How to Cast List To List

Question

I am using generic method to fill my dropdown for all types

below is my code.

the entity type are as follow

public class Role
{
    public string Id { get; set; }
    public string Name { get; set; }
}

public class DropDown
{
    public string Id { get; set; }
    public string Name { get; set; }
}

i am able to fetch data successfully at
var data = DataFetcher.FetchData<T>();

private static void Main( string[] args )
{
    List<DropDown> cities = BLL.GetDataList<City>();
    List<DropDown> states = BLL.GetDataList<State>();
    List<DropDown> roles = BLL.GetDataList<Role>();
}

public static class BLL
{
    public static List<DropDown> GetDataList<T>() where T : class ,new()
    {
        var data = DataFetcher.FetchData<T>();
        return data as List<DropDown>;
    }
}

I knew this cast data as List<DropDown> will fail,thats why its returning null back to calling method,

How can i cast Generic list to List of Known Type?

You have to ask yourself: how do I want to convert `T` to `DropDown`? — CodeCaster, Sep 28 '15 at 14:26
possible duplicate of [Convert a Generic List to specific type](http://stackoverflow.com/questions/12426426/convert-a-generic-list-to-specific-type) — Sinatr, Sep 28 '15 at 14:26
What does `FetchData` actually return? Is it convertible to `DropDown`? Perhaps you want to populate the `DropDown` with the data, instead of trying to convert it. — Yuval Itzchakov, Sep 28 '15 at 14:28
And why do you have your own `DropDown` type, aren't you using an UI framework? — CodeCaster, Sep 28 '15 at 14:32
Why are you using two different objects with the exact same properties? Is there any good reason for that? — Yuval Itzchakov, Sep 28 '15 at 14:33
@YuvalItzchakov Actually its an replica of my needin console application... Role is something that is only available till DataAcesslayer, but the dropdown will persist to web ... — Shekhar Pankaj, Sep 28 '15 at 14:35
I am now wondering whether all SO user abide the guidelines for downvoting !! — Shekhar Pankaj, Sep 29 '15 at 10:07

CodeCaster · Answer 1 · 2015-09-28T14:49:47.577

You have to ask yourself: how do I want to convert T to DropDown? If you can't answer this, the answer is: you can't.

I guess your DropDown class has an object Value property, that holds the dropdown value, and you wish to assign the data entity to that property.

Then you can project the list of data entities to DropDowns as such:

var data = DataFetcher.FetchData<T>();
return data.Select(d => new DropDown { Value = d }).ToList();

As for your edit: so you have at least one type, the displayed Role, that has an Id and Name property. But type T doesn't guarantee this, so you'd need to introduce an interface:

public interface INamedIdentifyableEntity
{
    string Id { get; set; }
    string Name { get; set; }
}

And apply this to your entities. Then introduce it as a generic constraint and do the mapping:

return data.Select(d => new DropDown
{ 
    Id = d.Id,
    Name = d.Name,
}).ToList();

But you don't want this, as here you are tying these two properties to dropdowns. Tomorrow you'll want an entity with Code instead of Id and Text instead of Name, so you'll have to add more interfaces, more overloads, and so on.

Instead you might want to use reflection, where you can specify the member names in the call:

List<DropDown> cities = BLL.GetDataList<City>(valueMember: c => c.CityCode, displayMember: c => c.FullCityname);

And use these member expressions to look up data's values and fill those into the DropDown.

However, you're then reinventing the wheel. Leave out your DropDown class entirely, and leave the dropdown generation to the front end, in this case MVC:

var cities = DataFetcher.FetchData<City>();

var selectList = new SelectList(cities.Select(c => new SelectListItem
{
    Selected = (c.Id == selectedCityId),
    Text = c.FullCityName,
    Value = c.CityCode,
});

Or:

var selectList = new SelectList(cities, "CityCode" , "FullCityName", selectedCityId);

Thanks ,your answer not exactly solved my problem but indeed it helped me to get insight of my own problem, Nice answer :) — Shekhar Pankaj, Sep 29 '15 at 10:05

score 1 · Answer 2 · answered Sep 28 '15 at 15:02

1

One solution is to use AutoMapper.

First create a map between your models like this:

AutoMapper.Mapper.CreateMap<Role, DropDown>();

Do the same thing for City and State classes if you need to.

Then you can use AutpMapper to convert your objects to DropDown like this:

public static List<DropDown> GetDataList<T>() where T : class ,new()
{
    var data = DataFetcher.FetchData<T>();
    return data.Select(x => AutoMapper.Mapper.Map<DropDown>(x)).ToList();
}

answered Sep 28 '15 at 15:02

Yacoub Massad

27,509
2
36
62

AFAIK, Automapper can map collections directly; something like `Mapper.Map>(DataFetcher.FetchData())` should work as well. – Dennis Sep 29 '15 at 06:20

score 0 · Answer 3 · answered Sep 28 '15 at 14:29

0

If I understood the question correctly, you could use Linq as follows.

return data.Cast<DropDown>().ToList();

answered Sep 28 '15 at 14:29

Codor

17,447
9
29
56

3

That only works if a cast from `T` to `DropDown` exists. – CodeCaster Sep 28 '15 at 14:30

How to Cast List To List

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