奇怪的委托参考行为

Question

I have a need to pass the results of a source function (which returns an IEnumerable ) through a list of other processing functions (that each take and return an IEnumerable ).

All is fine up to that point, but I also need to allow the processing functions to perform multiple loops over their input enumerables.

So rather than pass in IEnumerable<T> , I thought I would change the input parameter to Func<IEnumerable<T>> and allow each of the functions to restart the enumerable if required.

Unfortunately, I'm now getting a stack overflow where the final processing function is calling itself rather than passing the request back down the chain.

The example code is a bit contrived but hopefully gives you an idea of what I'm trying to achieve.

class Program
{
    public static void Main(string[] args)
    {
        Func<IEnumerable<String>> getResults = () => GetInputValues("A", 5);

        List<String> valuesToAppend = new List<String>();

        valuesToAppend.Add("B");
        valuesToAppend.Add("C");

        foreach (var item in valuesToAppend)
        {
             getResults = () => ProcessValues(() => getResults(),item);
        }

        foreach (var item in getResults())
        {
            Console.WriteLine(item);
        }

    }

    public static IEnumerable<String> GetInputValues(String value, Int32 numValues)
    {
        for (int i = 0; i < numValues; i++)
        {
            yield return value;
        }
    }

    public static IEnumerable<String> ProcessValues(Func<IEnumerable<String>> getInputValues, String appendValue)
    {
        foreach (var item in getInputValues())
        {
            yield return item + " " + appendValue;
        }
    }

}

Answer 1

getResults is captured as a variable , not a value. I don't really like the overall approach you're using here (it seems convoluted), but you should be able to fix the stackoverflow by changing the capture:

    foreach (var item in valuesToAppend)
    {
         var tmp1 = getResults;
         var tmp2 = item;
         getResults = () => ProcessValues(() => tmp1(),tmp2);
    }

On a side note: IEnumerable[<T>] is already kinda repeatable, you simply call foreach another time - is is IEnumerator[<T>] that (despite the Reset() ) isn't - but also, I think it is worth doing trying to do this without needing to ever repeat the enumeration, since in the general case that simply cannot be guaranteed to work.

---

Here's a simpler (IMO) implementation with the same result:

using System;
using System.Collections.Generic;
using System.Linq;
class Program {
    public static void Main() {
        IEnumerable<String> getResults = Enumerable.Repeat("A", 5);
        List<String> valuesToAppend = new List<String> { "B", "C" };
        foreach (var item in valuesToAppend) {
            string tmp = item;
            getResults = getResults.Select(s => s + " " + tmp);
        }
        foreach (var item in getResults) {
            Console.WriteLine(item);
        }
    }
}