What does "where T : class, new()" mean?

Question

Can you please explain to me what where T : class, new() means in the following line of code?

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

Answer 1

That is a constraint on the generic parameter T. It must be a class (reference type) and must have a public parameter-less default constructor.

That means T can't be an int, float, double, DateTime or any other struct (value type).

It could be a string, or any other custom reference type, as long as it has a default or parameter-less constructor.

Answer 2

Those are generic type constraints. In your case there are two of them:

where T : class 

Means that the type T must be a reference type (not a value type).

where T : new() 

Means that the type T must have a parameter-less constructor. Having this constraint will allow you to do something like T field = new T(); in your code which you wouldn't be able to do otherwise.

You then combine the two using a comma to get:

where T : class, new()