C# 弱引用 object 在“使用”块后存活

Question

Based on this previous question , I wanted to see if not disposing an HttpClient object creates leaks (which does, but nothing new here). However, what I've also found is that if I don't dispose a HttpClient object, the returned HttpResponseMessage is not garbage collected.

Here's the test. I've provided a reproducing fiddle here .

using System;
using System.Net.Http;

namespace HttpClientTest
{
    internal static class Program
    {
        private static T CallInItsOwnScope<T>(Func<T> getter)
        {
            return getter();
        }

        private static void Main()
        {
            var client = new HttpClient();
            var wRef2 = CallInItsOwnScope(() =>
            {
                using (var response = client.GetAsync(new Uri("https://postman-echo.com/get?foo1=bar1&foo2=bar2")).Result)
                {
                    return new WeakReference(response);
                }
            });
            GC.Collect();
            GC.WaitForPendingFinalizers();
            Console.WriteLine($"Alive: {wRef2.IsAlive}");

            client.Dispose();

            GC.Collect();
            GC.WaitForPendingFinalizers();
            Console.WriteLine($"Alive: {wRef2.IsAlive}");

            Console.ReadKey();
        }
    }
}

Compiling under .NET Framework 4.7.2, The output is as follows:

Alive: True
Alive: False

Edit: using .NET Core, I get the expected result:

Alive: False
Alive: False

My question is, why HttpResponseMessage is still alive after I disposed it, but only if I don't dispose it's creator?

(Looking at HttpClient 's sources I found that there is indeed a reference to the HttpResponseMessage being held, but this is deep into async land for me to really understand what's going on ( SetTaskCompleted -> TaskCompletionSource<>.TrySetResult() ))

Answer 1

First of all, we need look at this MSDN - HttpClient Class .

HttpClient is intended to be instantiated once per application, rather than per-use. See Remarks.

Anyway, here is what you missed.

public void CallServiceTest()
{
    var wRef2 = CallInItsOwnScope(() =>
    {
        // HttpClient -> HttpMessageInvoker(IDisposable), so it can be disposed.
        using (var client = new HttpClient())
        { 
            using (var response = client.GetAsync(new Uri("https://postman-echo.com/get?foo1=bar1&foo2=bar2")).Result)
            {
                return new WeakReference(response);
            }
        }
    });
    GC.Collect();
    GC.WaitForPendingFinalizers();
    Assert.IsFalse(wRef2.IsAlive);
}