Resolving ArgumentNullException in ASP.NET Core 5 MVC

Learn how to fix the common `ArgumentNullException: Value cannot be null. (Parameter 'items')` error in ASP.NET Core 5 MVC when creating a dropdown list in your forms.
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Resolving ArgumentNullException in ASP.NET Core 5 MVC: A Step-by-Step Guide

When developing applications with ASP.NET Core 5 MVC, you might encounter various exceptions during runtime. A common issue that developers face is the ArgumentNullException: Value cannot be null. (Parameter 'items') error. In this guide, we'll unpack the issue behind this error and explore how you can efficiently resolve it, enhancing your application development experience.

Understanding the Problem

The Scenario

Retrieve a List: You are trying to retrieve a list of user names to populate a dropdown on your create page.

Insert "Select" Option: You then insert an option for users to select.

Passing to View: You’re passing this list into ViewBag to be used in your view.

Submitting the Form: Upon form submission, you encounter the ArgumentNullException referencing the items parameter.

Diagnosing the Root Cause

In your case, you observed that the Prepared_By value was correctly displayed when you printed it in the POST method, but it caused an issue when attempting to create the dropdown:

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

Why Does This Happen?

This could occur because:

The list was only created in the GET method and not recreated or re-initialized in the POST method.

If the model state is invalid, and you return the view without reassigning the dropdown list, it remains null.

Implementing the Solution

Step 1: Create a Method for Dropdown Lists

To solve this problem, you can abstract the dropdown list logic into a separate method. This ensures that both the GET and POST methods have access to the same list.

Here’s how you can implement it:

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

Step 2: Update the GET Method

In your GET method, simply call the DropDowns() method to populate the lists:

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

Step 3: Update the POST Method

Do the same in your POST method:

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

Key Takeaways

By centralizing the logic for populating dropdowns into a DropDowns() method, you can easily ensure that your dropdown lists are always ready for both GET and POST requests.

Avoiding the issue of uninitialized ViewBag properties prevents runtime exceptions, simplifying debugging and enhancing user experience.

Conclusion

Handling the ArgumentNullException in ASP.NET Core 5 MVC is straightforward once you understand the flow of data between your controller and view. By effectively managing how and when your dropdown lists are created and populated, you'll streamline your form handling process and eliminate frustrating runtime errors. Happy coding!