Data Passing Techniques in ASP.NET Core MVC

1. Description

In ASP.NET Core MVC, data passing refers to the techniques used to send data from a controller action method to its corresponding view. There are several ways to do this, each with its own use case.

The primary methods are:

• Strongly-Typed Models: Passing a specific model object to the view. This is the most common and recommended approach.
• ViewData: A dictionary-like object that lets you pass data using string keys.
• ViewBag: A dynamic object that is a wrapper around ViewData. It allows you to create dynamic properties.
• TempData: A dictionary-like object similar to ViewData, but it persists data for one additional request. It's mainly used for passing data between redirects.

2. Why It Is Important

Choosing the right data-passing technique is important for writing clean, maintainable, and type-safe code.

• Strongly-typed models are the best practice because they provide compile-time type checking and IntelliSense support in the view, which helps prevent runtime errors.
• ViewData and ViewBag are useful for passing small, secondary pieces of data that don't belong in the main model.
• TempData is essential for the common "Post-Redirect-Get" pattern, where you need to display a status message (e.g., "Your form was submitted successfully!") after a redirect.

3. Real-World Examples

• Strongly-Typed Model: A controller action for a product details page would pass a Product object to the view.
• ViewData/ViewBag: Passing a page title or a small piece of UI-specific data, like ViewBag.PageTitle = "Product List";.
• TempData: After a user successfully creates a new product and is redirected to the product list page, TempData can be used to show a one-time success message: TempData["SuccessMessage"] = "Product created successfully!";.

4. Syntax & Explanation

Controller Action (ProductsController.cs)

using Microsoft.AspNetCore.Mvc;
using YourApp.Models; // Assuming you have a Product model
using System.Collections.Generic;

public class ProductsController : Controller
{
    public IActionResult Index()
    {
        // 1. Strongly-Typed Model
        var products = new List<Product>
        {
            new Product { Id = 1, Name = "Laptop", Price = 1200.00m },
            new Product { Id = 2, Name = "Mouse", Price = 25.00m }
        };

        // 2. ViewData
        ViewData["PageTitle"] = "Our Products";

        // 3. ViewBag
        ViewBag.ItemsInStock = 150;

        // Passing the model to the view
        return View(products);
    }

    [HttpPost]
    public IActionResult Create(Product product)
    {
        // ... logic to save the product ...

        // 4. TempData - for use after a redirect
        TempData["SuccessMessage"] = $"Product '{product.Name}' was successfully created.";
        
        return RedirectToAction("Index");
    }
}

View (Index.cshtml)

@* 1. Declare the strongly-typed model *@
@model List<YourApp.Models.Product>

@{
    // 2. Accessing ViewData
    ViewData["Title"] = ViewData["PageTitle"];
    
    // 3. Accessing ViewBag (no casting needed)
    var stockCount = ViewBag.ItemsInStock;
}

<h1>@ViewData["Title"]</h1>
<p>Total items in stock: @stockCount</p>

@* 4. Accessing TempData (check for null) *@
@if (TempData["SuccessMessage"] != null)
{
    <div class="alert alert-success">
        @TempData["SuccessMessage"]
    </div>
}

<h2>Product List</h2>
<table>
    <thead>
        <tr>
            <th>Name</th>
            <th>Price</th>
        </tr>
    </thead>
    <tbody>
        @* Accessing the strongly-typed Model *@
        @foreach (var product in Model)
        {
            <tr>
                <td>@product.Name</td>
                <td>@product.Price.ToString("C")</td>
            </tr>
        }
    </tbody>
</table>

Summary of Techniques

Technique	Type	Lifetime	Casting in View?	Use Case
Strongly-Typed Model	Model property (typed)	Current request	No	Primary data for the view (e.g., a list of products).
ViewData	ViewDataDictionary	Current request	Yes (e.g., (string)ViewData["Title"])	Small amounts of secondary data.
ViewBag	dynamic	Current request	No (but no IntelliSense)	Small amounts of secondary data, more concise syntax.
TempData	ITempDataDictionary	Current and next request	Yes (like ViewData)	Passing data between redirects (e.g., status messages).

5. Mini Practice Task

1. In a controller action, create a List of strings (e.g., a list of your favorite hobbies).
2. Pass this list to a view as a strongly-typed model.
3. In the same action, use ViewBag to pass your name to the view.
4. Use ViewData to pass the current date to the view.
5. In the view, display your name and the date from ViewBag and ViewData.
6. Use a foreach loop to render the list of hobbies from the Model in an unordered list (<ul>).