Resolving NullReferenceException in ASP.NET Core When Calling Controllers from Another Controller

Learn how to fix the `NullReferenceException` issue when using custom validation in ASP.NET Core MVC, specifically when controllers interact with each other.
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Resolving NullReferenceException in ASP.NET Core When Calling Controllers from Another Controller

When working with ASP.NET Core MVC applications, developers often encounter various challenges, particularly during model validation. One common issue arises when a controller attempts to call another controller, resulting in a frustrating NullReferenceException. This scenario particularly affects those utilizing custom validation attributes that interact with a database context. In this guide, we will explore the problem in detail and break down a practical solution that works seamlessly in your ASP.NET Core application.

Understanding the Problem

Imagine you have a main controller, MainController, that is responsible for handling different types of entities in an MVC view. Each entity is managed by its own controller. The main challenge occurs when you introduce custom validation attributes that rely on the database context (dbContext) to validate the data against the database. Here's the scenario:

The MainController calls the EntityController based on a discriminator value.

When you attempt to validate a model using the TryValidateModel method within the entity controller, a NullReferenceException is thrown.

What Causes the Exception?

This exception can typically occur due to one of the following reasons:

The controller instance may not be properly initialized when called from another controller.

The required services, like the dbContext, may not be available throughout the validation process.

This can lead to invalid assumptions in your code and a lot of head-scratching, especially when you find that invoking the individual controller directly works without issues.

A Practical Solution

After exploring different approaches, a tested solution to resolve this issue is to redirect the request from the MainController to the corresponding EntityController. Below, we detail the necessary steps to implement this solution effectively.

Step-by-Step Implementation

Identify the Logic in the Main Controller

In your MainController, instead of calling the EntityController directly, use the RedirectToAction method, which will ensure that the new request will be treated appropriately, keeping the necessary services (including your dbContext) available for validation.

Here’s an example of how to implement this approach:

[[See Video to Reveal this Text or Code Snippet]]

Update the Entity Controller

Your EntityController (in this case, ValveControlsController) should have a corresponding action method Put that receives the parameters passed from the MainController. This method will then perform any required validation.

[[See Video to Reveal this Text or Code Snippet]]

Why This Works

Fresh Request Context: By redirecting the request, you create a new context for the EntityController, which includes access to the services it needs, such as the database context.

Easier Management: This also simplifies the management of service dependencies within your application, ensuring that each controller operates in its intended context and scope.

Conclusion

Handling different types of entities through a main controller calling individual entity controllers can lead to complex scenarios, especially when custom validations are involved. However, by adopting a straightforward solution—redirecting requests to the appropriate action in the target controller—you can effectively avoid NullReferenceExceptions and streamline your application’s flow. This approach not only resolves immediate issues but also improves overall maintainability of your codebase.

If you encounter similar challenges in your ASP.NET Core projects, consider this