Added ProxyClient.SystemProxy

Based on the beginnings of a patch by @jwonagel.

MailKit/MailKit.csproj

@@ -114,6 +114,7 @@
     <Compile Include="Net\Proxy\Socks4Client.cs" />
     <Compile Include="Net\Proxy\Socks5Client.cs" />
     <Compile Include="Net\Proxy\SocksClient.cs" />
+    <Compile Include="Net\Proxy\WebProxyClient.cs" />
     <Compile Include="Net\Smtp\AsyncSmtpClient.cs" />
     <Compile Include="Net\Smtp\ISmtpClient.cs" />
     <Compile Include="Net\Smtp\SmtpAuthenticationSecretDetector.cs" />

MailKit/MailKitLite.csproj

@@ -119,6 +119,7 @@
     <Compile Include="Net\Proxy\Socks4Client.cs" />
     <Compile Include="Net\Proxy\Socks5Client.cs" />
     <Compile Include="Net\Proxy\SocksClient.cs" />
+    <Compile Include="Net\Proxy\WebProxyClient.cs" />
     <Compile Include="Net\Smtp\AsyncSmtpClient.cs" />
     <Compile Include="Net\Smtp\ISmtpClient.cs" />
     <Compile Include="Net\Smtp\SmtpAuthenticationSecretDetector.cs" />

MailKit/Net/Proxy/ProxyClient.cs

@@ -31,6 +31,10 @@
 using System.Net.Sockets;
 using System.Threading.Tasks;
 
+#if NET6_0_OR_GREATER
+using System.Net.Http;
+#endif
+
 namespace MailKit.Net.Proxy
 {
 	/// <summary>
@@ -45,6 +49,25 @@ namespace MailKit.Net.Proxy
 	/// </example>
 	public abstract class ProxyClient : IProxyClient
 	{
+#if NET6_0_OR_GREATER
+		static IProxyClient systemProxy;
+
+		/// <summary>
+		/// Get a client for the default system proxy.
+		/// </summary>
+		/// <remarks>
+		/// Gets a client for the default system proxy.
+		/// </remarks>
+		/// <value>A client for the default system proxy.</value>
+		public static IProxyClient SystemProxy {
+			get {
+				systemProxy ??= new WebProxyClient (HttpClient.DefaultProxy);
+
+				return systemProxy;
+			}
+		}
+#endif
+
 		/// <summary>
 		/// Initializes a new instance of the <see cref="T:MailKit.Net.Proxy.ProxyClient"/> class.
 		/// </summary>

