最近几天我在互联网上搜索了解决方案,并没有找到我想要的东西.基本上,这是我的问题：
>我有一个我需要实现的接口,它有一个返回IQueryable的方法(我没有访问接口,所以我不能改变它)
>我希望该方法返回(a)指向非常大的数据库表的IQueryable和(b)在相同实体类型的内存中计算的大IEnumerable的串联
>我不能做queryableA.Concat(enumerableB).Where(condition)因为它会尝试将整个数组发送到服务器(除此之外,我得到一个它只支持基本类型的异常)
>我无法执行enumerableB.Concat(queryableA).Where(condition)因为它会将整个表拉入内存并将其视为IEnumerable
因此,经过一些搜索,我认为我已经决定了解决这个问题的一个好方法是编写我自己的IQueryable的ConcatenatingQueryable实现,该实现需要两个IQueryable并在每个上独立执行Expression树,然后连接结果.但是,我似乎遇到了问题,因为它返回堆栈溢出.基于http://blogs.msdn.com/b/mattwar/archive/2007/07/30/linq-building-an-iqueryable-provider-part-i.aspx,这是我到目前为止实现的：
class Program
{
static void Main(string[] args)
{
var source1 = new[] { 1, 2 }.AsQueryable();
var source2 = new[] { -1, 1 }.AsQueryable();
var matches = new ConcatenatingQueryable<int>(source1, source2).Where(x => x <= 1).ToArray();
Console.WriteLine(string.Join(",", matches));
Console.ReadKey();
}
public class ConcatenatingQueryable<T> : IQueryable<T>
{
private readonly ConcatenatingQueryableProvider<T> provider;
private readonly Expression expression;
public ConcatenatingQueryable(IQueryable<T> source1, IQueryable<T> source2)
: this(new ConcatenatingQueryableProvider<T>(source1, source2))
{}
public ConcatenatingQueryable(ConcatenatingQueryableProvider<T> provider)
{
this.provider = provider;
this.expression = Expression.Constant(this);
}
public ConcatenatingQueryable(ConcatenatingQueryableProvider<T> provider, Expression expression)
{
this.provider = provider;
this.expression = expression;
}
Expression IQueryable.Expression
{
get { return expression; }
}
Type IQueryable.ElementType
{
get { return typeof(T); }
}
IQueryProvider IQueryable.Provider
{
get { return provider; }
}
public IEnumerator<T> GetEnumerator()
{
// This line is calling Execute below
return ((IEnumerable<T>)provider.Execute(expression)).GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return ((IEnumerable)provider.Execute(expression)).GetEnumerator();
}
}
public class ConcatenatingQueryableProvider<T> : IQueryProvider
{
private readonly IQueryable<T> source1;
private readonly IQueryable<T> source2;
public ConcatenatingQueryableProvider(IQueryable<T> source1, IQueryable<T> source2)
{
this.source1 = source1;
this.source2 = source2;
}
IQueryable<TS> IQueryProvider.CreateQuery<TS>(Expression expression)
{
var elementType = TypeSystem.GetElementType(expression.Type);
try
{
return (IQueryable<TS>)Activator.CreateInstance(typeof(ConcatenatingQueryable<>).MakeGenericType(elementType), new object[] { this, expression });
}
catch (TargetInvocationException tie)
{
throw tie.InnerException;
}
}
IQueryable IQueryProvider.CreateQuery(Expression expression)
{
var elementType = TypeSystem.GetElementType(expression.Type);
try
{
return (IQueryable)Activator.CreateInstance(typeof(ConcatenatingQueryable<>).MakeGenericType(elementType), new object[] { this, expression });
}
catch (TargetInvocationException tie)
{
throw tie.InnerException;
}
}
TS IQueryProvider.Execute<TS>(Expression expression)
{
return (TS)Execute(expression);
}
object IQueryProvider.Execute(Expression expression)
{
return Execute(expression);
}
public object Execute(Expression expression)
{
// This is where I suspect the problem lies, as executing the
