ASP MVC4 - Pass List to view via view model

问题:

I have a model Person (with among other fields the day of Birth) and I want to pass a list of all persons, together with the calculated age of each person, to the view

Therefor:

1. The view model

   public class vm_PersonList
   {
       public Person Person { get; set; } 
       public int age { get; set; }
   }
   

2. The controller action:

   public ActionResult PersonList()
   {
       ViewBag.Message = "My List";
   
       var list = new List<vm_PersonList>();
       var list_p = new vm_PersonList();
   
   
       foreach (var p in db.Person)
       {
           list_p.Person = p;
           //the age will be calculated based on p.birthDay, not relevant for the    
           //current question
           list_p.age = 23;
   
           list.Add(list_p);
       }
       return View(list);
   }
   

3. The view

   @model List<programname.Viewmodels.vm_PersonList>
   
   @foreach (var p in Model)
   {
       <tr>
           <td>
               @p.Person.FullName
           </td>
           <td>
               @p.age
           </td>  
       </tr>
   }
   

The Person table contains for example 6 entries. When debugging the application I see:

At the end of the controller action "list" contains correctly the 6 different Person entries

In the view, the "Model" contains 6 entries, but 6 times the last "database entry". Does anyone have a suggestion to solve this issue?

回答1:

You are using the same list_p instance over and over again inside the loop. So you are constantly updating its Person property. And since Person is a reference type you are modifying the same reference in memory. At the last iteration of the loop you are obviously replacing this reference with the last instance of Person which explains why you are seeing the same person in the view.

Try like this, seems lot easier:

public ActionResult PersonList()
{
    ViewBag.Message = "My List";
    var model = db.Person.Select(p => new vm_PersonList
    {
        Person = p,
        age = 23
    }).ToList();
    return View(model);
}

回答2:

You are working on the same instance of vm_PersonList. Move the instantiation of vm_PersonList into the loop

foreach (var p in db.Person)
{
    var list_p = new vm_PersonList();
    list_p.Person = p;
    //the age will be calculated based on p.birthDay, not relevant for the    
    //current question
    list_p.age = 23;

    list.Add(list_p);
}

回答3:

It's an issue with the scope of your list_p instance. Try changing your controller code to:

public ActionResult PersonList()
{
    ViewBag.Message = "My List";

    var list = db.Person
        .Select(p => new vm_PersonList
                     {
                         Person = p,
                         age = 23,
                     })
        .ToList();

    return View(list);
}