How to Resolve NullReferenceException for Drop-down Values in ASP.NET Core MVC with Mailkit

Discover how to efficiently handle `NullReferenceException` in your ASP.NET Core MVC application while using Mailkit to send emails, particularly concerning drop-down values.
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How to Resolve NullReferenceException for Drop-down Values in ASP.NET Core MVC with Mailkit

Developing applications with ASP.NET Core MVC can be a thrilling experience, but it can also present challenges, especially when working with external libraries like Mailkit. One common issue developers face is encountering a NullReferenceException when trying to access related data from a drop-down selection. In this post, we’ll explore a specific problem: handling NullReferenceException for a selected drop-down value in an ASP.NET Core MVC application that uses Mailkit.

The Problem

While creating a record using an ASP.NET Core 5.0 MVC application, you might run into a situation where you attempt to access a property of the ProductType, a drop-down value, after saving a record. Here's a simplified version of the situation:

Service Layer: You send an email after the user submits data using the form.

Drop-down Selection: You are using a foreign key relationship between the Sale and ProductType entities.

Error Encountered: After submitting the form, when you try to access sale.ProductType.ProductTypeDesc, it returns a NullReferenceException, which prevents the email from being sent.

This issue occurs because the related ProductType isn’t loaded after the Sale object is created and saved. The drop-down's foreign key is set, but the actual ProductType object is not automatically populated.

The Solution

To resolve this issue, we need to ensure that when we send the email, we are accessing an instance of the ProductType that is available and not null. The proposed solution involves modifying the way the application retrieves data right after saving the Sale record.

Step-by-Step Implementation

Save the Record:
First, ensure that you correctly add and save the Sale object as you've already done in your controller's Create method.

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

Retrieve the Included Data:
After saving, do a retrieval of the Sale object using the Include method to also fetch the ProductType.

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

Here, you're using Include to tell Entity Framework to also fetch the ProductType related to the Sale record based on its ID.

Update the Email Body:
Now, use the newSale object, which includes the fully populated ProductType, to set up your email content.

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

Note that I've corrected the property name from ProductTypeDecs to ProductTypeDesc based on the provided models (you might want to ensure there are no typos).

Conclusion

Implementing this straightforward change will help you avoid the NullReferenceException you encountered. By retrieving the full Sale entity with its associated ProductType after saving to the database, you gain access to all necessary properties for sending constructed email messages.

These small adjustments can significantly enhance the robustness of your application, ensuring a seamless user experience.

By following these steps, you can troubleshoot and resolve issues pertaining to the NullReferenceException in your ASP.NET Core MVC applications. Happy coding!