Modify the value of a property during PropertyChangedEventHandler

Question

I am writing an application that reads information on the form and transforms that data in various ways based on a set of rules. Rather than get into the why's of the scenario, I will just lay out a basic example of what I am trying to do. Please note, this whole business of using PropertyChangedEventHandler and INotifyPropertyChanged is very new to me and I am still reading and learning about it.

Here is a sample class I use to capture the first name and a message to the user:

class Names : INotifyPropertyChanged
{

    private string _firstName;
    public string firstName { get { return _firstName; } set { SetField(ref _firstName, value, "firstName"); } }

    private string _nameMessage;
    public string nameMessage { get { return _nameMessage; } set { SetField(ref _nameMessage, value, "nameMessage"); } }

    #region handle property changes
    public event PropertyChangedEventHandler PropertyChanged;
    protected bool SetField<T>(ref T field, T value, string propertyName)
    {
        //if the value did not change, do nothing.
        if (EqualityComparer<T>.Default.Equals(field, value)) return false;
        //the value did change, so make the modification.
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
    #endregion
}

Whenever the property for firstName changes, I want to automatically modify the value in the nameMessage property.

For example, the user types John , so the nameMessage property is set to Hello John .

This is a very simple example of what I am trying to accomplish but I am getting stuck because it is hard for me to understand how to get the pointer to that particular property for that index.

Here are my two questions: 1) What code should I write to get the changed value and modify it? 2) Does that business logic belong within the class, or should it be in a different class altogether?

Any pointers you can provide would be very helpful.

Thank you.

Answer 1

to cascade a property fro one to another, you could implement your firstName property to be implemented like this:

public string firstName 
{  
    get { return _firstName; } 
    set 
    { 
       if (SetField(ref _firstName, value))
       {
            // first name changed, so update the other field too
            this.nameMessage = string.Format("hello {0}", _firstName);
       }
    } 
}

you could also do this without every actually storing the nameMessage as a field. it could be generated on the fly:

public string nameMessage { get { return string.Format("hello {0}", _firstName); } }

and simply add an additional property change notification to the setter for first name:

    set 
    { 
       if (SetField(ref _firstName, value))
       {
            // first name changed, so update the other field too
            NotifyPropertyChanged("nameMessage");
       }
    } 

related: one trick is using the CallerMemberName annotation to provide a default value for the property name, so you don't have to pass that in your setfield calls

 protected bool SetField<T>(ref T field, T value, [CallerMemberName] string propertyName = null)