Resolving the Issue of ViewModel Not Returning Data in ASP.Net Core's Post Method

Discover the solution to the `ViewModel` not returning data during a POST request in ASP.Net Core. Learn how to correctly bind collections using Razor Pages.
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Resolving the Issue of ViewModel Not Returning Data in ASP.Net Core's Post Method

In the world of ASP.Net Core, working with ViewModels can sometimes lead to frustrating issues, especially when it comes to data binding during POST requests. A common problem developers face is when a ViewModel does not return the expected data to the POST method in the controller. In this guide, we’ll address why this happens and how to resolve it effectively.

Understanding the Problem

Imagine you have a registration page in your ASP.Net Core application. You've created a ViewModel that contains a list called allRegistrations which you expect to populate with data from previous registrations. However, when you submit the form, the number of registrations comes back as zero. This can be puzzling, especially if you've checked that the form should contain the necessary data.

The Cause of the Issue

The core of the issue lies in how ASP.Net Core model binding works. When you create a form with collection types, the names of the input elements need to match a specific structure for the model binder to correctly understand and map the data. If the naming convention does not match, the data will be lost, and you’ll see an empty collection upon submission.

Solution: Properly Naming the Form Inputs

Step-by-Step Approach

To fix this data binding issue, follow these guidelines:

Use Indexed Names: Each item in your collection should have an indexed name format. Specifically, the input names should look like this: allRegistrations[0].PropertyName. This tells ASP.Net Core how to group and bind the data back to your ViewModel.

Add Hidden Inputs: For properties of each registration in the allRegistrations collection, add hidden inputs to the form as follows:

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

Example Output HTML

When the above inputs are rendered, your HTML will look similar to this:

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

Conclusion

By carefully following these steps and ensuring your form elements are correctly named, you will successfully bind the allRegistrations collection to your ViewModel during the POST request. This change not only resolves the issue but also enhances the robustness of your ASP.Net Core application.

If you encounter similar binding problems in the future, remember to always check the naming conventions of your input fields. Happy coding!