ASP.NET MVC: When to use custom HTML helper methods vs Html.RenderAction?

Question

It's a little unclear for me on when to use a custom helper method and when to use RenderAction and also when to simply use ViewData instead. Some of their functions overlap slightly.

For example, if I were to create a Category navigation bar, would I create a new helper method and place that in some partial view? I had initially though of doing this, but I read on some blog to use RenderAction instead. I've just been thinking back and forth and could use some help not just with this example, but in general.

Assume the list of categories is coming from some data source.

Answer 1

The general guidelines that I follow are:

HtmlHelper methods:

1. Used to standardize markup. I use helpers to make sure my form fields, input buttons and image tags use consistent markup.
2. Used when the resulting markup is minimal. Small bits of text, form field markup, etc. I don't use helpers to render full domain objects.
3. Operate on a small number of discrete arguments. If I need to iterate over a collection and display something, that's a partial. If I need to take a large amount of input, that's a partial too.
4. Do not contain any business logic, just presentation logic. The arguments are usually objects in the solution domain, not the business/problem domain.
5. Are usually very general in scope and apply to large portions of the application.

Render partial:

1. Used when I want to decompose a large view into smaller pieces. The model should be a subset of the model of the "main" view.
2. Partial views are often used only by certain controllers or areas.

Render action:

1. Used when I want to create small chunks of functionality that can be composed in various ways.
2. Most often used to generate content that applies to many controllers or areas, such as navigation controls.

ViewData:

I'll use ViewData to track global data that applies to all views, such as the current user. If I need a consistent way of displaying this data I usually create a partial for it and then do RenderPartial() in a master page.

Answer 2

Firstly, this is probably clear, but let's say it: The category business logic (eg fetching data from a data source) should not be in Html-helper or in a user control: it should be done in a controller.

The difference between 1) RenderPartial / HtmlHelper vs. 2) RenderAction is in which controller this business logic is:

1. in the one controller action that does the whole page or
2. in a separate controller action specific to the partial view.

If you use your category data in pretty much every page I do not see it wrong to fetch it for each page on page-controller action level and pass it in the view data. Of course you would use some mechanism (custom model base class, extend controller, ...) so that you don't have the same category-fetching function call in each action (assuming you have lots).

If some page views choose to show the categories, some not and some perhaps have another category control with different business logic, then RenderAction is definitely better. Even in the above case, RenderAction is good: it separates the category fetching from other data in your controller actions.

Then whether to use RenderPartial or HtmlHelper... To me HtmlHelpers should be more generic and not specific to particular view or model, but this, I suppose, is more matter of taste than clear rule from MVC perspective: both should be just View-logic.

Answer 3

I would choose html helper methods when the scenario meets these criteria:

1. The arguments are not considered model data
2. It doesn't have to generate an excessive amount of markup
3. The html can be generated with just the arguments given to it

If you have an html helper method using model data or it has a lot of dependencies, its probably better as a RenderPartial or RenderAction.

Answer 4

I'm new also to using RenderAction

but when I need to load data for a specific piece of display I now go with RenderAction

Ideal for loading Tag cloud, which are displayed on every page but the data is not specific to a page.