CODEWITHSUNDEEP
asp.net-mvc
user1469655
user1469655

Reputation: 1091

Why do my viewmodel properties end up null or zero?

I'm working on my first MVC application, still figuring it all out.

I have a viewmodel which, at this point, is identical to my domain object. My controller builds the viewmodel and passes it to a view. The view only displays a some of the properties because I don't want primary and/or foreign keys displayed for the user yet, in the case of a primary key, I need to have the data in order to update / delete the database.

It appears that unless I use a viewmodel property in the view, it is set to default values (0 for numeric value types and null for reference types) when I pass the viewmodel back. Is this correct behavior?

I confirmed the viewmodel passed to the Edit view does contain all the properties (as I would expect).

The question - Once a view is rendered, what happens to the viewmodel? If my viewmodel contains properties that are not used in the view, do their values just disappear? For example, when I click the Edit actionlink to fire the Edit action on the controller, the viewmodel that gets passed to the action does not contain any properties unless they are visible on the screen. Why?

BTW, this is ASP.NET MVC 4 RC.

Upvotes: 0

Views: 2559

Answers (2)

Shyju
Shyju

Reputation: 218732

If your properties are in any helper methods which generates the input HTML element inside a form, It will be available in your HTTPost action method when you submit the form. If you are simply displaying it in a div/span , it is not going to get you the property values. That is how MVC model binding works.

Expect the values in your HttpPOST action if you use these HTML helpers

@Html.TextBoxFor

@Html.DropDownFor

@Html.EditorFor

@Html.HiddenFor

Dont expect the values if you use these

@Html.DisplayFor

Upvotes: 0

Darin Dimitrov
Darin Dimitrov

Reputation: 1038800

It appears that unless I use a viewmodel property in the view, it is set to default values (0 for numeric value types and null for reference types) when I pass the viewmodel back. Is this correct behavior?

Yes, when you invoke a controller action you need to pass all the properties that you want to be bound in the request. So for example if you are using a html <form> to call the action you need to use input fields. You could use hidden fields but they must be present, otherwise nothing is sent to the controller action

The question - Once a view is rendered, what happens to the viewmodel?

It falls out of scope and is eligible for garbage collected.

If my viewmodel contains properties that are not used in the view, do their values just disappear?

Absolutely. But even if you use those properties inside the view they disappear. For example if you only display the values inside the view but do not use input fields to send them back to the server when the form is submitted they will be gone as well.

For example, when I click the Edit actionlink to fire the Edit action on the controller, the viewmodel that gets passed to the action does not contain any properties unless they are visible on the screen. Why?

Because the view model no longer exists. It's gone and garbage collected. That's how the HTTP protocol works. It's stateless. Nothing is persisted between the requests. You will have to include whatever properties you want to be populated in the request either as POST form values or as query string parameters if you are using action links or whatever.

If the user is not supposed to modify those values inside the view you could then simply pass an id to the controller action which will allow for this controller action to retrieve the model from wherever it is stored (a database or something) using this id.

Upvotes: 6

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