介绍
这个问题非常有趣。问题在于，Funcs是委托，而Expressions是树形结构，它们是完全不同的结构。当您使用当前的扩展实现时，它会在每个步骤中为每个元素执行选择器，并正常工作。但是，当我们谈论实体框架和LINQ时，需要进行树遍历以将其转换为SQL查询。因此，这比Funcs要“稍微”困难一些（但我仍然喜欢表达式），并且下面描述了一些问题。
当您想要进行左外连接时，可以使用类似于以下内容的代码（取自此处：如何在JOIN扩展方法中实现左连接）
var leftJoin = p.Person.Where(n => n.FirstName.Contains("a"))
.GroupJoin(p.PersonInfo,
n => n.PersonId,
m => m.PersonId,
(n, ms) => new { n, ms = ms.DefaultIfEmpty() })
.SelectMany(z => z.ms.Select(m => new { n = z.n, m ));
这很不错，但不是我们需要的扩展方法。我猜你需要像这样的东西：
using (var db = new Database1Entities("..."))
{
var my = db.A.LeftOuterJoin2(db.B, a => a.Id, b => b.IdA,
(a, b) => new { a, b, hello = "Hello World!" });
}
创建这样的扩展程序有许多难点：
- 需要手动创建复杂的树形结构，编译器无法帮助我们。
- 像
Where、Select等方法需要使用反射。 - 匿名类型（!! 这里需要代码生成吗？我希望不需要）。
步骤
考虑两个简单的表：A（列：Id、Text）和B（列：Id、IdA、Text）。
外连接可以分为3个步骤实现：
// group join as usual + use DefaultIfEmpty
var q1 = Queryable.GroupJoin(db.A, db.B, a => a.Id, b => b.IdA,
(a, b) => new { a, groupB = b.DefaultIfEmpty() });
// regroup data to associated list a -> b, it is usable already, but it's
// impossible to use resultSelector on this stage,
// beacuse of type difference (quite deep problem: some anonymous type != TOuter)
var q2 = Queryable.SelectMany(q1, x => x.groupB, (a, b) => new { a.a, b });
// second regroup to get the right types
var q3 = Queryable.SelectMany(db.A,
a => q2.Where(x => x.a == a).Select(x => x.b),
(a, b) => new {a, b});
代码
好的，我不是一个很好的讲故事者，这里是我拥有的代码（抱歉我无法更好地格式化它，但它可以工作！）:
public static IQueryable<TResult> LeftOuterJoin2<TOuter, TInner, TKey, TResult>(
this IQueryable<TOuter> outer,
IQueryable<TInner> inner,
Expression<Func<TOuter, TKey>> outerKeySelector,
Expression<Func<TInner, TKey>> innerKeySelector,
Expression<Func<TOuter, TInner, TResult>> resultSelector)
{
var selectManies = typeof(Queryable).GetMethods()
.Where(x => x.Name == "SelectMany" && x.GetParameters().Length == 3)
.OrderBy(x=>x.ToString().Length)
.ToList();
var selectMany = selectManies.First();
var select = typeof(Queryable).GetMethods().First(x => x.Name == "Select" && x.GetParameters().Length == 2);
var where = typeof(Queryable).GetMethods().First(x => x.Name == "Where" && x.GetParameters().Length == 2);
var groupJoin = typeof(Queryable).GetMethods().First(x => x.Name == "GroupJoin" && x.GetParameters().Length == 5);
var defaultIfEmpty = typeof(Queryable).GetMethods().First(x => x.Name == "DefaultIfEmpty" && x.GetParameters().Length == 1);
var tuple = typeof(Tuple<,>).MakeGenericType(
typeof(TOuter),
typeof(IQueryable<>).MakeGenericType(
typeof(TInner)
)
);
var paramOuter = Expression.Parameter(typeof(TOuter));
var paramInner = Expression.Parameter(typeof(IEnumerable<TInner>));
var groupJoinExpression = Expression.Call(
null,
groupJoin.MakeGenericMethod(typeof (TOuter), typeof (TInner), typeof (TKey), tuple),
new Expression[]
{
Expression.Constant(outer),
Expression.Constant(inner),
outerKeySelector,
