C#

This satisfies your DRY principle concerns at a stroke. Now, one of the examples you give is age. As age can be a complex object, containing rules about what constitutes a valid age, you would probably want to make this a class as well:<pre lang="c#">public class Age : PositiveInt32Value
{
  protected override void ValidateValue(int value)
  {
    base.ValidateValue(value);
    if (value < 18) throw new ArgumentException("You are way too young to be using this site");
    if (value > 900) throw new ArgumentException("Who do you think you are? A Time Lord?");
  }
}

property PositiveInt32ValuProperty Int32 Value;

Higty wrote:

I must change many place if I insert OnPropertyChanged method call in your Value property.(If a class has many propeties like Value1,Value2,Value3...ect)

public Age Age = new Age();
public PositiveInt32Balance DaysSinceSomeEvent = new PositiveInt32Balance();
// Do something here....
Age.Value = -2; // An exception will be thrown here.

public int Value
{
    get { return _value; }
    set { _value = value; }
}

public int Value
{
    get { return _value; }
    set
    {
      OnPropertyChanging(value);
      ValidatingValue(value);
      _value = value;
      ValidatedValue(value);
      OnPropertyChanged(value);
    }
}

public int Value
{
    get { return _value; }
    set
    {
      ValidateValue(value);
      _value = value;
    }
}

property PositiveInt32ValuProperty Int32 Value;

Higty wrote:

I'm not interest in what you like or not.

public int MyProperty { get; private set; }

public property T MyReadOnlyProperty 
{ 
 get 
 { 
   return field; 
 } 
 private set 
 {
   field = value; 
 } 
}

public class Person
{
  property MyReadOnlyProperty Int32 Age;
}

public class Person
{
  public int MyProperty { get; private set; }
}

class SomeClass : INotifyPropertyChanged {
 public int Value { get; set; beforeset ValidateValue; afterset Notify; }
 public string Description { get; set; afterset Notify; }

 private int ValidateValue(string property, int value) { ... }
 private void Notify<T>(string property, T value) {
  // usual stuff here
 }
}