SmtpClient.Dispose throws System.NullReferenceException with OTEL

[vchirikov]

Describe the bug

MailService.Dispose throws System.NullReferenceException

Platform:

• OS: Windows
• .NET Runtime: 8.0.8
• Versions:

• <PackageVersion Include="MailKit" Version="4.7.1.1"/>
• <PackageVersion Include="MimeKit" Version="4.7.1"/>

Exception

System.NullReferenceException: Object reference not set to an instance of an object.
   at MailKit.Net.ClientMetrics.GetTags(Uri uri, Exception ex)
   at MailKit.Net.ClientMetrics.RecordClientDisconnected(Int64 startTimestamp, Uri uri, Exception ex)
   at MailKit.Net.Smtp.SmtpClient.RecordClientDisconnected(Exception ex)
   at MailKit.Net.Smtp.SmtpClient.Disconnect(String host, Int32 port, SecureSocketOptions options, Boolean requested)
   at MailKit.Net.Smtp.SmtpClient.Dispose(Boolean disposing)
   at MailKit.MailService.Dispose()
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.DisposeAsync()

Expected behavior

Should not throw on scope disposing.

Code Snippets

using MailKit.Net.Smtp;

var services = new ServiceCollection();

MailKit.Telemetry.Configure();
// do not use IMeterFactory constructor here, it's broken, use global metrics instead
services.AddScoped<SmtpClient>();

await using var globalSp = services.BuildServiceProvider();
await using (var scope = globalSp.CreateAsyncScope())
{
    var smtp = scope.ServiceProvider.GetRequiredService<SmtpClient>();
    if (!smtp.IsConnected) {
        await smtp.ConnectAsync(host, port, encOptions, default);
    }

    if (!smtp.IsAuthenticated) {
        await smtp.AuthenticateAsync(userName, password, default);
    }

    await smtp.SendAsync(mail, default);
    if (smtp.IsConnected) {
        await smtp.DisconnectAsync(quit: true, default);
    }
} // Dispose will throw NRE if you setup otel metrics

Additional context

If you want to reuse same SmtpClient across the DI scope it's expected that you place SmtpClient in disconnected state, but it will throw on scope Dispose because DisconnectAsync drops uri field, which is used in ClientMetrics.GetTags on disposing.

p.s. also ctor with IMeterFactory doesn't work with NRE too.

[jstedfast]

p.s. also ctor with IMeterFactory doesn't work with NRE too.

Are you calling new SmtpClient(..., meterFactory) after calling Telemetry.SmtpClient.Configure(meterFactory)? If so, it's probably because a meter has already been created with that name.

[vchirikov]

Are you calling new SmtpClient(..., meterFactory) after calling Telemetry.SmtpClient.Configure(meterFactory)?

No, firstly I tried just:

services.AddScoped(sp => new SmtpClient(new NullProtocolLogger(), sp.GetRequiredService<IMeterFactory>()));

And it throws on activation:

System.NullReferenceException: Object reference not set to an instance of an object.
   at MailKit.Net.ClientMetrics..ctor(Meter meter, String meterName, String an, String protocol)
   at MailKit.Telemetry.SmtpClient.CreateMetrics(Meter meter)
   at MailKit.Net.Smtp.SmtpClient..ctor(IProtocolLogger protocolLogger, IMeterFactory meterFactory)

because meter is null inside public ClientMetrics (Meter meter, string meterName, string an, string protocol), but looks like you already found the issue with ClientMetrics ctor + property instead of the method arg. :)

[jstedfast]

Gonna close this as fixed, then. I'll try to make a new release later this week (or this weekend).