Unexpected NullReference in API Handler

[pack-a-punch123]

🏷️ Discussion Type

Bug

Body

While handling API requests in our C# service, an intermittent NullReferenceException occurs in ApiHandler.ProcessRequest after JSON deserialization. Adding null checks didn't stop it. It appears under load and the stack trace points to line 142. Seeking debugging steps, likely root causes, and safe mitigation patterns.

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[lgustavopuga]

Hi there! Intermittent NullReferenceException errors under load are classic signs of concurrency issues or nested nulls. Since basic null checks aren't working, here is what is likely happening and how to fix it:

1. Thread Safety / Shared State (Most Likely)

If your handler or its injected dependencies are registered as a Singleton but manage state, concurrent requests can modify or nullify a reference between your null check and line 142.

• Fix: Ensure all dependencies are stateless or scoped correctly (Transient/Scoped). Avoid mutable static fields.

2. Nested Properties

The root object might be non-null, but a nested property inside your deserialized JSON model could be null.

• Fix: Use C#’s null-conditional operator to traverse safely:
  var value = request?.Data?.Details?.Value;

3. Deferred Execution (LINQ)

If line 142 runs a LINQ query, execution is deferred. If the underlying collection changes under load before iteration, it throws an NRE.

• Fix: Force immediate evaluation using .ToList() or .ToArray().

Hope this helps you squash it!