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ServicePoint.ConnectionLimit default behavior with loopback changed unexpectedly

Symptoms

The limit for HTTP connections per endpoint is controlled by the ServicePointManager.DefaultConnectionLimit property. This value defaults to 2. In the .NET Framework 4.7 and earlier versions, the limit applied only to non-loopback addresses such as http://www.microsoft.com. For loopback addresses such as http://localhost, the limit for connections was always int.MaxValue (2,147,483,647) unless changed by calling the ServicePoint.ConnectionLimit API.

In the .NET Framework 4.7.1 when using HttpClient APIs, the connection limit for loopback addresses now matches the limit for non-loopback addresses. Thus, the default limit is 2. This can cause applications to run slower or hang when doing multiple, parallel, requests to the http://localhost addresses.

Here is code that shows the problem:

using System;
using System.Net;
using System.Net.Http;

namespace ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine($"ServicePointManager.DefaultConnectionLimit: {ServicePointManager.DefaultConnectionLimit}");

            var uriLoopback = new Uri("http://localhost");
            var uriExternal = new Uri("http://www.microsoft.com");

            Console.WriteLine("Before using HttpClient APIs");
            ServicePoint spLoopback = ServicePointManager.FindServicePoint(uriLoopback);
            Console.WriteLine($"{uriLoopback.AbsoluteUri}, ConnectionLimit (should be {int.MaxValue}): {spLoopback.ConnectionLimit}");

            ServicePoint spExternal = ServicePointManager.FindServicePoint(uriExternal);
            Console.WriteLine($"{uriExternal.AbsoluteUri}, ConnectionLimit (should be {ServicePointManager.DefaultConnectionLimit}): {spExternal.ConnectionLimit}");

            Console.WriteLine("Use HttpClient APIs");
            var client = new HttpClient();
            try
            {
                HttpResponseMessage response = client.GetAsync(uriLoopback).Result;
            }
            catch (Exception)
            {
                // Ignore any network error since there is probably not a loopback server present.
            }

            Console.WriteLine("After using HttpClient APIs");

            // BUG - due to the bug in .NET Framework 4.7.1, the ConnectionLimit for this loopback ServicePoint is changed
            // unexpectedly.
            Console.WriteLine($"{uriLoopback.AbsoluteUri}, ConnectionLimit (should be {int.MaxValue}): {spLoopback.ConnectionLimit}");
        }
    }
}

Cause

Changes in .NET Framework 4.7.1 to the System.Net.Http.HttpClientHandler class caused this problem.

Resolution

To work around the problem using HttpClient APIs, you can use the following code to increase the connection limit. This code should be added before calling any HttpClient APIs.

ServicePointManager.DefaultConnectionLimit = 20; // Actual value should be based on your requirements.

Note: This will also change the limits for non-loopback addresses.

More information

This issue was fixed in the August 2018 Security and Quality Rollup. Installing this or later updates will correct the problem.

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