How to Fix 'A second operation started on this context before a previous operation completed' in Entity Framework Core?

Entity Framework Core: Resolving "A second operation started on this context before a previous operation completed"

The error message "A second operation started on this context before a previous operation completed" in Entity Framework Core typically indicates that multiple threads are attempting to access the same DbContext instance concurrently. This can occur when the DbContext is registered as a scoped service, which creates a new instance for each request.

Scope of DbContext Registration

By default, Entity Framework Core registers the DbContext as a scoped service. This means that a new instance of the DbContext is created for each HTTP request or scoped service. In a multithreaded environment, this can lead to the error message in question.

Transient DbContext Registration

To resolve this issue, it is recommended to register the DbContext as a transient service. This ensures that a new instance is created for each individual request handler:

services.AddTransient<MyContext>();

Alternatively, you can use ServiceLifetime.Transient:

services.AddDbContext<MyContext>(ServiceLifetime.Transient);

Downside of Transient Registration

Registering the DbContext as transient has its drawbacks. Entities managed by the context cannot be persisted across multiple method calls or classes that use the same DbContext instance.

Other Potential Causes

In addition to transient DbContext registration, other potential causes of the error include:

• Asynchronous Lambda Expressions: Using async lambda expressions in Entity Framework Core queries can cause concurrency issues. Consider using Task.FromResult instead.
• Nested Concurrency Checks: If you have nested concurrency checks within your code, this can also lead to the error.

Additional Information

For more details on DbContext lifetime and thread safety, refer to the Entity Framework Core documentation:

• [DbContext Lifetime Management](https://docs.microsoft.com/en-us/ef/core/dbcontext-configuration/lifetime-management)
• [Thread Safety](https://docs.microsoft.com/en-us/ef/core/miscellaneous/thread-safety)

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