Add Item to List T Generic method

Question

i'm not sure even it's possible but here the problem i'm trying to add item to List<T> as follow

public static SelectList  ToSelectList<T>(List<T> addlist) where T : new ()
{
    addlist.Insert(0, new T { Id = -1, Name = "SELECT" });
    var list = new SelectList(addlist, "Id", "Name");
    return list;                
}

new T { Id = -1, Name = "SELECT" } throwing error is it possible to add item to List<T>?

How do you expect it to work? There are classes in this world which do not have `Id` and `Name` properties. What if `T` is `System.IO.Stream`, then what is it expected to do ? — Yeldar Kurmangaliyev, Aug 02 '17 at 05:09
Your code further constrains T in that it requires properties Id and Name. — StuartLC, Aug 02 '17 at 05:10
those instance i pass do have Id and Name i'm not sure to use for all classes, so let's say i do have base class with those two properties — Gayan, Aug 02 '17 at 05:10
@GayanRanasinghe You can create an interface like `INamedEntity` with `Id` and `Name` properties and add `where T : INamedEntity, new()` constraint. — Yeldar Kurmangaliyev, Aug 02 '17 at 05:11
you should use polymorphism, i.e use constraint to base class where all classes with Id and Name inheriting that base class. — M.kazem Akhgary, Aug 02 '17 at 05:12
you should declare Interface type with Id and name of the T. that way only you should access the properties. and assign the values — umasankar, Aug 02 '17 at 05:13
thanks umasanka, M.kazem Akhgary , Yeldar Kurmangaliyev, StuartLC and this is for those who minus voted if i do have proper explanation i never post here for the help if you do not have solution it doesn't mean question should get minus vote. thanks — Gayan, Aug 02 '17 at 05:16
I upvoted to balance out whoever downvoted your question. I honestly don't know what the downvote is for. — Devin L., Aug 02 '17 at 05:39

Andrew Shepherd · Accepted Answer · 2017-08-02T05:24:08.650

The problem is that, via the generic constraints, you have declared T as any object with a default constructor.

The compiler performs type checking at compile time, and T does not neccessarily have the properties Id or Name.

A solution is to

Create an interface which does have Id and Name,
Modify every compatible class so it implements this interface.
Add another generic constraint to your function, requiring the type parameter to implement this interface.

A compiling example:

public interface IEntity
{
    int Id {get; set; }
    string Name {get; set; }
}

class Widget : IEntity 
{
    public int Id {get; set; }
    public string Name {get; set; }    

    public string SomeOtherProperty { get; set; }
}

public static SelectList  ToSelectList<T>(List<T> addlist) where T : IEntity, new ()
{
    addlist.Insert(0, new T { Id = -1, Name = "SELECT" });
    var list = new SelectList(addlist, "Id", "Name");
    return list;

}

// In your code
List<Widget> widgetList = new List<Widget>();
ToSelectList(widgetList);

Gilad Green · Answer 2 · 2017-08-02T05:17:58.667

The problem with your code is that you do not know what T is and what properties it has. new is not enough as your generic constraint. All it specifies it:

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor

If you want to go and just instantiate an object of type T then see: Create instance of generic type?.

But what might be better is just to create an interface with those properties, specify that your function gets a list of that type and then just instantiate an object of that type:

public static SelectList ToSelectList(List<YourInterface> addlist)
{
    addlist.Insert(0, new YourDerived { Id = -1, Name = "SELECT" });
    var list = new SelectList(addlist, "Id", "Name");
    return list;    
}

Add Item to List T Generic method

2 Answers2