What does "T : new()" mean with generics?

Question

i want to understand that code. I think T must be IContinentFactory's implemented class but i don't understand to end of the new() keyword.

class AnimalWorld<T> : IAnimalWorld where T : IContinentFactory, new()
{
.....
}

[MSDN](http://msdn.microsoft.com/en-us/library/sd2w2ew5(v=vs.110).aspx) — Tim Schmelter, Feb 17 '13 at 19:39
(There is no generic interface here and the other interfaces are just noise around the real question.) — , Feb 17 '13 at 20:15
As a subtlety to consider with the answers given: `struct`s also satisfy the `new()` constraint. It is an interesting question as to whether `struct`s actually have a public parameterless constructor - and IIRC the C# and CLI specifications disagree on this question: but crucially: `struct`s always satisfy this constraint. — Marc Gravell, Feb 17 '13 at 20:32
Does this answer your question? [What does new() mean?](https://stackoverflow.com/questions/4236854/what-does-new-mean) — Heretic Monkey, Mar 30 '22 at 12:31

score 6 · Answer 1 · answered Feb 17 '13 at 19:39

6

T: new() means that type T has to have a parameter-less constructor.

By that you actually specify that you can write T param = new T(); in your implementation of AnimalWorld<T>

answered Feb 17 '13 at 19:39

eyossi

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+1 Only answer to mention that the constraint allows code which uses `T` to do something it otherwise could not. – supercat Feb 18 '13 at 17:14

score 5 · Accepted Answer · answered Feb 17 '13 at 19:39

5

new() mean that T must have default(parameterless) ctor.

Constraints on Type Parameters (C# Programming Guide)

answered Feb 17 '13 at 19:39

Hamlet Hakobyan

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Jeppe Stig Nielsen · Answer 3 · 2013-02-17T19:51:17.200

4

The constraint new() means that the type T must have a public parameterless instance constructor. This includes all value types, but not all classes. No interface or delegate type can have such a constructor. When the new() constraint is present, T can never be an abstract class.

When new() is present, the following code is allowed inside the class:

T instance = new T();

edited Feb 17 '13 at 19:51

answered Feb 17 '13 at 19:39

Jeppe Stig Nielsen

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score 3 · Answer 4 · answered Feb 17 '13 at 19:40

class AnimalWorld<T> : IAnimalWorld where T : IContinentFactory, new()

Here is what the declaration means:

AnimalWorld is a class with a generic type parameter T
The class AnimalWorld must implement IAnimalWorld
The type parameter T must implement IContinentFactory
The class for the type parameter T must have a no-argument constructor (that's what the new is for).

What does "T : new()" mean with generics?

4 Answers4