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From Pluralsight/ASP.NET Core 6 Fundamentals

ViewModels

ViewModels (strongly typed)

If you specify a model type in the view, we call this a ViewModel. An instance of the ViewModel is passed to the view from the action.

ViewModels are either stored in Models/ or Viewmodels/.

Address.cs

namespace WebApplication1.ViewModels
{
    public class Address
    {
        public string Name { get; set; }
        public string Street { get; set; }
        public string City { get; set; }
        public string State { get; set; }
        public string PostalCode { get; set; }
    }
}

Define a model with @model. Use the model with @Model:

@model WebApplication1.ViewModels.Address

<h2>Contact</h2>
<address>
    @Model.Street<br>
    @Model.City, @Model.State @Model.PostalCode<br>
    <abbr title="Phone">P:</abbr> 425.555.0100
</address>

The controller passes the viewmodel instance to the view as a parameter:
.cs

public IActionResult Contact()
{
    ViewData["Message"] = "Your contact page.";

    var viewModel = new Address()
    {
        Name = "Microsoft",
        Street = "One Microsoft Way",
        City = "Redmond",
        State = "WA",
        PostalCode = "98052-6399"
    };

    return View(viewModel);
}

ViewData (weakly typed)

Since ViewData (and ViewBag) are dynamically resolved at runtime, they don’t offer compile-time type checking.

ViewData is a ViewDataDictionary object accessed through string keys. ViewData object values must be cast to specific types when extracted.

.cs

public IActionResult SomeAction()
{
    ViewData["Greeting"] = "Hello";
    ViewData["Address"]  = new Address()
    {
        Name = "Steve",
        Street = "123 Main St",
        City = "Hudson",
        State = "OH",
        PostalCode = "44236"
    };

    return View();
}

.cshtml

@{
    // Since Address isn't a string, it requires a cast.
    var address = ViewData["Address"] as Address;
}

@ViewData["Greeting"] World!

<address>
    @address.Name<br>
    @address.Street<br>
    @address.City, @address.State @address.PostalCode
</address>

[ViewData] Attribute

Properties on controllers or Razor Page models marked with [ViewData] have their values stored and loaded from the ViewDataDictionary:

.cs

public class HomeController : Controller
{
    [ViewData]
    public string Title { get; set; }

    public IActionResult About()
    {
        Title = "About Us";
        ViewData["Message"] = "Your application description page.";

        return View();
    }
}

.cshtml

<!DOCTYPE html>
<html lang="en">
<head>
    <title>@ViewData["Title"] - WebApplication</title>
    ...

ViewBag

ViewBags are a way to pass data to a view.

ViewBag is a Microsoft.AspNetCore.Mvc.ViewFeatures.Internal.DynamicViewData object:

  • It allows creation via dot notation (@ViewBag.SomeKey = value or object)
  • It does not require casting
  • It is simpler to check for nulls (@ViewBag.Person?.Name)

.cs

public IActionResult SomeAction()
{
    // since ViewBag is dynamic, you can add properties of any name to the object
    ViewBag.Greeting = "Hello"; 
    ViewBag.Address  = new Address()
    {
        Name = "Steve",
        Street = "123 Main St",
        City = "Hudson",
        State = "OH",
        PostalCode = "44236"
    };

    return View();
}

.cshtml

// the View can then access the data in its properties:
@ViewBag.Greeting World! 

<address>
    @ViewBag.Address.Name<br>
    @ViewBag.Address.Street<br>
    @ViewBag.Address.City, @ViewBag.Address.State @ViewBag.Address.PostalCode
</address>