TextBlock does not update after assigning the value

Question

I am making a datagrid inside a window, where two columns are DockPanels with a TextBlock and a Button. When a user selects a file/folder, the TextBlock receives the value and displays it. However, it does not reflect it right away in my case. When I double click on the TextBlock, it suddenly shows. What am I missing here?

XAML

<DataGrid  Name="copierDataGrid" Margin="30,50,30,100"
               ItemsSource="{Binding SelectedAsm}" SelectedItem="{Binding Selected}" AutoGenerateColumns="False" CanUserResizeColumns="False" >
        <DataGrid.Columns>
            <DataGridTextColumn Header="Prefix" Width="70" Binding="{Binding prefCol, TargetNullValue=''}"/>
            <DataGridTemplateColumn Width="235" CanUserResize="False" Header="Select File">
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <DockPanel DockPanel.Dock="Top">
                            <TextBlock Name="fileLocColtxtBox" Width="185" Margin="0,0,0,0" TextWrapping="Wrap" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Text="{Binding fileLocCol, TargetNullValue=''}"/>
                            <Button x:Name="brwsFileBtn" Content="..." Width="30" Margin="0,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Click="brwsFileBtn_Click"/>
                        </DockPanel>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>
            <DataGridTextColumn Header="Suffix" Width="70" Binding="{Binding suffCol, TargetNullValue=''}"/>
            <DataGridTemplateColumn Header="Target location" Width="*" CanUserResize="False"  >
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <DockPanel DockPanel.Dock="Top">
                            <TextBlock Name="destLocColtxtBox" Width="180" Margin="0,0,0,0" TextWrapping="Wrap" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Text="{Binding destLocCol, TargetNullValue=''}"/>
                            <Button x:Name="brwsFileBtn" Content="Browse" Width="50" Margin="0,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Click="brwsFolderBtn_Click"/>
                        </DockPanel>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>

            </DataGridTemplateColumn>

        </DataGrid.Columns>

    </DataGrid>

XAML.CS

public itemAsm Selected { get; set; }
    public ObservableCollection<itemAsm> SelectedAsm { get; set; }
    public unitCopierFrm()
    {
        InitializeComponent();

        SelectedAsm = new ObservableCollection<itemAsm>();
        DataContext= this;}
    public class itemAsm
    {
        public string prefCol { get; set; }

        public string fileLocCol { get; set; }

        public string suffCol { get; set; }

        public string destLocCol { get; set; }
    }


    }private void brwsFolderBtn_Click(object sender, RoutedEventArgs e)
    {
        if (Selected !=null)
        {
            System.Windows.Forms.FolderBrowserDialog dlgFolderBrowser = new System.Windows.Forms.FolderBrowserDialog();

            dlgFolderBrowser.Description = "Select the destination folder";
            dlgFolderBrowser.RootFolder = System.Environment.SpecialFolder.MyComputer;
            dlgFolderBrowser.ShowDialog();

            string dirPath = dlgFolderBrowser.SelectedPath;

            //if (dlgFolderBrowser.ShowDialog == Windows)
            if (dirPath != null)
            {
                Selected.destLocCol = dirPath;
            }
        }

    }

Answer 1

itemAsm should implement INotifyPropertyChanged and raise the PropertyChanged event to notify the view whenever the data-bound property has been set to a new value:

public class itemAsm : INotifyPropertyChanged
{
    private string _prefCol;
    public string prefCol
    {
        get { return _prefCol; }
        set { _prefCol = value; NotifyPropertyChanged(); }
    }

    private string _fileLocCol;
    public string fileLocCol
    {
        get { return _fileLocCol; }
        set { _fileLocCol = value; NotifyPropertyChanged(); }
    }

    private string _suffCol;
    public string suffCol
    {
        get { return _suffCol; }
        set { _suffCol = value; NotifyPropertyChanged(); }
    }

    private string _destLocCol;
    public string destLocCol
    {
        get { return _destLocCol; }
        set { _destLocCol = value; NotifyPropertyChanged(); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

You may also want to consider renaming the properties according to the C# naming conventions.