MailKit/Net/Proxy/WebProxyClient.cs

@@ -0,0 +1,194 @@
+//
+// WebProxyClient.cs
+//
+// Author: Jeffrey Stedfast <[email protected]>
+//
+// Copyright (c) 2013-2024 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+#if NET6_0_OR_GREATER
+
+using System;
+using System.IO;
+using System.Net;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MailKit.Net.Proxy
+{
+	/// <summary>
+	/// A proxy client that makes use of a <see cref="IWebProxy"/>.
+	/// </summary>
+	/// <remarks>
+	/// A proxy client that makes use of a <see cref="IWebProxy"/>.
+	/// </remarks>
+	internal class WebProxyClient : ProxyClient
+	{
+		readonly IWebProxy proxy;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Proxy.WebProxyClient"/> class.
+		/// </summary>
+		/// <remarks>
+		/// Initializes a new instance of the <see cref="T:MailKit.Net.Proxy.ProxyClient"/> class.
+		/// </remarks>
+		/// <param name="proxy">The web proxy.</param>
+		/// <exception cref="ArgumentNullException">
+		/// <paramref name="proxy"/> is <c>null</c>.
+		/// </exception>
+		public WebProxyClient (IWebProxy proxy) : base ("System", 0)
+		{
+			if (proxy is null)
+				throw new ArgumentNullException (nameof (proxy));
+
+			this.proxy = proxy;
+		}
+
+		static Uri GetTargetUri (string host, int port)
+		{
+			string scheme;
+
+			switch (port) {
+			case 25: case 465: case 587: scheme = "smtp"; break;
+			case 110: case 995: scheme = "pop"; break;
+			case 143: case 993: scheme = "imap"; break;
+			default: scheme = "http"; break;
+			}
+
+			return new Uri ($"{scheme}://{host}:{port}");
+		}
+
+		static NetworkCredential GetNetworkCredential (ICredentials credentials, Uri uri)
+		{
+			if (credentials is NetworkCredential network)
+				return network;
+
+			return credentials.GetCredential (uri, "Basic");
+		}
+
+		static ProxyClient GetProxyClient (Uri proxyUri, ICredentials credentials)
+		{
+			var credential = GetNetworkCredential (credentials, proxyUri);
+
+			if (proxyUri.Scheme.Equals ("https", StringComparison.OrdinalIgnoreCase))
+				return new HttpsProxyClient (proxyUri.Host, proxyUri.Port, credential);
+
+			if (proxyUri.Scheme.Equals ("http", StringComparison.OrdinalIgnoreCase))
+				return new HttpProxyClient (proxyUri.Host, proxyUri.Port, credential);
+
+			throw new NotImplementedException ($"The default system proxy does not support {proxyUri.Scheme}.");
+		}
+
+		/// <summary>
+		/// Connect to the target host.
+		/// </summary>
+		/// <remarks>
+		/// Connects to the target host and port through the proxy server.
+		/// </remarks>
+		/// <returns>The connected network stream.</returns>
+		/// <param name="host">The host name of the target server.</param>
+		/// <param name="port">The target server port.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="host"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="port"/> is not between <c>0</c> and <c>65535</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// The <paramref name="host"/> is a zero-length string.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.Net.Sockets.SocketException">
+		/// A socket error occurred trying to connect to the remote host.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override Stream Connect (string host, int port, CancellationToken cancellationToken = default)
+		{
+			ValidateArguments (host, port);
+
+			var targetUri = GetTargetUri (host, port);
+			var proxyUri = proxy.GetProxy (targetUri);
+
+			if (proxyUri is null) {
+				// Note: if the proxy URI is null, then it means that the proxy should be bypassed.
+				var socket = SocketUtils.Connect (host, port, LocalEndPoint, cancellationToken);
+				return new NetworkStream (socket, true);
+			}
+
+			var proxyClient = GetProxyClient (proxyUri, proxy.Credentials);
+
+			return proxyClient.Connect (host, port, cancellationToken);
+		}
+
+		/// <summary>
+		/// Asynchronously connect to the target host.
+		/// </summary>
+		/// <remarks>
+		/// Asynchronously connects to the target host and port through the proxy server.
+		/// </remarks>
+		/// <returns>The connected network stream.</returns>
+		/// <param name="host">The host name of the target server.</param>
+		/// <param name="port">The target server port.</param>
+		/// <param name="cancellationToken">The cancellation token.</param>
+		/// <exception cref="System.ArgumentNullException">
+		/// <paramref name="host"/> is <c>null</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentOutOfRangeException">
+		/// <paramref name="port"/> is not between <c>0</c> and <c>65535</c>.
+		/// </exception>
+		/// <exception cref="System.ArgumentException">
+		/// The <paramref name="host"/> is a zero-length string.
+		/// </exception>
+		/// <exception cref="System.OperationCanceledException">
+		/// The operation was canceled via the cancellation token.
+		/// </exception>
+		/// <exception cref="System.Net.Sockets.SocketException">
+		/// A socket error occurred trying to connect to the remote host.
+		/// </exception>
+		/// <exception cref="System.IO.IOException">
+		/// An I/O error occurred.
+		/// </exception>
+		public override async Task<Stream> ConnectAsync (string host, int port, CancellationToken cancellationToken = default)
+		{
+			ValidateArguments (host, port);
+
+			var targetUri = GetTargetUri (host, port);
+			var proxyUri = proxy.GetProxy (targetUri);
+
+			if (proxyUri is null) {
+				// Note: if the proxy URI is null, then it means that the proxy should be bypassed.
+				var socket = await SocketUtils.ConnectAsync (host, port, LocalEndPoint, cancellationToken).ConfigureAwait (false);
+				return new NetworkStream (socket, true);
+			}
+
+			var proxyClient = GetProxyClient (proxyUri, proxy.Credentials);
+
+			return await proxyClient.ConnectAsync (host, port, cancellationToken);
+		}
+	}
+}
+
+#endif

