Question

Using MVVM in Silverlight, on my View I have:

<TextBox Text="{Binding Path=Temperature,Mode=TwoWay}" />

MyViewModel has:

public MyModel Model {get{...}set{...}}//In my code I do have the property changed events for this
public string Temperature
{
    get
    {
        return Model.Temperature.ToString();
    }
    set
    {
        double test;
        if(double.TryParse(value, out test))
        {
            Model.Temperature = test;
        }
        else
        {
            Model.Temperature = 0D;
        }
    }
}

void Model_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
    switch(e.PropertyName)
    {
        case "Temperature":
            OnPropertyChanged("Temperature");
            break;
    }
}

MyModel has:

private double _temperature;
public double Temperature
{
    get
    {
        return _temperature;
    }
    set
    {
        _temperature = value; OnPropertyChanged("Temperature");
    }
}

If the user enters something that would not parse to a double, the Model.Temperature property gets set to 0, and I would like the TextBox to also change to 0. I thought the OnPropertyChanged events would take care of that. Any ideas whats happening?

Answer 1

EDIT:
This is caused because the Text will not call the get during the call to set. This could cause a bad infinite loop. You have a few choices.

Delay firing property changed for the error condition:

public string Temperature
{
    get { return Model.Temperature.ToString(); }
    set
    {
        double test;
        if (double.TryParse(value, out test))
        {
            Model.Temperature = test;
        }
        else
        {
            Model.Temperature = 0D;
            Deployment.Current.Dispatcher.BeginInvoke(() => OnPropertyChanged("Temperature"));
        }
    }
}

Use a NumericTextBox

Implement IDataErrorInfo on your ViewModel and set ValidatesOnDataErrors on your TextBox to true

<TextBox Text="{Binding Temperature, Mode=TwoWay, ValidatesOnDataErrors=True}"/>

OLD:
Make sure that your binding allow for two way binding. Without it, you cannot set the TextProperty of the TextBlock.

<TextBlock Text="{Binding Temperature, Mode=TwoWay}"/>