Constraints on type parameters – new()

As per MSDN, the new() constraint is used to ensure that the type argument must have a public parameterless constructor. However, consider the example given below (taken from the same page).

public class Employee
{
    public Employee(string name, int id)
    {
        Name = name;
        ID = id;
    }
    public string Name { get; set; }
    public int ID { get; set; }
}

class EmployeeList<T> where T : Employee, new()
{

}

Here, the Employee type does not have a parameterless/default constructor, yet this code compiles successfully. Can someone please elaborate on the usage of this constraint and why this works?

>Solution :

The code compiles because it’s entirely possible to have a type derived from Employee with a parameterless constructor – and that’s what the constraint says. The constraint is not trying to say that Employee itself has a parameterless constructor, and indeed an attempt to use EmployeeList<Employee> would fail as the constraint isn’t satisfied.

As example of what would be valid:

public class GeneratedEmployee : Employee
{
    public GeneratedEmployee() : base(GenerateName(), GenerateId())
    {
    }

    private static string GenerateName()
    {
        // Implementation here
    }

    private static int GenerateId()
    {
        // Implementation here
    }
}

At that point, it’s fine to create an EmployeeList<GeneratedEmployee>, and assuming the EmployeeList<> class uses new() somewhere in the implementation, it would call the parameterless constructor of GeneratedEmployee.

That said, it’s a pretty odd constraint, as I wouldn’t expect you to really want to create an employee without specifying the name and ID.