C# Language Type constraints (new-keyword)


Example

By using the new() constraint, it is possible to enforce type parameters to define an empty (default) constructor.

class Foo
{
    public Foo () { }
}

class Bar
{
    public Bar (string s) { ... }
}

class Factory<T>
    where T : new()
{
    public T Create()
    {
        return new T();
    }
}

Foo f = new Factory<Foo>().Create(); // Valid.
Bar b = new Factory<Bar>().Create(); // Invalid, Bar does not define a default/empty constructor.

The second call to to Create() will give compile time error with following message:

'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 'Factory'

There is no constraint for a constructor with parameters, only parameterless constructors are supported.