For the ViewBag, I heard it was a no-no to use. I would assume have the content from the ViewBag should be incorporated into a view model?

Question:

Is my assumption above the best practice. (Not to use a ViewBag and second to have it in the view model)
Are there situations where a ViewBag is absolutely necessary?

标签： asp.net-mvc razor viewbag

9条回答

贪生不怕死

2楼-- · 2019-01-08 12:56

ViewBag is a dynamic dictionary. So when using ViewBag to transfer data between action methods and views, your compiler won't be able to catch if you make a typo in your code when trying to access the ViewBag item in your view. Your view will crash at run time :(

Generally it is a good idea to use a view model to transfer data between your action methods and views. view model is a simple POCO class which has properties specific to the view. So if you want to pass some additional data to view, Add a new property to your view model and use that.Strongly typed Views make the code cleaner and more easy to maintain. With this approach, you don't need to do explicit casting of your viewbag dictionary item to some types back and forth which you have to do with view bag.

public class ProductsForCategoryVm
{
  public string CategoryName { set;get; }
  public List<ProductVm> Products { set;get;}    
}
public class ProductVm
{
  public int Id {set;get;} 
  public string Name { set;get;}
}

And in your action method, create an object of this view model, load the properties and send that to the view.

public ActionResult Category(int id)
{
  var vm= new ProductsForCategoryVm();
  vm.CategoryName = "Books";
  vm.Products= new List<ProductVm> {
     new ProductVm { Id=1, Name="The Pragmatic Programmer" },
     new ProductVm { Id=2, Name="Clean Code" }
  }
  return View(vm);
}

And your view, which is strongly typed to the view model,

@model ProductsForCategoryVm
<h2>@Model.CategoryName</h2>
@foreach(var item in Model.Products)
{
    <p>@item.Name</p>
}

Dropdown data ?

A lot of tutorials/books has code samples which uses ViewBag for dropdown data. I personally still feel that ViewBag's should not be used for this. It should be a property of type List<SelectListItem> in your view model to pass the dropdown data. Here is a post with example code on how to do that.

Are there situations where a ViewBag is absolutely necessary?

There are some valid use cases where you can(not necessary) use ViewBag to send data. For example, you want to display something on your Layout page, you can use ViewBag for that. Another example is ViewBag.Title (for the page title) present in the default MVC template.

public ActionResult Create()
{
   ViewBag.AnnouncementForEditors="Be careful";
   return View();
}

And in the layout, you can read the ViewBag.AnnouncementForEditors

<body>
<h1>@ViewBag.AnnouncementForEditors</h1>
<div class="container body-content">
    @RenderBody()
</div>
</body>

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孤傲高冷的网名

3楼-- · 2019-01-08 12:58

If you cannot re-design EXISTING ViewModel use ViewBag.

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Summer. ? 凉城

4楼-- · 2019-01-08 12:59

2.Are there situations where a ViewBag is absolutely necessary?

In some case you'll need to share your data from the Controller across the layouts, views and partial views. In this case, ViewBag is very helpful and I doubt there's any better way.

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来，给爷笑一个

5楼-- · 2019-01-08 13:03

1) Is my assumption above the best practice. (Not to use a ViewBag and second to have it in the view model)

You should use viewmodels instead of passing data via ViewBag as much as possible.

2) Are there situations where a ViewBag is absolutely necessary?

There is no situation where a ViewBag is absolutely necessary. However, there are some data I personally prefer using ViewBag instead of View Model. For example, when I need to populate a dropdown box for predefined values (i.e Cities), I use ViewBag for carrying SelectListItem array to view. I prefer not to pollute my ViewModels with this data.

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做自己的国王

6楼-- · 2019-01-08 13:04

If there were no use cases for it, it wouldn't be implemented in the first place. Yes you can do everything with ViewModels, but what if you don't really need one? One such scenario is editing entities. You can pass DTO directly as a model.

@model CategoryDto
<div class="md-form form-sm">
    <input asp-for="Name" class="form-control">
    <label asp-for="Name">("Category Name")</label>
</div>

But what if you want to select Category parent? Entity DTO ideally holds only it's own values, so to populate select list you use ViewBag

<select asp-for="ParentId" asp-items="ViewBag.ParentList">
    <option value="">None</option>
</select>

Why do this? Well if you have 50 types of entities each with some sort of select from different values, you just avoided creating 50 extra ViewModels.

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Viruses.

7楼-- · 2019-01-08 13:09

1) Is my assumption above the best practice. (Not to use a ViewBag and second to have it in the view model)

Yes.

2) Are there situations where a ViewBag is absolutely necessary?

No. Everything that you have stored in a ViewBag could go into the view model passed to the view.

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