Why does the c# compiler emit Activator.CreateInstance when calling new in with a generic type with a new() constraint?

Question

When you have code like the following:

static T GenericConstruct() where T : new()
{
    return new T();
}

The C# compiler insist

Answer 1

This is a little bit faster, since the expression is only compiled once:

public class Foo where T : new()
{
    static Expression> x = () => new T();
    static Func f = x.Compile();

    public static T build()
    {
        return f();
    }
}

Analyzing the performance, this method is just as fast as the more verbose compiled expression and much, much faster than new T() (160 times faster on my test PC) .

For a tiny bit better performance, the build method call can be eliminated and the functor can be returned instead, which the client could cache and call directly.

public static Func BuildFn { get { return f; } }