在asp.net中返回DropdownList部分视图的通用方法

Question

I am working on a controller's action which should be able to return a dropdown list. Arguments for that are parent Entity type, Parent entity Id and Child Entity type.

 public ActionResult Dropdown<TParent,TChild>(int id) where TParent : class where TChild:class
 { 
     var dropdownList = new DropdownList<TParent,TChild>(id);
     return PartialView();
 }

DropdownList class looks like this.

public class DropdownList<TParentEntity, TChildEntity> where TParentEntity : EntityObject where TChildEntity : EntityObject
{
    public List<SelectListItem> ListItems;

    public DropdownList(int parentId)
    {
        DatabaseEntities db = new DatabaseEntities();
        TParentEntity parentEntity = db.Set<TParentEntity>().Find(parentId);
        DbEntityEntry parentEntityEntry = db.Entry(parentEntity);

        ListItems.Add(new SelectListItem { Value = "-1", Selected = false, Text = "Select " + typeof(TChildEntity).Name });
        //foreach (dynamic entity in childrenEntities)
        //{
        //    ListItems.Add(new SelectListItem
        //    {
        //        Value = Convert.ToString(entity.Id),
        //        Text = entity.Name,
        //        Selected = false
        //    });
        //}

    }
}

I have no idea how to proceed and I am stuck. I cannot find any solution on google as well. Basically I want to create a class where I will pass the Id of an Parent Entity, the type of parent Entity and type of Child Entity. I want it to return the dropdown list of child entities.

Answer 1

You can introduce interface

public interface IDropdownOption
{
      public int Value {get; set;}
      public int Text {get; set;}
}

Implement this interface on all the entity for which you need to show the drop down list.

you can then change the signature of your function like below

/* Change Constraints on Type Parameters to allow only IDropdownOption */
public class DropdownList<TParentEntity, TChildEntity> where TParentEntity : EntityObject where TChildEntity : EntityObject, IDropdownOption
{

public List<SelectListItem> ListItems;
public DropdownList(int parentId)
{
    DatabaseEntities db = new DatabaseEntities();
    TParentEntity parentEntity = db.Set<TParentEntity>().Find(parentId);
    DbEntityEntry parentEntityEntry = db.Entry(parentEntity);

    ListItems.Add(new SelectListItem { Value = "-1", Selected = false, Text = "Select " + typeof(TChildEntity).Name });
    /* assuming you can load child entities as list then you just convert it like below */
ListItems.AddRange(childrenEntities.OfType<IDropdownOption>().Select(s => new SelectItem
{
Text = s.Text, Value = s.Value
}));

}
}

Answer 2

I was able to get it by using NavigationProperty name. I had to pass parent and child types, parent Id and Navigation Property name. So the calling method will be like

var dropdownList = new DropdownList<TParent,TChild>(id, navigationProperty);

Then the DrodownList class is like

 public class DropdownList<TParent,TChild> where TParent : class
{
    public List<SelectListItem> ListItems;
    public DropdownList(int id, string navigationProperty)
    {
        ListItems = new List<SelectListItem>();
        DatabaseEntities db = new DatabaseEntities();
        TParent parentEntity = db.Set<TParent>().Find(id);
        DbEntityEntry<TParent> parentEntityEntry = db.Entry(parentEntity);
        DbCollectionEntry<TParent,TChild> navigationPropertyCollection = parentEntityEntry.Collection(navigationProperty).Cast<TParent, TChild>();
        ListItems.Add(new SelectListItem { Value = "-1", Selected = false, Text = "Select " + navigationProperty });
        foreach (dynamic entity in navigationPropertyCollection.CurrentValue)
        {
            ListItems.Add(new SelectListItem
            {
                Value = Convert.ToString(entity.Id),
                Text = entity.Name,
                Selected = false
            });
        }
    }
}

This solution works fine but I am still looking for a better approach where you don't have to pass navigation property name.

parentEntityEntry.Collection(navigationProperty)

Above method accepts both string as a navigation property name and an Expression which can derive navigation property name from the child type.

Any idea for expression that need be passed ?