将LINQ查询的枚举限制为仅一个 [英] Restricting the enumerations of LINQ queries to One Only

问题描述

我有一个LINQ查询，它不应被多次枚举，并且我想避免错误地将其枚举两次.我可以使用任何扩展方法来确保自己免受此类错误的侵害吗?我正在考虑这样的事情:

I have a LINQ query that should NOT be enumerated more than once, and I want to avoid enumerating it twice by mistake. Is there any extension method I can use to ensure that I am protected from such a mistake? I am thinking about something like this:

var numbers = Enumerable.Range(1, 10).OnlyOnce();
Console.WriteLine(numbers.Count()); // shows 10
Console.WriteLine(numbers.Count()); // throws InvalidOperationException: The query cannot be enumerated more than once.

我想要此功能的原因是因为我有一个无数的任务，旨在逐步实例化并运行任务，而在控制下缓慢地枚举它.我已经犯了两次运行任务的错误，因为我忘记了它是一个不同的可枚举的，而不是 数组.

The reason I want this functionality is because I have an enumerable of tasks, that is intended to instantiate and run the tasks progressivelly, while it is enumerated slowly under control. I already made the mistake to run the tasks twice because I forgot that it's a differed enumerable and not an array.

var tasks = Enumerable.Range(1, 10).Select(n => Task.Run(() => Console.WriteLine(n)));
Task.WaitAll(tasks.ToArray()); // Lets wait for the tasks to finish...
Console.WriteLine(String.Join(", ", tasks.Select(t => t.Id))); // Lets see the completed task IDs...
// Oups! A new set of tasks started running!

推荐答案

我想避免两次错误地枚举它.

I want to avoid enumerating it twice by mistake.

您可以用一个被两次枚举时抛出的集合来包装该集合.

You can wrap the collection with a collection that throws if it's enumerated twice.

例如:

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

namespace ConsoleApp8
{
    public static class EnumExtension
    {
        class OnceEnumerable<T> : IEnumerable<T>
        {
            IEnumerable<T> col;
            bool hasBeenEnumerated = false;
            public OnceEnumerable(IEnumerable<T> col)
            {
                this.col = col;
            }

            public IEnumerator<T> GetEnumerator()
            {
                if (hasBeenEnumerated)
                {
                    throw new InvalidOperationException("This collection has already been enumerated.");
                }
                this.hasBeenEnumerated = true;
                return col.GetEnumerator();
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return GetEnumerator();
            }
        }

        public static IEnumerable<T> OnlyOnce<T>(this IEnumerable<T> col)
        {
            return new OnceEnumerable<T>(col);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
             var col = Enumerable.Range(1, 10).OnlyOnce();

             var colCount = col.Count(); //first enumeration
             foreach (var c in col) //second enumeration
             {
                 Console.WriteLine(c);
             }
        }
    }
}

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