Why is My MVC Model Binder Failing to Populate a List of Objects from a Form?

Troubleshooting MVC Model Binding Issues with Lists of Objects

This article addresses a common problem in MVC applications: failure of the model binder to correctly populate a list of objects from form data. The symptom is a null list in the controller action, despite seemingly correct form submission.

The Root Cause: Missing Indices

The core issue lies in the lack of indexing in form elements. Without indices, the model binder cannot differentiate individual elements within the list. For example, consider this scenario: the form initially contains elements without unique identifiers.

<code><!-- Incorrect: Missing indices --></code>

The Solution: Leveraging EditorTemplates

The solution involves using EditorTemplates to automatically generate correctly indexed form elements. This eliminates manual indexing and ensures proper model binding.

Implementation Steps:

1. Create an EditorTemplates Folder: Create a new folder named "EditorTemplates" within your Views folder. This folder should reside within the same directory as your other view files.

2. Create a Strongly-Typed View: Inside the EditorTemplates folder, create a strongly-typed view file whose name matches your model class (e.g., PlanCompareViewModel.cshtml).

3. Migrate Partial View Content: Move the HTML content from your original partial view into this newly created EditorTemplate.

4. Utilize EditorForModel(): Modify your parent view to use the EditorForModel() helper method. This helper will automatically render the form elements, correctly indexed, based on your model.

@model IEnumerable<plancompareviewmodel>
@using (Html.BeginForm("ComparePlans", "Plans", FormMethod.Post, new { id = "compareForm" }))
{
    <div>
        @Html.EditorForModel()
    </div>
}

By employing this approach, the model binder will correctly interpret and populate the list of objects in your controller action. The EditorForModel() helper dynamically generates the necessary indices for each element, resolving the binding issue.

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