Handling DbContext Threading Issues in .NET Core with EF Core

net | 2021-08-20

🚩 The Problem

While working on a .NET Core application using Entity Framework Core (EF Core) as ORM,

we faced some threading issues due to multiple DbContext usage, JWT authentication, and API concurrency.

📈 Request & Response Flow

Request → 
JWT Middleware to grab user object → 
Authorization Filter to stop multiple logins → 
Touch Controller → 
Repository → 
Execute Query → 
Response

🛑 Issue and Solution

Common Exceptions

1. Connection Closed Exception

An exception occurred while **iterating over** the results of a query for context type. 
The connection is closed

Cause: A queryable object was still in memory while a second query was attempted.
Solution: Always ensure you materialize (e.g., .ToList(), .FirstOrDefault()) any queryable object before running another query.

2. Parallel Operation Exception

One or more errors occurred. (A second operation started on this context before a previous operation completed. This is usually caused by different threads using the same instance of DbContext.)

Cause: EF Core does not allow multiple parallel operations on the same DbContext.
Solution:
- Always await asynchronous operations before making another database call.
- Do not share the same DbContext across threads.

🛠️ Troubleshooting Tips

✅ Reduce API calls: Consolidate similar frontend requests to minimize concurrent database usage.
✅ Await all async calls: Ensure each query is awaited before proceeding.
✅ Register DbContext as Transient:

  services.AddDbContext<AppDbContext>(options => options.UseSqlServer(connectionString), ServiceLifetime.Transient);