// Expression.Constant from above here will call Enumerate again,
// which then calls this, and... you get the point
dynamic results1 = source1.Provider.Execute(expression);
dynamic results2 = source2.Provider.Execute(expression);
return results1.Concat(results2);
}
}
internal static class TypeSystem
{
internal static Type GetElementType(Type seqType)
{
var ienum = FindIEnumerable(seqType);
if (ienum == null)
return seqType;
return ienum.GetGenericArguments()[0];
}
private static Type FindIEnumerable(Type seqType)
{
if (seqType == null || seqType == typeof(string))
return null;
if (seqType.IsArray)
return typeof(IEnumerable<>).MakeGenericType(seqType.GetElementType());
if (seqType.IsGenericType)
{
foreach (var arg in seqType.GetGenericArguments())
{
var ienum = typeof(IEnumerable<>).MakeGenericType(arg);
if (ienum.IsAssignableFrom(seqType))
{
return ienum;
}
}
}
var ifaces = seqType.GetInterfaces();
if (ifaces.Length > 0)
{
foreach (var iface in ifaces)
{
var ienum = FindIEnumerable(iface);
if (ienum != null)
return ienum;
}
}
if (seqType.BaseType != null && seqType.BaseType != typeof(object))
{
return FindIEnumerable(seqType.BaseType);
}
return null;
}
}
}
我对这个界面没有多少经验,而且从这里开始做什么有点遗失.有没有人对如何做到这一点有任何建议？如果需要,我也完全放弃这种方法.
重申一下,我得到一个StackOverflowException,堆栈跟踪只是上面两条注释行之间的一堆调用,每对调用之间都有“[External Code]”.我添加了一个使用两个微小枚举的示例Main方法,但您可以想象这些是需要很长时间枚举的较大数据源.
非常感谢您的帮助！
最佳答案 当您分解传递给IQueryProvider的表达式树时,您将看到LINQ方法的调用链.请记住,LINQ通常通过链接扩展方法来工作,其中前一个方法的返回值作为第一个参数传递给下一个方法.
如果我们在逻辑上遵循这一点,那意味着链中的第一个LINQ方法必须具有源参数,并且从代码中可以看出它的来源实际上是完全相同的IQueryable. (你的ConcatenatingQueryable).
当你构建它时,你几乎已经有了这个想法 – 你只需要再向前迈一小步.我们需要做的是重新指出第一个LINQ方法使用实际的源,然后允许执行遵循其自然路径.
以下是一些示例代码：
public object Execute(Expression expression)
{
var query1 = ChangeQuerySource(expression, Expression.Constant(source1));
var query2 = ChangeQuerySource(expression, Expression.Constant(source2));
dynamic results1 = source1.Provider.Execute(query1);
dynamic results2 = source2.Provider.Execute(query2);
return Enumerable.Concat(results1, results2);
}
private static Expression ChangeQuerySource(Expression query, Expression newSource)
{
// step 1: cast the Expression as a MethodCallExpression.
// This will usually work, since a chain of LINQ statements
// is generally a chain of method calls, but I would not
// make such a blind assumption in production code.
var methodCallExpression = (MethodCallExpression)query;
// step 2: Create a new MethodCallExpression, passing in
// the existing one's MethodInfo so we're calling the same
// method, but just changing the parameters. Remember LINQ
// methods are extension methods, so the first argument is
// always the source. We carry over any additional arguments.
query = Expression.Call(
methodCallExpression.Method,
new Expression[] { newSource }.Concat(methodCallExpression.Arguments.Skip(1)));
// step 3: We call .AsEnumerable() at the end, to get an
// ultimate return type of IEnumerable<T> instead of
// IQueryable<T>, so we can safely use this new expression
// tree in any IEnumerable statement.
query = Expression.Call(
typeof(Enumerable).GetMethod("AsEnumerable", BindingFlags.Static | BindingFlags.Public)
.MakeGenericMethod(
TypeSystem.GetElementType(methodCallExpression.Arguments[0].Type)
),
query);
return query;
}
