Add more tests for SmtpClient (#114690)

* Add STARTTLS tests

* Partition tests from SmtpClientTest class by area

* Add more tests

* Update src/libraries/System.Net.Mail/tests/Functional/SmtpClientAttachmentTest.cs

Co-authored-by: Copilot <[email protected]>

* Fix nondeterministic test

---------

Co-authored-by: Copilot <[email protected]>

Author: rzikm

src/libraries/System.Net.Mail/tests/Functional/LoopbackServerTestBase.cs

@@ -0,0 +1,179 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Net.NetworkInformation;
+using System.Net.Security;
+using System.Security.Authentication;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using System.Net.Mail.Tests;
+using System.Threading;
+using System.Threading.Tasks;
+using Xunit;
+using Xunit.Sdk;
+using Xunit.Abstractions;
+
+namespace System.Net.Mail.Tests
+{
+    public enum SendMethod
+    {
+        Send,
+        SendAsync,
+        SendMailAsync
+    }
+
+    public interface ISendMethodProvider
+    {
+        static abstract SendMethod SendMethod { get; }
+    }
+
+    public struct SyncSendMethod : ISendMethodProvider
+    {
+        public static SendMethod SendMethod => SendMethod.Send;
+    }
+
+    public struct AsyncSendMethod : ISendMethodProvider
+    {
+        public static SendMethod SendMethod => SendMethod.SendAsync;
+    }
+
+    public struct SendMailAsyncMethod : ISendMethodProvider
+    {
+        public static SendMethod SendMethod => SendMethod.SendMailAsync;
+    }
+
+    public abstract class LoopbackServerTestBase<T> : IDisposable
+        where T : ISendMethodProvider
+    {
+        protected LoopbackSmtpServer Server { get; private set; }
+        protected ITestOutputHelper Output { get; private set; }
+
+        private SmtpClient _smtp;
+
+        protected SmtpClient Smtp
+        {
+            get
+            {
+                return _smtp ??= Server.CreateClient();
+            }
+        }
+
+        public LoopbackServerTestBase(ITestOutputHelper output)
+        {
+            Output = output;
+            Server = new LoopbackSmtpServer(Output);
+        }
+
+        private Task<Exception?> SendMailInternal(MailMessage msg, CancellationToken cancellationToken, bool? asyncExpectDirectException)
+        {
+            switch (T.SendMethod)
+            {
+                case SendMethod.Send:
+                    try
+                    {
+                        Smtp.Send(msg);
+                        return Task.FromResult<Exception>(null);
+                    }
+                    catch (Exception ex)
+                    {
+                        return Task.FromResult(ex);
+                    }
+
+                case SendMethod.SendAsync:
+                    TaskCompletionSource<Exception?> tcs = new TaskCompletionSource<Exception?>();
+                    SendCompletedEventHandler handler = null!;
+                    handler = (s, e) =>
+                    {
+                        Smtp.SendCompleted -= handler;
+
+                        if (e.Error != null)
+                        {
+                            tcs.SetResult(e.Error);
+                        }
+                        else if (e.Cancelled)
+                        {
+                            tcs.SetResult(new OperationCanceledException("The operation was canceled."));
+                        }
+                        else
+                        {
+                            tcs.SetResult(null);
+                        }
+                    };
+                    Smtp.SendCompleted += handler;
+                    try
+                    {
+                        Smtp.SendAsync(msg, tcs);
+
+                        if (asyncExpectDirectException == true)
+                        {
+                            Assert.Fail($"No exception thrown");
+                        }
+
+                        return tcs.Task;
+                    }
+                    catch (Exception ex) when (ex is not XunitException)
+                    {
+                        Smtp.SendCompleted -= handler;
+
+                        if (asyncExpectDirectException == false)
+                        {
+                            Assert.Fail($"Expected exception via callback, got direct: {ex}");
+                        }
+
+                        return Task.FromResult(ex);
+                    }
+
+                case SendMethod.SendMailAsync:
+                    try
+                    {
+                        Task task = Smtp.SendMailAsync(msg, cancellationToken);
+
+                        if (asyncExpectDirectException == true)
+                        {
+                            Assert.Fail($"No exception thrown");
+                        }
+
+                        return task.ContinueWith(t => t.Exception?.InnerException);
+                    }
+                    catch (Exception ex) when (ex is not XunitException)
+                    {
+                        if (asyncExpectDirectException == false)
+                        {
+                            Assert.Fail($"Expected stored exception, got direct: {ex}");
+                        }
+
+                        return Task.FromResult(ex);
+                    }
+
+                default:
+                    throw new ArgumentOutOfRangeException();
+            }
+        }
+
+        protected async Task SendMail(MailMessage msg, CancellationToken cancellationToken = default)
+        {
+            Exception? ex = await SendMailInternal(msg, cancellationToken, null);
+            Assert.Null(ex);
+        }
+
+        protected async Task<TException> SendMail<TException>(MailMessage msg, CancellationToken cancellationToken = default, bool unwrapException = true, bool asyncDirectException = false) where TException : Exception
+        {
+            Exception? ex = await SendMailInternal(msg, cancellationToken, asyncDirectException);
+
+            if (unwrapException && T.SendMethod != SendMethod.Send && typeof(TException) != typeof(SmtpException))
+            {
+                ex = Assert.IsType<SmtpException>(ex).InnerException;
+            }
+
+            return Assert.IsType<TException>(ex);
+        }
+
+        protected static string GetClientDomain() => IPGlobalProperties.GetIPGlobalProperties().HostName.Trim().ToLower();
+
+        public virtual void Dispose()
+        {
+            _smtp?.Dispose();
+            Server?.Dispose();
+        }
+    }
+}