new() keyword at the end of method declaration

Question

A workmate just gave me some C# classes that I must use in a .NET application.
There's a typo that I have never seen, and I can't found any explication on the internet...

Here's the code :

public void GoTo<TView>() where TView : Form, new()
{
    var view = Activator.CreateInstance<TView>();

    //si on vient de creer une startup view alors on affiche l'ancienne
    //la reference a la nouvelle sera detruite en sortant de la fonction GoTo
    if (view is StartupForm)
    {
        ShowView(_StartupForm);
    }
    else ShowView(view);

}

What is the new() keyword for, right at the end of the method declaration ?

https://msdn.microsoft.com/en-us/library/sd2w2ew5.aspx – Dennis_E Jan 15 '16 at 14:01 — Dennis_E, Jan 15 '16 at 14:01

score 11 · Answer 1 · answered Jan 15 '16 at 14:01

11

It is type parameter constraint. Literally it means TView must have a public parameterless constructor.

answered Jan 15 '16 at 14:01

Andrey Korneyev

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As an addition to this answer you can use following in your code: var variable = new TView(); while without new() constraint its not allowed by C# compiler. – serhiyb Jan 15 '16 at 14:05

Rahul Tripathi · Accepted Answer · 2016-01-15T14:11:15.643

See the MSDN:

The new constraint specifies that any type argument in a generic class declaration must have a public parameterless constructor. To use the new constraint, the type cannot be abstract.

So when you say:

void myMethod<T>(T item) where T : class, new();

then it means that you are putting a constraint on generic parameter T. So T should be a reference type and cannot be a value type(int, float, double etc). Also T should have a public parameter-less default constructor.

score 0 · Answer 3 · answered Jan 15 '16 at 14:04

This is a type parameter constraint, specifically a constuctor-constraint, detailed in Section 10.1.5 of the C# Language Specification.

If the where clause for a type parameter includes a constructor constraint (which has the form new() ), it is possible to use the new operator to create instances of the type (§7.6.10.1). Any type argument used for a type parameter with a constructor constraint must have a public parameterless constructor (this constructor implicitly exists for any value type) or be a type parameter having the value type constraint or constructor constraint (see §10.1.5 for details).

This is just a way of guaranteeing that the type passed in can be constructed with a parameterless constructor.

new() keyword at the end of method declaration

3 Answers3