create interface with method that return generic object

public interface IHelloWorld
{
  T SayHello<T>();
}
 
public class HelloWorld : IHelloWorld
{
    public T SayHello<T>()
    {
        if (typeof(T) == typeof(string))
            return (T)(object)"Hello, World!";
        else
            return default(T);
    }
}