c# - 为什么此 Queryable.Where 调用会更改 Queryable 的类型参数？

Question

重现问题的代码

我遇到了一个 IQueryable.Where<TSource> 的情况调用返回 IQueryable<TOther>哪里TOther != TSource 。我整理了一些示例代码来重现它:

using System;
using System.Collections.Generic;
using System.Linq;

namespace IQueryableWhereTypeChange {
    class Program {
        static void Main( string[] args ) {
            var ints = new List<ChildQueryElement>();

            for( int i = 0; i < 10; i++ ) {
                ints.Add( new ChildQueryElement() { Num = i, Value = i.ToString() } );
            }

            IQueryable<ChildQueryElement> theIQ = ints.AsQueryable();

            Object theObj = theIQ;

            Type theObjElementType = ( (IQueryable<ParentQueryElement>) theObj ).ElementType;
            Type theObjGenericType = ( (IQueryable<ParentQueryElement>) theObj ).GetType().GetGenericArguments()[ 0 ];

            var iQ = ( (IQueryable<ParentQueryElement>) theObj );

            var copy = iQ;

            Type copyType1 = copy.GetType().GetGenericArguments()[ 0 ];
            Type elementType1 = copy.ElementType;

            copy = copy.Where( qe1 => true );

            Type copyType2 = copy.GetType().GetGenericArguments()[ 0 ];
            Type elementType2 = copy.ElementType;

            Console.WriteLine( "theObjElementType : " + theObjElementType.ToString() );
            Console.WriteLine( "theObjGenericType : " + theObjGenericType.ToString() );
            Console.WriteLine( "copyType1 : " + copyType1.ToString() );
            Console.WriteLine( "elementType1 : " + elementType1.ToString() );
            Console.WriteLine( "copyType2 : " + copyType2.ToString() );
            Console.WriteLine( "elementType2 : " + elementType2.ToString() );
        }
    }

    public class ParentQueryElement {
        public int Num { get; set; }
    }
    public class ChildQueryElement : ParentQueryElement {
        public string Value { get; set; }
    }
}

该程序的输出是:

theObjElementType : IQueryableWhereTypeChange.ChildQueryElement    
theObjGenericType : IQueryableWhereTypeChange.ChildQueryElement    
copyType1 : IQueryableWhereTypeChange.ChildQueryElement  
elementType1 : IQueryableWhereTypeChange.ChildQueryElement  
copyType2 : IQueryableWhereTypeChange.ParentQueryElement  
elementType2 : IQueryableWhereTypeChange.ParentQueryElement 

代码结果摘要

因此，我们存储 IQueryable<ChildQueryElement>在 Object ，然后将对象转换为 IQueryable<ParentQueryElement> ，其中子类型继承自父类型。此时对象存储在Object中变量仍然知道它是子类型的集合。然后我们调用Queryable.Where上，但返回的对象不再知道它包含子类型，并认为它只包含父类型。

问题

为什么会发生这种情况？除了跳过将其存储在对象中的步骤之外，还有什么方法可以避免这种情况吗？我问这个问题是因为我正在处理一个第三方 API，它要求我向它传递 Object ，而且我不想重写一堆第三方代码。

更新了示例代码

在得到 Jon Skeet 的一些建议后，我尝试了这个示例代码，它使用动态变量进行复制。更换Main的主体具有以下内容:

var ints = new List<ChildQueryElement>();

for( int i = 0; i < 10; i++ ) {
    ints.Add( new ChildQueryElement() { Num = i, Value = i.ToString() } );
}

IQueryable<ChildQueryElement> theIQ = ints.AsQueryable();

Object theObj = theIQ;

Type theObjElementType = ( (IQueryable<ParentQueryElement>) theObj ).ElementType;
Type theObjGenericType = ( (IQueryable<ParentQueryElement>) theObj ).GetType().GetGenericArguments()[ 0 ];

var iQ = ( (IQueryable<ParentQueryElement>) theObj );

dynamic copy = iQ;

Type copyType1 = copy.GetType().GetGenericArguments()[ 0 ];
Type elementType1 = ((IQueryable)copy).ElementType;

Expression<Func<ParentQueryElement, bool>> del = qe => true;

copy = Queryable.Where( copy, del );

Type copyType2 = copy.GetType().GetGenericArguments()[ 0 ];
Type elementType2 = ((IQueryable)copy).ElementType;

Console.WriteLine( "theObjElementType : " + theObjElementType.ToString() );
Console.WriteLine( "theObjGenericType : " + theObjGenericType.ToString() );
Console.WriteLine( "copyType1 : " + copyType1.ToString() );
Console.WriteLine( "elementType1 : " + elementType1.ToString() );
Console.WriteLine( "copyType2 : " + copyType2.ToString() );
Console.WriteLine( "elementType2 : " + elementType2.ToString() );

不幸的是，输出保持不变。

Answer 1

Why does this happen?

因为Where调用正在接收 ParentQueryElement 的类型参数如TSource 。它根据 TSource 创建结果作为一个新对象...所以你最终会得到一些“了解”ParentQueryElement 的东西而不是ChildQueryElement .

无需进入 LINQ 即可轻松演示这一点:

using System;

public interface IWrapper<out T>
{
    T Value { get; }
}

public class Wrapper<T> : IWrapper<T>
{
    private readonly T value;

    public Wrapper(T value)
    {
        this.value = value;
    }

    public T Value { get { return value; } }
}

class Program
{
    static void Main(string[] args)
    {
        IWrapper<string> original = new Wrapper<string>("foo");
        IWrapper<object> original2 = original;        
        IWrapper<object> rewrapped = Rewrap(original2);

        Console.WriteLine(original2.GetType()); // Wrapper<string>
        Console.WriteLine(rewrapped.GetType()); // Wrapper<object>
    }

    static IWrapper<T> Rewrap<T>(IWrapper<T> wrapper)
    {
        return new Wrapper<T>(wrapper.Value);
    }    
}

Is there any way I can avoid this, other than skipping the step where it gets stored in an object?

嗯，您可以调用Where动态地，此时将在执行时推断类型参数:

dynamic copy = ...;

Expression<Func<ChildQueryElement, bool>> filter = qe1 => true;
// Can't call an extension method "on" dynamic; call it statically instead
copy = Queryable.Where(copy, filter);

请注意，表达式树类型需要为 Func<ChildQueryElement, bool>还有...我不清楚这对您来说是否会成为问题。