// ViewModel
public double ProgressBarValue
{
    get
    {
        if (Automation != null)
            return Automation.ProgressBarValue;
        else
            return double.NaN;
    }
    set { }
}

// Model
private string _output;
public string Output
{
    get { return _output; }
    set
    {
        if (_output == value)
            return;
        _output = value;
        RaisePropertyChanged("Output");
    }
}