There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:
// File1.cs
using System;
namespace Outer.Inner
{
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now imagine that someone adds another file (File2.cs) to the project that looks like this:
// File2.cs
namespace Outer
{
class Math
{
}
}
The compiler searches Outer before looking at those using directives outside the namespace, so it finds Outer.Math instead of System.Math. Unfortunately (or perhaps fortunately?), Outer.Math has no PI member, so File1 is now broken.
This changes if you put the using inside your namespace declaration, as follows:
// File1b.cs
namespace Outer.Inner
{
using System;
class Foo
{
static void Bar()
{
double d = Math.PI;
}
}
}
Now the compiler searches System before searching Outer, finds System.Math, and all is well.
Some would argue that Math might be a bad name for a user-defined class, since there's already one in System; the point here is just that there is a difference, and it affects the maintainability of your code.
It's also interesting to note what happens if Foo is in namespace Outer, rather than Outer.Inner. In that case, adding Outer.Math in File2 breaks File1 regardless of where the using goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using directive.
Best Answer
I strongly disagree with the concept that the Model should not implement the
INotifyPropertyChanged. This interface is not UI specific! It simply informs of a change. Indeed, WPF heavily uses this to identify changes, but that doesn't mean it is an UI interface. I would compare it to the following comment: "A tire is a car accessory". Sure it is, but bikes, buses, etc. also use it. In summary, do not take that interface as an UI thing.Having said that, it doesn't necessarily mean I believe that the Model should be providing notifications. In fact, as a rule of thumb, the model should not implement this interface, unless it is necessary. In most cases where no server data is pushed to the client app, the model can be stale. But if listening to financial market data, then I do not see why the model cannot implement the interface. As an example, what if I have non-UI logic such as a service that when it receives a Bid or Ask price for a given value it issues an alert (ex. through an email) or places an order? This could be a possible clean solution.
However, there are different ways of achieving things, but I would always argue in favor of simplicity and avoid redundancy.
What is better? Defining events on a collection or property changes on the view model and propagating it to the model or having the view intrinsically update the model (through the view model)?
The bottom line whenever you see someone claiming that "you can't do this or that" it is a sign they do not know what they are talking about.
It really depends on your case and in fact MVVM is a framework with lots of issues and I am yet to see a common implementation of MVVM across the board.
I wish I had more time to explain the many flavors of MVVM and some solutions to common problems - mostly provided by other developers, but I guess I will have to do it another time.