C# 使用联接的相同LINQ查询,但条件不同
我有一些非常类似的LINQ查询:C# 使用联接的相同LINQ查询,但条件不同,c#,linq,C#,Linq,我有一些非常类似的LINQ查询: var city = "Hamburg"; var query1 = from c in DB.Customers join address in DB.Addresses on c.ID equals address.CustomerID where address.City == city select c; var query2 = from c in DB.Customers join addres
var city = "Hamburg";
var query1 =
from c in DB.Customers
join address in DB.Addresses on c.ID equals address.CustomerID
where address.City == city
select c;
var query2 =
from c in DB.Customers
join address in DB.Addresses on c.ID equals address.CustomerID
where address.City.StartsWith(city)
select c;
etc.
我想使用循环创建查询以减少冗余:
var city = "Hamburg";
var expressions = new Expression<Func<string, bool>>[] {
a => a == city,
a => a.StartsWith(city)
};
for(int i = 1; i <= 2; i++) {
queries.Add(
from c in DB.Customers
join address in DB.Addresses on c.ID equals address.CustomerID
where expressions[i](address.City)
select c
);
}
var city=“汉堡”;
变量表达式=新表达式[]{
a=>a==城市,
a=>a.StartsWith(城市)
};
对于(inti=1;i,您可以在公共查询中保存公共连接部分
var query = from c in DB.Customers
join address from DB.Addresses on c.ID equals address.CustomerID
select new {A=a, C=c};
var query1 = query.Where ( item => item.A.temp == item.C.test).Select (item => item.C);
var query2 = query.Where ( item => item.A.temp2 == item.C.test2).Select (item => item.C);
您可以直接使用Join
方法,而不是使用查询语法:
public IQueryable<Customer> FindCustomers(Expression<Func<Customer, Address, bool>> predicate)
{
return DB.Customers.Join(DB.Addresses, c => c.ID, a => d.CustomerID, (c, a) => new { Address = a, Customer = c})
.Where(pair => predicate(pair.Address))
.Select(pair => pair.Customer)
}
然后,您可以将阵列创建为:
IQueryable<Customer>[] queries = expressions.Select(expr => FindCustomers(expr)).ToArray();
IQueryable[]querys=expressions.Select(expr=>FindCustomers(expr)).ToArray();
var city=“汉堡”;
//谓词应该接受地址
变量表达式=新表达式[]{
a=>a.City==City,
a=>a.City.StartsWith(城市)
};
foreach(表达式中的var谓词){
查询。添加(
DB.Customers.Join(
DB.Addresses.Where(谓词),//此处筛选
c=>c.ID,
a=>a.CustomerID,
(c,a)=>c)//返回客户
));
}
var查询=(
从f到in
新函数[]{
x=>x.City==City,
x=>x.City.StartsWith(城市)
}
选择
来自数据库中的c
数据库中的联接地址。c.ID上的地址等于address.CustomerID
其中f(地址)
选择c
).ToArray();
您正在创建一个表达式数组
,这意味着您最终将生成一个表达式数组
;您不需要将委托设置为表达式数组
您不需要foreach的
如果以后要将更多表达式添加到查询
,请将ToArray()
更改为ToList()
如果要在生成查询之前修改Func
,请为其定义一个局部变量,只需替换Func[]
老实说,这看起来像是一个存储过程的作业。您期望的结果是什么?一个列表,i
lists?@Hogan我希望i
列表包含通过不同表达式过滤的客户。另外,Expression?在EF中,join
几乎不需要。您没有像Customer.Addresses这样的导航属性吗?问题是谓词需要访问地址的属性,但当我加入(c,a)时=>c
我只能在谓词中使用客户,还是我遗漏了什么?@Dennis-我已将其更改为使用地址而不是客户
。谢谢。什么是协会奖金,我刚收到通知。
IQueryable<Customer>[] queries = expressions.Select(expr => FindCustomers(expr)).ToArray();
var city = "Hamburg";
// predicate should accept Address
var expressions = new Expression<Func<Address, bool>>[] {
a => a.City == city,
a => a.City.StartsWith(city)
};
foreach(var predicate in expressions) {
queries.Add(
DB.Customers.Join(
DB.Addresses.Where(predicate), // filtering here
c => c.ID,
a => a.CustomerID,
(c, a) => c) // return customer
));
}
var queries=(
from f in
new Func<Address, bool>[] {
x => x.City==city,
x => x.City.StartsWith(city)
}
select
from c in DB.Customers
join address in DB.Addresses on c.ID equals address.CustomerID
where f(address)
select c
).ToArray();