Question

The type argument must have a public parameterless constructor. When used together with other constraints, the new() constraint must be specified last.

Can you guys give me a sample scenario when this constraint is needed?

Answer 1

This is essentially what the new() constraint boils down to:

class Factory<T> where T : new()
{
    public T Create()
    {
        return new T();
        //     ^^^^^^^
        //     this requires the new() type constraint.
    }
}

---

Now, you’re not allowed to pass arguments to the constructor. If you nevertheless want to initialize the newed object, you could achieve this e.g. by introducing a further constraint:

interface ILikeBananas
{
    double GreenBananaPreferenceFactor { get; set; }
}

class Factory<T> where T : ILikeBananas, new()
{
    public T Create(double greenBananaPreferenceFactor)
    {
        ILikeBananas result = new T();
        result.GreenBananaPreferenceFactor = greenBananaPreferenceFactor;

        return (T)result;
        //     ^^^^^^^^^
        //     freely converting between ILikeBananas and T is permitted
        //     only because of the interface constraint.
    }
}

---

Note that another way of instantiating an object is via Activator.CreateInstance, which gives you some more freedom, such as passing arguments directly to a constructor.

Activator.CreateInstance does not strictly require the new() constraint; however, the type being instantiated still needs to provide a suitable constructor.