Question

This isn’t legal:

public class MyBaseClass {   public MyBaseClass() {}   public MyBaseClass(object arg) {} }   public void ThisIsANoNo<T>() where T : MyBaseClass {   T foo = new T('whoops!'); } 

In order to do this, you have to do some reflection on the type object for T or you have to use Activator.CreateInstance. Both are pretty nasty. Is there a better way?

Answer 1

You can’t constrain T to have a particular constructor signature other than an empty constructor, but you can constrain T to have a factory method with the desired signature:

public abstract class MyBaseClass {     protected MyBaseClass() {}     protected abstract MyBaseClass CreateFromObject(object arg); }  public void ThisWorksButIsntGreat<T>() where T : MyBaseClass, new() {     T foo = new T().CreateFromObject('whoopee!') as T; } 

However, I would suggest perhaps using a different creational pattern such as Abstract Factory for this scenario.