ASP.net MVC - One ViewModel per View or per Action?

Question

Is it a better idea to have a single ViewModel per view or one per controller action?

Example:

public ProjectController : Controller
{
    public ActionResult Edit(int id)
    {
        var project = ...;

        return View(new ProjectEditViewModel(project));
    }

    [HttpPost]
    public ActionResult Edit(ProjectEditViewModel model)
    {
    }

    **OR**

    [HttpPost]
    public ActionResult Edit(Project model)
    {
    }

    [HttpPost]
    public ActionResult Edit(ProjectEditPostViewModel model)
    {
    }
}

Here are the three options, which is best?

1. Use the same ViewModel for my POST/GET actions.
2. Use a ViewModel for my GET action and my domain model for my POST action.
3. Use a different ViewModel for GET and a different ViewModel for POST.

Answer 1

Using a different view model for the GET and POST actions is the best and most flexible design. But using the same view model for the GET and POST actions also works in 90% of the cases and it is fine a good design. So if using the same view model works in your scenario don't hesitate to reuse it like this.

In the case where different view models are used for the GET and POST actions there is still some relation between those classes: inheritance or composition.