Question

I’m working on a WP7 app and I’ve had some trouble updating a TextBlock bound to a property. I’m new to MVVM, and C# in general, so I’m not sure what I’m doing wrong.

In the end I have solved this problem, but I don’t understand why my solution works (always fun …), so I’d really appreciate your guidance.

In my app’s Model, I originally had something like this:

// Broken
namespace MyApp.MyModel
{
    public class MetaData : INotifyPropertyChanged
    {
        private StatusType status;
        public StatusType Status
        {
            get { return status; }
            set
            {
                status = value;
                statusMessage = ConvertStatusToSomethingMeaningful(value);
            }
        }

        private string statusMessage;
        public string StatusMessage
        {
            get { return statusMessage; }
            private set
            {
                statusMessage = value;
                // This doesn't work
                NotifyPropertyChanged("StatusMessage");
            }
        }

        ...
    }
}

Status is a enum, and when it’s set by my app, it also sets StatusMessage too (which is a more human readable description to show the user). My View’s TextBlock is bound to StatusMessage, but it doesn’t update using the above code.

However, if I move NotifyPropertyChanged("StatusMessage") into Status, my View’s TextBlock updates like it should. However, I don’t understand why this works when the original code above doesn’t?

// Fixed
namespace MyApp.MyModel
{
    public class MetaData : INotifyPropertyChanged
    {
        private StatusType status;
        public StatusType Status
        {
            get { return status; }
            set
            {
                status = value;
                StatusMessage = ConvertStatusToSomethingMeaningful(value);
                // This works
                NotifyPropertyChanged("StatusMessage");
            }
        }

        public string StatusMessage { get; private set; }

        ...
    }
}

Many thanks in advance for helping a newbie out 🙂

Answer 1

Issue in this line:

 statusMessage = ConvertStatusToSomethingMeaningful(value);

StatusMessage setter is never called (NotifyPropertyChanged(“StatusMessage”) called exactly there)

 StatusMessage = ConvertStatusToSomethingMeaningful(value);

will be the right call

Probably my implementation of this would be next:

 namespace MyApp.MyModel
 {
      public class MetaData : INotifyPropertyChanged
      {
           private StatusType status;
           public StatusType Status
           {
                get { return status; }
                set
                {
                     if (status != value)
                     {
                          status = value;
                          NotifyPropertyChanged("Status");
                          NotifyPropertyChanged("StatusMessage");
                     }                
                }
           }

           public string StatusMessage
           {
                get { return ConvertStatusToSomethingMeaningful(status); }
           }

      ...
      }
 }