Issue with DataGrid Selection in WPF Application

Did you also analyze the problem, trying to reproduce it in a new simple project (which can be shared to demonstrate the question)? Maybe the issue is caused by other parts of the code.

By the way, the .NET 5 is old and out of support.

Hi, @Kamyab Faghih. Welcome to Microsoft Q&A. 

I tested it in .Net 5 environment using the following code, selectedRow and index could be successfully updated according to the selected row.

MainWindow.xaml

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <DataGrid x:Name="dgrdMain" AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Id" Binding="{Binding Id}"></DataGridTextColumn>
                <DataGridTextColumn Header="Name" Binding="{Binding Name}"></DataGridTextColumn>
            </DataGrid.Columns>
        </DataGrid>
        <Button Grid.Row="1" Height="50" Width="100" Content="Clike Me" Click="ModifyProductSellPrices"></Button>
    </Grid>

MainWindow.xaml.cs

    public partial class MainWindow : Window
    {
        MainWindowViewModel viewModel = new MainWindowViewModel();
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = viewModel;
            dgrdMain.ItemsSource = viewModel.productSizes;
        }

        private void ModifyProductSellPrices(object sender, RoutedEventArgs e)
        {
            var selectedRow = (ProductSize)dgrdMain.SelectedValue;
            int index = dgrdMain.SelectedIndex;
            if (selectedRow != null && index != -1)
            {
                MessageBox.Show(selectedRow?.ToString() + "\n" + index.ToString());
            }
            else {
                MessageBox.Show("Unselected Item");
            }
        }
    }

MainWindowViewModel.cs

    public class MainWindowViewModel
    {
        public ObservableCollection<ProductSize> productSizes = new ObservableCollection<ProductSize>() { 
            new ProductSize(){ Id = 1 , Name = "AA"},
            new ProductSize(){ Id = 2 , Name = "BB"}
        };
    }

ProductSize.cs

    public class ProductSize
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public override string ToString()
        {
            return "ProductSize:" + Id + "--" + Name;
        }
    }

You could first check whether there are other additional settings in DataGrid that cause incorrect selectedRow or index capture, such as adjusting SelectedIndex and SelectedValue in SelectionChanged event .

If you find that selectedRow or index is correct during debugging, you could consider whether AppWindows.ModifyProductSellPrices modify = new ModifyProductSellPrices(selectedRow); has an error when outputting.