Fixed: IExceptionHandler Exceptions Aren't Logged in ASP.NET Core 10

My IExceptionHandler turned database errors into a 503, and the client got exactly what it should. Then I looked at the log for the error it had handled, and the middleware's line wasn't there. That's a change in .NET 10, and one option brings the line back.

What I Was Doing

My .NET C# Web API runs on ASP.NET Core 10. An exception handler answers 503 when the database can't do something right now:

src/Dewiride.Inventory.Api/Infrastructure/DatabaseUnavailableExceptionHandler.cs (excerpt)

public async ValueTask<bool> TryHandleAsync(
    HttpContext httpContext,
    Exception exception,
    CancellationToken cancellationToken)
{
    if (!IsTemporary(exception))
    {
        return false;
    }

It's registered in Program.cs, along with the middleware:

src/Dewiride.Inventory.Api/Program.cs (excerpt)

builder.Services.AddExceptionHandler<DatabaseUnavailableExceptionHandler>();

An order then waited too long for a row lock.

What I Expected

A 503 for the client, and in the log an An unhandled exception has occurred while executing the request. line with the stack, as I'd seen for every other exception.

What I Got

The 503 was right:

Response

HTTP/1.1 503 Service Unavailable
Content-Type: application/problem+json

{
  "type": "https://tools.ietf.org/html/rfc9110#section-15.6.4",
  "title": "The database can't do this right now.",
  "status": 503,
  "detail": "Try again in 5 seconds. Send the same Idempotency-Key, so a change can't be applied twice.",
  "traceId": "00-be9c3937ef6ae66c81c2ae2a3339273f-670d2b6b06441546-00"
}

The log had only EF Core's line about the failed command, and nothing from the middleware:

API log (the errors)

fail: Microsoft.EntityFrameworkCore.Database.Command[20102]

An exception that doesn't come from a database command would leave no error in the log at all.

What I Checked

The handler's result. TryHandleAsync had returned true, because the response was the handler's 503.

The docs. It's a documented breaking change: "Starting in .NET 10, if IExceptionHandler.TryHandleAsync returns true, then exception diagnostics are no longer recorded by default." So it only affects the exceptions a handler has answered.

Why It Happens

Microsoft's reasoning, on the same page: "ASP.NET Core now follows the behavior expected by users by suppressing diagnostics when IExceptionHandler handles the exception." For an expected error that you've fully dealt with, that's reasonable. For an outage answered with a 503, I want it in the log.

The Fix

Set SuppressDiagnosticsCallback, as the page shows. It's asked for each handled exception, and returning false keeps the diagnostics:

src/Dewiride.Inventory.Api/Program.cs (excerpt)

app.UseExceptionHandler(new ExceptionHandlerOptions
{
    // Keep logging the exceptions an IExceptionHandler answers, so an outage shows up in the logs.
    SuppressDiagnosticsCallback = _ => false,
});

After the Fix

The same 503, and now the log has the middleware's line:

API log (the errors)

fail: Microsoft.EntityFrameworkCore.Database.Command[20102]
fail: Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[1]
An unhandled exception has occurred while executing the request.
MySql.Data.MySqlClient.MySqlException (0x80004005): Lock wait timeout exceeded; try restarting transaction

Common Mistakes

Mistake	What happens	Do this instead
Logging by hand in every handler	It works, and you lose the middleware's request details	Use the callback
Returning false from TryHandleAsync to get the log	The client gets a 500	Return true, and use the callback
Assuming no errors in the log means no errors	Handled ones are hidden	Check after upgrading to .NET 10

Troubleshooting

Symptom	Likely cause	Fix
SuppressDiagnosticsCallback doesn't exist	The app isn't on .NET 10	Earlier versions log by default
Some handled exceptions should stay quiet	The callback returns false for all	Decide from its context argument
The handler never runs	No UseExceptionHandler, or it's registered too late	How to Write an IExceptionHandler in ASP.NET Core

Conclusion

Since .NET 10, an exception handled by an IExceptionHandler leaves no diagnostics from the middleware. If you want them, set SuppressDiagnosticsCallback = _ => false.

Additional Resources

• Exception diagnostics are suppressed when IExceptionHandler.TryHandleAsync returns true — the breaking change.
• Handle errors in ASP.NET Core APIs — exception handlers.

Related

• How to Write an IExceptionHandler in ASP.NET Core
• How to Return 503 with a Retry-After Header in ASP.NET Core
• Fixed: "MySqlException: Lock wait timeout exceeded" with EF Core and MySQL