System.InvalidOperationException : Sequence contains more than one element for DotNet Testcontainer

Question

Error Log:

Standard Output Messages:
[testcontainers.org 00:00:03.98] Reuse is an experimental feature. For more information, visit: https://dotnet.testcontainers.org/api/resource_reuse/
-> warning: The previous ScenarioContext was already disposed.
   Failed EmailAddressesAreValidatedByTheForgottenPasswordScreen("it uses unquoted backslashes",null) [26 ms]
Error Message:
System.InvalidOperationException : Sequence contains more than one element
TearDown : System.NullReferenceException : Object reference not set to an instance of an object.
Stack Trace:
 at System.Linq.ThrowHelper.ThrowMoreThanOneElementException()
 at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Boolean& found)
 at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source)
 at DotNet.Testcontainers.Containers.DockerContainer.UnsafeCreateAsync(CancellationToken ct) in /_/src/Testcontainers/Containers/DockerContainer.cs:line 441
 at DotNet.Testcontainers.Containers.DockerContainer.StartAsync(CancellationToken ct) in /_/src/Testcontainers/Containers/DockerContainer.cs:line 306
 at [MYCOMPANY].[MYAPP].UI.Tests.Hooks.ConfigurationHooks.SetConfiguration(IObjectContainer objectContainer) in /home/runner/actions-runner/_work/[MYAPP]/[MYAPP]/Tests/[MYCOMPANY].[MYAPP].UI.Tests/Hooks/ConfigurationHooks.cs:line 48
 at Reqnroll.Bindings.AsyncMethodHelper.ConvertTaskOfT(Task task, Boolean getValue)
 at Reqnroll.Bindings.BindingDelegateInvoker.InvokeDelegateAsync(Delegate bindingDelegate, Object[] invokeArgs, ExecutionContextHolder executionContext)
 at Reqnroll.Bindings.BindingInvoker.InvokeBindingAsync(IBinding binding, IContextManager contextManager, Object[] arguments, ITestTracer testTracer, DurationHolder durationHolder)
 at Reqnroll.Infrastructure.TestExecutionEngine.InvokeHookAsync(IAsyncBindingInvoker invoker, IHookBinding hookBinding, HookType hookType)
 at Reqnroll.Infrastructure.TestExecutionEngine.FireEventsAsync(HookType hookType)
 at Reqnroll.Infrastructure.TestExecutionEngine.FireEventsAsync(HookType hookType)
 at Reqnroll.Infrastructure.TestExecutionEngine.OnFeatureStartAsync(FeatureInfo featureInfo)
 at Reqnroll.TestRunner.OnFeatureStartAsync(FeatureInfo featureInfo)
 at [MYCOMPANY].[MYAPP].UI.Tests.Features.PasswordAndEmailValidationFeature.TestInitializeAsync()
 at NUnit.Framework.Internal.TaskAwaitAdapter.GenericAdapter`1.BlockUntilCompleted()
 at NUnit.Framework.Internal.MessagePumpStrategy.NoMessagePumpStrategy.WaitForCompletion(AwaitAdapter awaiter)
 at NUnit.Framework.Internal.AsyncToSyncAdapter.Await[TResult](TestExecutionContext context, Func`1 invoke)
 at NUnit.Framework.Internal.AsyncToSyncAdapter.Await(TestExecutionContext context, Func`1 invoke)
 at NUnit.Framework.Internal.Commands.SetUpTearDownItem.RunSetUpOrTearDownMethod(TestExecutionContext context, IMethodInfo method)
 at NUnit.Framework.Internal.Commands.SetUpTearDownItem.RunSetUp(TestExecutionContext context)
 at NUnit.Framework.Internal.Commands.BeforeAndAfterTestCommand.<>c__DisplayClass1_0.<Execute>b__0()
 at NUnit.Framework.Internal.Commands.DelegatingTestCommand.RunTestMethodInThreadAbortSafeZone(TestExecutionContext context, Action action)
--TearDown
 at Reqnroll.Infrastructure.TestExecutionEngine.OnScenarioEndAsync()
 at Reqnroll.TestRunner.OnScenarioEndAsync()
 at [MYCOMPANY].[MYAPP].UI.Tests.Features.PasswordAndEmailValidationFeature.TestTearDownAsync()
 at NUnit.Framework.Internal.TaskAwaitAdapter.GenericAdapter`1.GetResult()
 at NUnit.Framework.Internal.AsyncToSyncAdapter.Await[TResult](TestExecutionContext context, Func`1 invoke)
 at NUnit.Framework.Internal.AsyncToSyncAdapter.Await(TestExecutionContext context, Func`1 invoke)
 at NUnit.Framework.Internal.Commands.SetUpTearDownItem.RunSetUpOrTearDownMethod(TestExecutionContext context, IMethodInfo method)
 at NUnit.Framework.Internal.Commands.SetUpTearDownItem.RunTearDown(TestExecutionContext context)

The above error message occurs when I try to setup a TestContainer in my automation test script for running a MailDev server to intercept email for a mail sender for users logins/password resets for my company.

