default text in textbox wpf - wpf

Question

I have a TextBox, its Text property is bound to a property of type double named Grade. I also have a CheckBox, when the CheckBox is checked I want the Grade to take an auto calculated value (i.e. automatically set to MaxScore/Count of questions). If the CheckBox is not checked then I want to set and change the Grade Manually. My question is how can I implement this?
<TextBox Height="23"
Visibility="{Binding Path=Visible2, Converter={StaticResource boolToVis}, UpdateSourceTrigger=PropertyChanged}"
Text="{Binding Path=Grade,UpdateSourceTrigger =PropertyChanged,Mode=TwoWay}"
HorizontalAlignment="Left" Margin="376,453,0,0"
Name="textBox3" VerticalAlignment="Top" Width="120" />
and i bind this to:
public double Grade
{
get
{
return grade;
}
set
{
grade = value;
OnPropertyChanged("Grade");
foreach (ExaminationQuestion exaq in
this.Examination.ExaminationQuestions)
{
if (exaq.Question.Guid == SelectedQuestionDropList.Guid)
{
exaq.Grade = value;
}
}
}
}
Thanks

Answer 1

in your VM, have another public Property Auto and make sure your Checkbox binds to it and so does your TextBox (so that it becomes readonly/disabled/hidden). And my default AutoCheck will be true

Answer 2

<StackPanel Orientation="Horizontal" VerticalAlignment="Top">
<Label Content="Grade" />
<TextBox Height="23" Width="120"
Text="{Binding Path=GradeDisplay}"
IsEnabled="{Binding EnableBox}"
/>
<Label Content="Auto?" />
<CheckBox IsChecked="{Binding IsAuto}" />
</StackPanel>
And for the View Model
public bool EnableBox { get; set; }
public string GradeDisplay
{
get
{
if (EnableBox)
return Grade.ToString();
else
return "AUTO";
}
set
{
double result;
if (double.TryParse(value, out result))
Grade = result;
NotifyPropertyChanged("GradeDisplay");
}
}
private bool _IsAuto;
public bool IsAuto
{
get
{
return _IsAuto;
}
set
{
_IsAuto = value;
EnableBox = !value;
NotifyPropertyChanged("GradeDisplay");
NotifyPropertyChanged("EnableBox");
}
}
private double _Grade;
private double Grade
{
set
{
_Grade = value;
}
get
{
if (IsAuto)
{
// CODE TO GET AUTO GRADE
return 0.0;
}
else
{ // RETURN MANUALLY SET GRADE
return _Grade;
}
}
}
// CTOR
public MainWindowViewModel()
{
IsAuto = false;
}