Add SocketsHttpHandler.ConnectCallback

[geoffkizer]

Background and Motivation

We recently removed System.Net.Connections from 5.0. One feature we lost was the ConnectionFactory property on SocketsHttpHandler, which allowed you to plug in your own transport that SocketsHttpHandler would run over.

This new API provides the same capability to replace the transport without relying on System.Net.Connections. When you set this property, your callback will be invoked whenever SocketsHttpHandler needs to create a new connection. You can use this callback to create the Stream however you want -- e.g. use an in-memory stream, bind to a particular local address, etc.

Proposed API

public class SocketsHttpHandler
{
    public Func<DnsEndPoint, HttpRequestMessage, CancellationToken, ValueTask<Stream>>? ConnectCallback;
}

The DnsEndPoint argument identifies the server we are trying to connect to. The HttpRequestMessage argument allows you to inspect the initial HttpRequestMessage for this connection -- though you should be aware that the connection can be used for many requests, not just this initial one. We expect most users will only need the DnsEndPoint.

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[terrajobst]

Video

• We should add a context class that passes in the parameters so that we can add more values over time
• We originally had SocketsHttpConnectionContext as get/set POCO but that would mean that all properties would have to be nullable, which would be super annoying to use

namespace System.Net.Http
{
    public sealed class SocketsHttpConnectionContext
    {
        public DnsEndPoint DnsEndPoint { get; }
        public HttpRequestMessage RequestMessage { get; }
    }
    public sealed class SocketsHttpHandler : HttpMessageHandler
    {
        public Func<SocketsHttpConnectionContext, CancellationToken, ValueTask<Stream>>? ConnectCallback { get; set; }
    }
}