Bind Model to Winforms DataGridView, can not get old value when property changed

.NET version

.Net 8.0 Preview 4

Did it work in .NET Framework?

No

Did it work in any of the earlier releases of .NET Core or .NET 5+?

No response

Issue description

I have a Model (INotifyPropertyChanged) and bound to a DataGridView by BindingList, Once I edited a property of model instance, DataGridView’s cell value updated before PropertyChanged raied. I have no idea why.

I am going to listen to BindingList’s ListChanged (ItemChanged) event and get new value and old value of updating property, but DataGridView’s cell value updated before raising this event, so that I can not get old value from DatGridView’s cell.

Does anyone has some others solution to get both of new and old values when model’s property changed? Great thanks!

Steps to reproduce

Here is my code:

public partial class Form1 : Form
{
    public abstract class BindableBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler? PropertyChanged;

        protected bool SetProperty<TValue>(ref TValue field, TValue newValue, [CallerMemberName] string propertyName = "")
        {
            if (EqualityComparer<TValue>.Default.Equals(field, newValue))
            {
                return false;
            }

            var dataGridView = Application.OpenForms.OfType<Form1>().First().dataGridView1;
            if (dataGridView.RowCount > 0)
                Debug.Print($"Before set value to field: {dataGridView.Rows[0].Cells[0].Value}");
            field = newValue;
            if (dataGridView.RowCount > 0)
                Debug.Print($"After set value to field But before raise PropertyChanged event: {dataGridView.Rows[0].Cells[0].Value}");
            this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
            if (dataGridView.RowCount > 0)
                Debug.Print($"After raise PropertyChanged event: {dataGridView.Rows[0].Cells[0].Value}");
            return true;
        }
    }

    public class Model : BindableBase
    {
        private int age;

        public int Age { get => age; set => this.SetProperty(ref this.age, value); }
    }

    BindingList<Model> Models = new();

    public Form1()
    {
        this.InitializeComponent();
        this.dataGridView1.AutoGenerateColumns = true;
        this.dataGridView1.DataSource = this.Models;
        this.dataGridView1.Click += this.DataGridView1_Click;
    }

    private void DataGridView1_Click(object? sender, EventArgs e)
    {
        switch (this.dataGridView1.RowCount)
        {
            case 0:
                this.Models.Add(new Model() { Age = 10 });
                break;
            default:
                this.Models[0].Age = DateTime.Now.Microsecond;
                break;
        }
    }

Here is some logs, you can see that Cell value was updated when set value to field and before raising PropertyChanged event;

Before set value to field: 10
After set value to field But before raise PropertyChanged event: 478
After raise PropertyChanged event: 478

Before set value to field: 478
After set value to field But before raise PropertyChanged event: 744
After raise PropertyChanged event: 744

[tbolon]

I don’t think there is anything wrong in this behavior: the DataGridView does not store a copy of your value, he just use the model’s value when asked, so when you call dataGridView.Rows[0].Cells[0].Value, the datagridview calls Model.Age on your model, which returns the new value since you just updated the field.

The only goal of this event is to indicate the control that the value has been updated “recently”, and that the control should reread the value and update its display accordingly (for example). There is no concept of previous/new value.

INotifyPropertyChanging was added for this exact case: if you need to get the previous and the new value, then you have should implement and subscribe to this event.

In my opinion, storing a value before/after change should be handled by your model directly (like storing the previous value in a different field, exposed as a “PreviousAge” property for example), and not depending on the datagridview, the UI, or the databinding engine itself.

[Olina-Zhang]

@Olina-Zhang can one of your team please look into this?

We got the same result as the code provided in this issue. But needs our engineering team to look into it about codes.