How to "return" an error message from a C# property setter to the user?

Question

I have a windows form that allows the user to input an integer value. This value is used to update a member of a separate class (foo) so long as it falls within a certain valid range. If the user inputs something invalid, I want an error message to tell the user that the default value for the parameter will be used.

In order to accomplish this, I have been error checking the user input within the code for the windows form before letting foo set the field in question, and simply displaying a message if the input fails the check.

But what I would like to do is send the raw user input to a property setter within the class foo and have the property setter handle all the error checking. For other functions in foo that may require a message be displayed on the user interface, I am simply returning a string, but to my knowledge there is no way to return any sort of status or value from a property setter. Is there a graceful way to bubble up a status message of some sort from a property setter to a different function? I don't want foo to modify the user interface directly.

Thanks in advance for any help or tips, and I apologize if my question is too vague or should be asked differently as I am new to StackOverflow

Answer 1

I fixed my problem (as well as removed the need to return strings from nearly all my functions) by implementing INotifyPropertyChanged in my class foo and raising a PropertyChanged event to be caught by the windows form whenever it was necessary to let the user know something had changed.

in class foo-

public class foo : INotifyPropertyChanged {

    public event PropertyChangedEventHandler PropertyChanged;
    public int bar;

    private void NotifyPropertyChanged(string type) {
        if (PropertyChanged != null) {
            PropertyChanged(this, new PropertyChangedEventArgs(type));
        }
    }

    private void test(){
        bar = 1;
        NotifyPropertyChanged("changed int");
    }

}

in the windows form:

public partial class GUI : Form {
     foo fooinstance = new foo();

     public GUI(){
           InitializeComponent();
           fooinstance.PropertyChanged += doEvent;
     }

     private void doEvent(object sender, PropertyChangedEventArgs e){
        foo updated = sender as foo;
        if (object.ReferenceEquals(e.PropertyName, "changed int")) {
            ShowWhatChanged(updated.bar); //show on GUI
        }
     }

}

EDIT: A more graceful and cleaner way that I ended up using was to utilize callbacks. The GUI passed a function pointer into foo upon instantiation and foo used it as a delegate to pass strings back to the GUI whenever necessary. No events required:

public class foo {

    public delegate void UpdateCallback(string msg);
    private UpdateCallback _ucb;
    public foo(UpdateCallback cb){
        _ucb = cb;
    }

    private void test(){
        if(_ucb != null) {
           _ucb("Message Here");
        }
    }
}

public partial class GUI : Form {

     public GUI(){
           InitializeComponent();
           foo fooinstance = new foo(showmessage);

     }

     private void showmessage(string msg){
           //do whatever with the message
     }
}