假设我有一个左外连接,如下所示:
from f in Foo
join b in Bar on f.Foo_Id equals b.Foo_Id into g
from result in g.DefaultIfEmpty()
select new { Foo = f, Bar = result }
如何使用扩展方法表达相同的任务?如。
Foo.GroupJoin(Bar, f => f.Foo_Id, b => b.Foo_Id, (f,b) => ???)
.Select(???)
你可以创建这样的扩展方法:
public static IEnumerable<TResult> LeftOuterJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source, IEnumerable<TInner> other, Func<TSource, TKey> func, Func<TInner, TKey> innerkey, Func<TSource, TInner, TResult> res)
{
return from f in source
join b in other on func.Invoke(f) equals innerkey.Invoke(b) into g
from result in g.DefaultIfEmpty()
select res.Invoke(f, result);
}
对于一个表Bar与一个表Foo对Foo的连接(左外)。Foo_Id = Bar。Foo_Id用lambda表示:
var qry = Foo.GroupJoin(
Bar,
foo => foo.Foo_Id,
bar => bar.Foo_Id,
(x,y) => new { Foo = x, Bars = y })
.SelectMany(
x => x.Bars.DefaultIfEmpty(),
(x,y) => new { Foo=x.Foo, Bar=y});
将Marc Gravell的答案转换为一个扩展方法,我做了以下工作。
internal static IEnumerable<Tuple<TLeft, TRight>> LeftJoin<TLeft, TRight, TKey>(
this IEnumerable<TLeft> left,
IEnumerable<TRight> right,
Func<TLeft, TKey> selectKeyLeft,
Func<TRight, TKey> selectKeyRight,
TRight defaultRight = default(TRight),
IEqualityComparer<TKey> cmp = null)
{
return left.GroupJoin(
right,
selectKeyLeft,
selectKeyRight,
(x, y) => new Tuple<TLeft, IEnumerable<TRight>>(x, y),
cmp ?? EqualityComparer<TKey>.Default)
.SelectMany(
x => x.Item2.DefaultIfEmpty(defaultRight),
(x, y) => new Tuple<TLeft, TRight>(x.Item1, y));
}
虽然接受的答案工作,是很好的Linq对象它困扰我的SQL查询不只是一个直接的左外连接。
下面的代码依赖于允许您传递表达式并将它们调用到查询的LinqKit项目。
static IQueryable<TResult> LeftOuterJoin<TSource,TInner, TKey, TResult>(
this IQueryable<TSource> source,
IQueryable<TInner> inner,
Expression<Func<TSource,TKey>> sourceKey,
Expression<Func<TInner,TKey>> innerKey,
Expression<Func<TSource, TInner, TResult>> result
) {
return from a in source.AsExpandable()
join b in inner on sourceKey.Invoke(a) equals innerKey.Invoke(b) into c
from d in c.DefaultIfEmpty()
select result.Invoke(a,d);
}
它可以这样使用
Table1.LeftOuterJoin(Table2, x => x.Key1, x => x.Key2, (x,y) => new { x,y});
你可以创建这样的扩展方法:
public static IEnumerable<TResult> LeftOuterJoin<TSource, TInner, TKey, TResult>(this IEnumerable<TSource> source, IEnumerable<TInner> other, Func<TSource, TKey> func, Func<TInner, TKey> innerkey, Func<TSource, TInner, TResult> res)
{
return from f in source
join b in other on func.Invoke(f) equals innerkey.Invoke(b) into g
from result in g.DefaultIfEmpty()
select res.Invoke(f, result);
}
要实现两个数据集的连接,不需要使用组连接方法。
内连接:
var qry = Foos.SelectMany
(
foo => Bars.Where (bar => foo.Foo_id == bar.Foo_id),
(foo, bar) => new
{
Foo = foo,
Bar = bar
}
);
对于左连接,只需添加DefaultIfEmpty()
var qry = Foos.SelectMany
(
foo => Bars.Where (bar => foo.Foo_id == bar.Foo_id).DefaultIfEmpty(),
(foo, bar) => new
{
Foo = foo,
Bar = bar
}
);
EF和LINQ to SQL正确转换为SQL。
对于LINQ to Objects,最好使用GroupJoin,因为它在内部使用Lookup。但是如果您正在查询DB,那么跳过GroupJoin是AFAIK作为性能。
Personlay对我来说,这种方式比GroupJoin()更具可读性。