Setting Default Value to a property on c# [duplicate]

Question

Possible Duplicate:
Setting the default value of a DateTime Property to DateTime.Now inside the System.ComponentModel Default Value Attrbute

Im creating a control like a calendar, and I want user to set de year they want to view. But If they don't, the property of SelectedYear must be the actual year.

I try setting this Default Value like this:

[DefaultValue(DateTime.Today.Year)]
public int SelectedYear{
    get{
        return _selYear;
    }
    set{
        _selYear = value;
        UpdateCalendar();
    }
}

And I got the following error

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

Answer 1

The Year property of DateTime is a calculated value but you're trying to embed it into an attribute which is an immutable value stored in the binary. These two concepts aren't compatible.

What you want to do here is have a marker to note that the value isn't set yet and to return the current Year in that case. This is a good place to use a nullable

int? _selYear;
public int SelectedYear {
  get { return _selYear ?? DateTime.Today.Year; }
  set { 
    _selYear = value;
    UpdateCalendar();
  }
}

Answer 2

Use AmbientValueAttribute to indicate that when you set the property to the default value, it actually means to get the current date.

int _selYear = 0;

[AmbientValue(0)]
[DefaultValue(0)]
public int SelectedYear{
    get{
        if (_selYear == 0)
            return DateTime.Today.Year;
        return _selYear;
    }
    set{
        _selYear = value;
        UpdateCalendar();
    }
}

If you want to use the 0 value, then you can make it nullable, but never return null, only allow setting null:

int _selYear = null;

[AmbientValue(null)]
[DefaultValue(null)]
public int? SelectedYear{
    get{
        return _selYear ?? DateTime.Today.Year;
    }
    set{
        _selYear = value;
        UpdateCalendar();
    }
}

Now, it is the null value that represents the current year.

The AmbientValueAttribute means: if you set this property to that value, when you get the property, another value will be returned from the environment, and that the return value in that case may vary, just like the current date.

Answer 3

You should initialize a value of a property explicitly. The DefaultValue attribute does not initialize property by the value it just provide metadata which value is default.

You can initialize it in ctor of a type or you can write a custom code which will reflect a type and initialize properties by the values from the type metadata.