Repository 简化实现多条件查询

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 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>，示例代码：

using System.Linq.Expressions;public IQueryable Get(int id, string name, string address, Status? status, DateTime createTime){    Expression> 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'))