Why Does My Entity Framework Core Application Throw a 'Second Operation Started' Error, and How Can I Fix It?

Entity Framework Core: Resolving "Second Operation Started" Error during Execution

In a recently encountered issue while developing with ASP.Net Core 2.0 and Entity Framework Core, an error occurred: "A second operation started on this context before a previous operation completed." This exception, thrown by InvalidOperationException, indicates that multiple operations were initiated on the same context before the previous operation had finished.

Examining the codebase, a method responsible for populating a list of entities with various properties and related data was identified as the culprit. The problematic method included queries to incorporate related entities using Include statements and mapping these entities to view models.

The underlying issue was rooted in the manner the DbContext was resolved and utilized throughout the application. When employing IoC (Inversion of Control) for dependency injection, the DbContext should be registered as Transient. This entails using the following code snippet:

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

or

services.AddTransient<MyContext>();

instead of:

services.AddDbContext<MyContext>();

The default behavior is to add the DbContext as scoped, which can lead to conflicts when multiple threads access the context concurrently.

Furthermore, asynchronous operations and async lambda expressions can exacerbate this issue. Using Transient for the DbContext grants each class its instance, precluding any changes to an entity across multiple classes.

For further insights into this exception, refer to the following resource: [https://docs.microsoft.com/en-us/ef/core/miscellaneous/warnings/second-operation-started](https://docs.microsoft.com/en-us/ef/core/miscellaneous/warnings/second-operation-started)

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