where T new() (parameterless constructor)

Context: The new() constraint requires that the generic type has a public parameterless constructor. It allows creating instances of the type inside the generic class. This constraint must be the last if combined with others.

Usage Example

using System;

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

public class Person
{
    public string Name { get; set; }
    public Person() => Name = "Unknown";
}

public class NoDefaultCtor
{
    public NoDefaultCtor(string name) { }
}

class Program
{
    static void Main()
    {
        var personFactory = new Factory<Person>();
        Person p = personFactory.Create();
        Console.WriteLine(p.Name); // Unknown

        // This does not compile: NoDefaultCtor has no parameterless constructor
        // var errorFactory = new Factory<NoDefaultCtor>();
    }
}

Output console

dotnet run
Unknown

Important notes

• new() cannot be used with struct (structs always have an implicit default constructor, but the syntax is allowed).
• If multiple constraints, new() must be last.

Real-world usage example

Activator.CreateInstance<T>() – Has the same parameterless constructor requirement.
See .NET docs on new().

Related sections

• 11.4.2 where T : class
• 11.4.7 Multiple constraints