 4 AND (`x`.`status` = 0) AND (`x`.`create_time` == '2017-04-25T16:24:29.769+08:00'))
原文来源：https://www.cnblogs.com/xishuai/p/repository-query-linq-expression.html
