Wpf MVVM best practices in designing a viewmodel - wpf

Question

I'm using WPF and am currently designing the getters/setters for my viewmodels. My question is should I be applying the changes to the model as soon as the user changes it (via the setter) or should I be copying the changes over to the model only when the Save() method is called? Eg:
Model
public class Customer {
string Name { get; set; }
int Age { get; set; }
}
ViewModel
public class CustomerVM {
//getters and setters are bound to the model.
public string Name {
get
{
return model.Name;
}
set {
model.Name = value;
}
}
public int Age {
get {
return model.Age;
}
set {
model.Age = value;
}
}
public Customer model { get; set; }
public CustomerVM(Customer model) {
SetModel(model);
}
private void SetModel(Customer model) {
this.model = model;
}
public void Save() {
CustomerService.Update(model);
}
}
is this preferred over..
public class CustomerVM {
string name;
public string Name {
get
{
return name;
}
set {
name = value;
}
}
int age;
public int Age {
get {
return age;
}
set {
age = value;
}
}
public Customer model { get; set; }
public CustomerVM(Customer model) {
SetModel(model);
}
private void SetModel(Customer model) {
//values are copied over to VM when upon initialization
this.model = model;
this.name = model.Name;
this.age = model.Age;
}
public void Save() {
//values are copied to the model when user saves
model.Name = name;
model.Age = age;
CustomerService.Update(model);
}
}

Answer 1

From an end-user perspective both will function exactly the same, i.e. the data will be saved when the Save method is invoked. If this is the correct behaviour for your app, you should use the simplest implementation, which is the first one.
I can see the second implementation being useful if you want to add 'undo' functionality, i.e. allow the user to reset the property values to the current model value.
Anyhow, I typically start with a view model that adapts the model values until I need somethng ore complex.