返回 IEnumerable 的扩展方法的控制台输出<T>

Question

I'm using Visual Studio 2013.

When the following code is executed, ForEach2 outputs to the console, but there is no output by ForEach . I was surprised that I thought they would have the same result. Is this a specification, or is something wrong in our environment? What are the specifications, if any?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var array = new[] { 1, 2, 3 };
            Console.WriteLine("ForEach:");
            array.ForEach(a => Console.WriteLine(a));
            Console.WriteLine("ForEach2:");
            array.ForEach2(a => Console.WriteLine(a));
            Console.ReadKey();
        }
    }

    static class Extension
    {
        public static IEnumerable<T> ForEach<T>(this IEnumerable<T> target, Action<T> action)
        {
            foreach (var item in target)
            {
                action(item);
                yield return item;
            }
        }

        public static void ForEach2<T>(this IEnumerable<T> target, Action<T> action)
        {
            foreach (var item in target)
            {
                action(item);
            }
        }
    }
}

When I set a breakpoint, it doesn't seem to get into ForEach in the first place. I tried to suppress optimization with [MethodImpl (MethodImplOptions.NoOptimization)] but the result was the same.

Answer 1

The yield statement in the ForEach method means the method will not be executed until you iterate over the items in the IEnumerable<T> returned by ForEach . You can see this is true by putting a breakpoint on the action(item); line in your code - the breakpoint is not hit.

If you want the code to execute you will need to add a foreach for the values returned from ForEach . For example:

static void Main(string[] args)
{
    var array = new[] { 1, 2, 3 };
    Console.WriteLine("ForEach:");
    array.ForEach(a => Console.WriteLine(a));

    // add this line
    foreach (var b in array.ForEach(a => Console.WriteLine(a))) { }

    Console.WriteLine("ForEach2:");
    array.ForEach2(a => Console.WriteLine(a));
    Console.ReadKey();
}

The new line will cause the ForEach to execute and yield each result. This will cause the action(item) to execute and print the result to the console.

The remarks in the yield docs page tell you that you need to use foreach or LINQ to consume the results of a yield statement.

Answer 2

You've defined a generator method, but since you haven't enumerated the results, nothing happens. Which do you want, the callback or the enumerator?

        public static IEnumerable<T> ForEach<T>(this IEnumerable<T> target)
        {
            foreach (var item in target)
            {
                yield return item;
            }
        }

        Console.WriteLine("ForEach:");
        foreach (var item in array.ForEach())
            Console.WriteLine(item);