Here's my codebase:

[BeforeFeature(Order = TestRunSequence.RegisterObjectContainer)]
public static async Task SetConfiguration(IObjectContainer objectContainer)
{
  var configuration = new ConfigurationBuilder()
    .SetBasePath(AppDomain.CurrentDomain.BaseDirectory)
    .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
    .AddEnvironmentVariables() // Load environment variables
    .AddUserSecrets<IdentityProviderConfiguration>()
    .Build();
  objectContainer?.RegisterInstanceAs(configuration);

  var mailDevContainer= new ContainerBuilder()
    .WithImage("maildev/maildev:latest")
    .WithPortBinding(1080, true)
    .WithPortBinding(1025, true)
    .WithWaitStrategy(Wait.ForUnixContainer().UntilPortIsAvailable(1080))
    .WithReuse(true)
    .WithLabel("maildev-container", "MailDev")
    .WithPortBinding(1080, 1080)
    .WithPortBinding(1025, 1025)
    .Build();

  if (!isStarted)
  {
      await mailDevContainer.StartAsync();
      isStarted = true;
  }

  objectContainer?.RegisterInstanceAs(mailDevContainer);

  ***(More code irrelevant to this issue is here)***
}

It seems like it's complaining about Line 48 (Tests/[MYCOMPANY].[MYAPP].UI.Tests/Hooks/ConfigurationHooks.cs:line 48) ; which reflects the line await mailDevContainer.StartAsync();

I don't know why this is occurring, but I'm also seeing an error before this mentioning Error Message: Docker.DotNet.DockerApiException : Docker API responded with status code=InternalServerError, response={"message":"failed to set up container networking: driver failed programming external connectivity on endpoint adoring_hopper (397aeaf67af9014dc4341dfedc0494e52219ceb718d00fce6160bd81e8c69a74): Bind for 0.0.0.0:1025 failed: port is already allocated"} so maybe this is affecting things? Can you guys help identify this?

So I would try to run those containers without binding or use automatic one instead, I remember that testcontainers has this feature. Just to see if this the source of the problem. — Monsieur Merso
– Monsieur Merso, Commented Jul 14 at 9:19
The error you're encountering comes from this line: WithPortBinding(1025, 1025). It binds port 1025 from the container to port 1025 on your host. Since ports can only be used by one process at a time, something else or another container is already using port 1025 on your host. As @monsieur-merso pointed out, you should use the random host port feature instead. There are plenty of examples of this in the Testcontainers docs. — Andre Hofmeister
– Andre Hofmeister, Commented Jul 21 at 13:30
Wish that I could. it's a company firewall issue. Only certain ports are whitelisted and this is the only one — Raisus
– Raisus, Commented Jul 22 at 8:37

Monsieur Merso · Accepted Answer · 2025-07-14 09:29:41Z

1

It seems that exception that you get is result of failed environment setup. As you already mentioned - there was a failure that prevented network of containers to setup correctly due to port already in use.

I remember I had such issue once and it was solved with automatic bind. Actually testcontainers documentation mentions it, see #2.

Avoid assigning static port bindings to containers. Instead, use random assigned host ports _builder.WithPortBinding(ushort, true) and retrieve the mapped port with _container.GetMappedPublicPort(ushort).

It solves lots of pain points managing ports in a test container setup.

answered Jul 14 at 9:29

Monsieur Merso

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2 Comments

Raisus Jul 22 at 8:39

Unfortunately my company only whitelists certain ports through its firewall, and I can't just use a random ushort port and hope. Insted I ensured that the port would be free by killing all processes on that port

Monsieur Merso Jul 22 at 8:58

glad you found a solution. good job.

Raisus · Accepted Answer · 2025-07-22 08:42:56Z

I found a solution that works for this issue, in which I ensured that the port I wanted to use was actually free and available to use by killing all processes trying to use that port before starting my tests.

I am doing this over trying to ushort a random port as the Testcontainer documentation mentions because I am doing this for my company, who only allow certain ports whitelisted through their firewall.

Here's how I am ensuring that my docker is clear of all ports on 1025/1080:

          # Clean up any containers created during testing
          docker stop $(docker ps -q) 2>/dev/null || true
          docker rm $(docker ps -a -q) 2>/dev/null || true
          docker network prune -f
          docker volume prune -f
          
          # Kill any remaining processes on port 14333
          sudo lsof -ti:14333 | xargs -r sudo kill -9 2>/dev/null || true
          
          # Force cleanup of any lingering SQL Server containers
          docker ps -a --filter "ancestor=mcr.microsoft.com/mssql/server" -q | xargs -r docker rm -f 2>/dev/null || true
          
          # Clean up testcontainer networks and volumes
          docker network ls --filter "name=testcontainers" -q | xargs -r docker network rm 2>/dev/null || true
          docker volume ls --filter "name=testcontainers" -q | xargs -r docker volume rm 2>/dev/null || true

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