C# Language Generics Type constraints (new-keyword)

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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.

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