问题描述
我用((的ObjectQuery)的IQueryable).ToTraceString()
来获取和调整就是要通过LINQ执行的SQL代码。
I use ((ObjectQuery)IQueryable).ToTraceString()
to obtain and tweak SQL code that is going to be executed by LINQ.
我的问题是,不像大多数的IQueryable方法IQueryable.Count为这样定义的:
My problem is that unlike most IQueryable methods IQueryable.Count as defined like this:
public static int Count(this IQueryable source) {
return (int)source.Provider.Execute(
Expression.Call(
typeof(Queryable), "Count",
new Type[] { source.ElementType }, source.Expression));
}
执行查询,无需编译并返回IQueryable的。
我希望通过这样的事情做的伎俩:
executes query without compiling and returning IQueryable.I wanted to do the trick by something like this:
public static IQueryable CountCompile(this IQueryable source) {
return source.Provider.CreateQuery(
Expression.Call(
typeof(Queryable), "Count",
new Type[] { source.ElementType }, source.Expression));
}
但随后的createQuery给了我以下异常:
But then CreateQuery gives me the following exception:
LINQ到实体查询表达式只能从实现的IQueryable接口实例构造。
推荐答案
这是我来到了,当我试图让相同的实际工作答案。唯一的例外说,所以答案似乎很简单只能从实现了IQueryable的接口实例构造:返回一个可查询的东西。这是可能的返回时,一个 .Count之间的()
?是的!
Here's an actual working answer I came up with when I tried to so the same. The exception says "can only be constructed from instances that implement the IQueryable interface", so the answer seems simple: return a queryable something. Is that possible when returning a .Count()
? Yes!
public partial class YourObjectContext
{
private static MethodInfo GetMethodInfo(Expression<Action> expression)
{
return ((MethodCallExpression)expression.Body).Method;
}
public IQueryable<TResult> CreateScalarQuery<TResult>(Expression<Func<TResult>> expression)
{
return QueryProvider.CreateQuery<TResult>(
Expression.Call(
method: GetMethodInfo(() => Queryable.Select<int, TResult>(null, (Expression<Func<int, TResult>>)null)),
arg0: Expression.Call(
method: GetMethodInfo(() => Queryable.AsQueryable<int>(null)),
arg0: Expression.NewArrayInit(typeof(int), Expression.Constant(1))),
arg1: Expression.Lambda(body: expression.Body, parameters: new[] { Expression.Parameter(typeof(int)) })));
}
}
要使用它:
var query = context.CreateScalarQuery(() => context.Entity.Count());
MessageBox.Show(((ObjectQuery)query).ToTraceString());
基本上,这确实是子查询包裹非IQueryable的查询。它把从假人在新的INT [] {1} .AsQueryable()
选择环境中的查询到
Basically, what this does is wrap a non-IQueryable query in a subselect. It transforms the query into
from dummy in new int[] { 1 }.AsQueryable()
select context.Entity.Count()
除了让上下文的QueryProvider处理查询。生成的SQL是相当多的,你应该期待什么:
except lets the context's QueryProvider handle the query. The generated SQL is pretty much what you should expect:
SELECT
[GroupBy1].[A1] AS [C1]
FROM ( SELECT
COUNT(1) AS [A1]
FROM [dbo].[Entity] AS [Extent1]
) AS [GroupBy1]
这篇关于如何获取ToTraceString为IQueryable.Count的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!