innerKeySelector,
Expression.Lambda(
Expression.New(
tuple.GetConstructor(tuple.GetGenericArguments()),
new Expression[]
{
paramOuter,
Expression.Call(
null,
defaultIfEmpty.MakeGenericMethod(typeof (TInner)),
new Expression[]
{
Expression.Convert(paramInner, typeof (IQueryable<TInner>))
}
)
},
tuple.GetProperties()
),
new[] {paramOuter, paramInner}
)
}
);
var tuple2 = typeof (Tuple<,>).MakeGenericType(typeof (TOuter), typeof (TInner));
var paramTuple2 = Expression.Parameter(tuple);
var paramInner2 = Expression.Parameter(typeof(TInner));
var paramGroup = Expression.Parameter(tuple);
var selectMany1Result = Expression.Call(
null,
selectMany.MakeGenericMethod(tuple, typeof (TInner), tuple2),
new Expression[]
{
groupJoinExpression,
Expression.Lambda(
Expression.Convert(Expression.MakeMemberAccess(paramGroup, tuple.GetProperty("Item2")),
typeof (IEnumerable<TInner>)),
paramGroup
),
Expression.Lambda(
Expression.New(
tuple2.GetConstructor(tuple2.GetGenericArguments()),
new Expression[]
{
Expression.MakeMemberAccess(paramTuple2, paramTuple2.Type.GetProperty("Item1")),
paramInner2
},
tuple2.GetProperties()
),
new[]
{
paramTuple2,
paramInner2
}
)
}
);
var paramTuple3 = Expression.Parameter(tuple2);
var paramTuple4 = Expression.Parameter(tuple2);
var paramOuter3 = Expression.Parameter(typeof (TOuter));
var selectManyResult2 = selectMany
.MakeGenericMethod(
typeof(TOuter),
typeof(TInner),
typeof(TResult)
)
.Invoke(
null,
new object[]
{
outer,
Expression.Lambda(
Expression.Convert(
Expression.Call(
null,
select.MakeGenericMethod(tuple2, typeof(TInner)),
new Expression[]
{
Expression.Call(
null,
where.MakeGenericMethod(tuple2),
new Expression[]
{
selectMany1Result,
Expression.Lambda(
Expression.Equal(
paramOuter3,
Expression.MakeMemberAccess(paramTuple4, paramTuple4.Type.GetProperty("Item1"))
),
paramTuple4
)
}
),
Expression.Lambda(
Expression.MakeMemberAccess(paramTuple3, paramTuple3.Type.GetProperty("Item2")),
paramTuple3
)
}
),
typeof(IEnumerable<TInner>)
),
paramOuter3
),
resultSelector
}
);
return (IQueryable<TResult>)selectManyResult2;
}
使用方法
再次使用：
db.A.LeftOuterJoin2(db.B, a => a.Id, b => b.IdA,
(a, b) => new { a, b, hello = "Hello World!" });
看到这个，你可能会想这一切的 SQL 查询语句会有多长？但是猜猜看，它其实很简短：
SELECT * FROM table_name;
SELECT
1 AS [C1],
[Extent1].[Id] AS [Id],
[Extent1].[Text] AS [Text],
[Join1].[Id1] AS [Id1],
[Join1].[IdA] AS [IdA],
[Join1].[Text2] AS [Text2],
N'Hello World!' AS [C2]
FROM [A] AS [Extent1]
INNER JOIN (SELECT [Extent2].[Id] AS [Id2], [Extent2].[Text] AS [Text], [Extent3].[Id] AS [Id1], [Extent3].[IdA] AS [IdA], [Extent3].[Text2] AS [Text2]
FROM [A] AS [Extent2]
LEFT OUTER JOIN [B] AS [Extent3] ON [Extent2].[Id] = [Extent3].[IdA] ) AS [Join1] ON [Extent1].[Id] = [Join1].[Id2]
希望对您有所帮助。
IQueryable实际上 是 一个IEnumerable，因此对于IEnumerable工作的方法也应该适用于这个实例，你尝试过使用适用于IEnumerable的方法，然后通过调用.AsQueryable()进行简单的强制转换为IQueryable吗？ - aevitas