Using MVVM toolkit for connecting two properties (NotifyPropertyChangedFor)

Question

I have a question about C# new MVVM toolkit.

I wonder why this code works:

string MyProp => inputSystemName;  

[ObservableProperty]
[NotifyPropertyChangedFor(nameof(MyProp))]
private string inputSystemName;
partial void OnInputSystemNameChanged(string? oldValue, string newValue)
{
    Debug.WriteLine($"InputSystemName changed from {oldValue} to {newValue}");
    Debug.WriteLine($"MyProp is now {MyProp}");
}

But this piece of code does not update the value of myProp:

private string myProp = String.Empty;
public string MyProp
{
    get => myProp;
    set => myProp = value;
}  

[ObservableProperty]
[NotifyPropertyChangedFor(nameof(MyProp))]
private string inputSystemName;
partial void OnInputSystemNameChanged(string? oldValue, string newValue)
{
    Debug.WriteLine($"InputSystemName changed from {oldValue} to {newValue}");
    Debug.WriteLine($"MyProp is now {MyProp}");
}

It could potentially be my lack of understanding how the notify property works, but isn't it updating another property?

Thanks!

Answer 1

That attribute doesn't set the property, it just raises an event to notify that it changed. So you'd need something like this snippet below to actually set the property.

        [ObservableProperty]
        [NotifyPropertyChangedFor(nameof(MyProp))]
        private string inputSystemName;
        partial void OnInputSystemNameChanged(string? oldValue, string newValue)
        {
            Debug.WriteLine($"InputSystemName changed from {oldValue} to {newValue}");
            Debug.WriteLine($"MyProp is now {MyProp}");
            MyProp = newValue; // set the new value to the related property
        }

That attribute will generate the following code

public string? InputSystemName
{
    get => inputSystemName;
    set
    {
        if (SetProperty(ref inputSystemName, value))
        {
            OnPropertyChanged("MyProp");
        }
    }
}

You can see more here https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/observableproperty#notifying-dependent-properties