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> ?

Answer 1

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);

Answer 2

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;    
}