Question

i never use new Constraint because the use is not clear to me. here i found one sample but i just do not understand the use. here is the code

class ItemFactory<T> where T : new()
{
    public T GetNewItem()
    {
        return new T();
    }
}

public class ItemFactory2<T> where T : IComparable, new()

{
}

so anyone please make me understand the use of new Constraint with small & easy sample for real world use. thanks

Answer 1

In addition to Darin’s answer, something like this would fail because Bar does not have a parameterless constructor

   class ItemFactory<T> where T : new()
   {
      public T GetNewItem()
      {
         return new T();
      }
   }

   class Foo : ItemFactory<Bar>
   {

   }

   class Bar
   {
      public Bar(int a)
      {

      }
   }

Actual error is: 'Bar' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'ItemFactory<T>'

The following would also fail:

class ItemFactory<T>
{
    public T GetNewItem()
    {
        return new T();
    }
}

Actual error is: Cannot create an instance of the variable type 'T' because it does not have the new() constraint