UnitTests/Net/Proxy/WebProxyClientTests.cs

@@ -0,0 +1,151 @@
+//
+// WebProxyClientTests.cs
+//
+// Author: Jeffrey Stedfast <[email protected]>
+//
+// Copyright (c) 2013-2024 .NET Foundation and Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+//
+
+using System.Net;
+
+using MailKit.Net.Proxy;
+
+namespace UnitTests.Net.Proxy
+{
+	[TestFixture]
+	public class WebProxyClientTests
+	{
+		const int ConnectTimeout = 5 * 1000; // 5 seconds
+
+		[Test]
+		public void TestArgumentExceptions ()
+		{
+			var credentials = new NetworkCredential ("user", "password");
+			var proxy = ProxyClient.SystemProxy;
+
+			Assert.Throws<ArgumentNullException> (() => new WebProxyClient (null));
+
+			Assert.That (proxy.ProxyPort, Is.EqualTo (1080));
+			Assert.That (proxy.ProxyHost, Is.EqualTo ("System"));
+			Assert.That (proxy.ProxyCredentials, Is.Null);
+
+			Assert.Throws<ArgumentNullException> (() => proxy.Connect (null, 80));
+			Assert.Throws<ArgumentNullException> (() => proxy.Connect (null, 80, ConnectTimeout));
+			Assert.ThrowsAsync<ArgumentNullException> (async () => await proxy.ConnectAsync (null, 80));
+			Assert.ThrowsAsync<ArgumentNullException> (async () => await proxy.ConnectAsync (null, 80, ConnectTimeout));
+
+			Assert.Throws<ArgumentException> (() => proxy.Connect (string.Empty, 80));
+			Assert.Throws<ArgumentException> (() => proxy.Connect (string.Empty, 80, ConnectTimeout));
+			Assert.ThrowsAsync<ArgumentException> (async () => await proxy.ConnectAsync (string.Empty, 80));
+			Assert.ThrowsAsync<ArgumentException> (async () => await proxy.ConnectAsync (string.Empty, 80, ConnectTimeout));
+
+			Assert.Throws<ArgumentOutOfRangeException> (() => proxy.Connect ("www.google.com", 0));
+			Assert.Throws<ArgumentOutOfRangeException> (() => proxy.Connect ("www.google.com", 0, ConnectTimeout));
+			Assert.ThrowsAsync<ArgumentOutOfRangeException> (async () => await proxy.ConnectAsync ("www.google.com", 0));
+			Assert.ThrowsAsync<ArgumentOutOfRangeException> (async () => await proxy.ConnectAsync ("www.google.com", 0, ConnectTimeout));
+
+			Assert.Throws<ArgumentOutOfRangeException> (() => proxy.Connect ("www.google.com", 80, -ConnectTimeout));
+			Assert.ThrowsAsync<ArgumentOutOfRangeException> (async () => await proxy.ConnectAsync ("www.google.com", 80, -ConnectTimeout));
+		}
+
+		[Test]
+		public void TestConnect ()
+		{
+			var proxy = ProxyClient.SystemProxy;
+			Stream stream = null;
+
+			try {
+				stream = proxy.Connect ("www.google.com", 80);
+			} catch (TimeoutException) {
+				Assert.Inconclusive ("Timed out.");
+			} catch (Exception ex) {
+				Assert.Fail (ex.Message);
+			} finally {
+				stream?.Dispose ();
+			}
+		}
+
+		[Test]
+		public async Task TestConnectAsync ()
+		{
+			var proxy = ProxyClient.SystemProxy;
+			Stream stream = null;
+
+			try {
+				stream = await proxy.ConnectAsync ("www.google.com", 80);
+			} catch (TimeoutException) {
+				Assert.Inconclusive ("Timed out.");
+			} catch (Exception ex) {
+				Assert.Fail (ex.Message);
+			} finally {
+				stream?.Dispose ();
+			}
+		}
+
+		[Test]
+		public void TestConnectViaWebProxy ()
+		{
+			using (var server = new HttpProxyListener ()) {
+				server.Start (IPAddress.Loopback, 0);
+
+				var credentials = new NetworkCredential ("username", "password");
+				var webProxy = new WebProxy (new Uri ($"http://{server.IPAddress}:{server.Port}"), true, null, credentials);
+
+				var proxy = new WebProxyClient (webProxy);
+				Stream stream = null;
+
+				try {
+					stream = proxy.Connect ("www.google.com", 80, ConnectTimeout);
+				} catch (TimeoutException) {
+					Assert.Inconclusive ("Timed out.");
+				} catch (Exception ex) {
+					Assert.Fail (ex.Message);
+				} finally {
+					stream?.Dispose ();
+				}
+			}
+		}
+
+		[Test]
+		public async Task TestConnectViaWebProxyAsync ()
+		{
+			using (var server = new HttpProxyListener ()) {
+				server.Start (IPAddress.Loopback, 0);
+
+				var credentials = new NetworkCredential ("username", "password");
+				var webProxy = new WebProxy (new Uri ($"http://{server.IPAddress}:{server.Port}"), true, null, credentials);
+
+				var proxy = new WebProxyClient (webProxy);
+				Stream stream = null;
+
+				try {
+					stream = await proxy.ConnectAsync ("www.google.com", 80, ConnectTimeout);
+				} catch (TimeoutException) {
+					Assert.Inconclusive ("Timed out.");
+				} catch (Exception ex) {
+					Assert.Fail (ex.Message);
+				} finally {
+					stream?.Dispose ();
+				}
+			}
+		}
+	}
+}