mvc3组合框默认选择的值

Question

I'm using this approach using viewmodel for manipulating values inside combo boxes. Now, I'm struggle to select as default value actual selected value used in create action, not the first one from combo. I know this is fourth parameter in SelectList but I do not know how to use actual UserGroupId cause it's give me an error when using like these

var model = new UserViewModel 
    { 
        UserGroups = new SelectList(GetAllUsers(), "UserId", "Name", UserGroupId) 
    } 
    return View(model); 

---

 public class UserViewModel 
    { 
        public int UserGroupId { get; set; } 
        public IEnumerable<SelectListItem> UserGroups { get; set; } 
    } 

Answer 1

Like this:

var model = new UserViewModel 
{ 
    // preselct an item in the dropdown whose value equals 5
    // This means that inside your `GetAllUsers()` collection you must
    // have an element with UserId=5 and this element will automatically be
    // preselcted. Here you could put any value of course
    UserGroupId = 5,
    UserGroups = new SelectList(GetAllUsers(), "UserId", "Name") 
};
return View(model);

and in your view:

@model UserViewModel
...
@Html.DropDownListFor(x => x.UserGroupId, Model.UserGroups)

Answer 2

As for the 4th parameter, it needs to be an actual instance that is part of the collection returned by GetAllUsers()

For example:

var users = GetAllUsers();
var defaultUser = users[0];
var model = new UserViewModel 
{ 
     UserGroups = new SelectList(users, "UserId", "Name", defaultUser) 
};

return View(model);

Answer 3

You may want to manually build your SelectList rather than using the SelectList constructor. That will give you the control you need over handling the "UserId" and the "Name". This also addresses the issue of how to add an integer record id to the select list.

Lets start with a method that builds and returns a List<SelectListItem>

public static List<SelectListItem> UserGroups(int userGroupId, int selectedUserId)
{
    var userGroupsQuery = from u in dbContext.Users
                          where u.UserGroupId == userGroupId
                          select u;

    var userSelectList= new List<SelectListItem>();

    foreach (var user in userGroupsQuery.Users)
    {
        userSelectList.Add(
            new SelectListItem
            {
                Text = userGroup.Name,
                Value = userGroup.UserId.ToString(CultureInfo.InvariantCulture),
                Selected = (user.UserId == selectedUserId)
            };
    }

    return userSelectList;
}

Now we have a select list that we can use so lets use it in our action method.

int selectedUserId = 1000;
int userGroupId = 5;

var userViewModel = new UserViewModel
{
    UserGroups = ListLookup.UserGroups(userGroupId, selectedUserId)
}

return View(userViewModel);

And then in the view.

@Html.DropDownList("UserId", Model.Users)

The key here is that the Value property in the select list must be a string, not an integer. Also note that you cannot call to string inside a lambda expression by default (hence the use of a foreach loop). Most SelectList examples do not show how to use the Selected property to select a particular value in the SelectList item.

I hope this helps :)