How to validate for an empty fields in wpf using c# class form?

Hello,

Use below,

namespace Feedmill_Weighbridge.Presentation
{
    /// 

    /// Interaction logic for UCFinishedProduct.xaml
    /// 
    public partial class UCFinishedProduct : UserControl, IFinishedProductView, INotifyPropertyChanged, IDataErrorInfo
    {
        public FinishedProductPresenter presenter;
        public event PropertyChangedEventHandler PropertyChanged;
       
private string _topSeal;
    public string TopSeal
    {
        get { return _topSeal; }
        set
        {
            _topSeal = value;
            OnPropertyChanged(nameof(TopSeal));
        }
    }

    private string _bottomSeal;
    public string BottomSeal
    {
        get { return _bottomSeal; }
        set
        {
            _bottomSeal = value;
            OnPropertyChanged(nameof(BottomSeal));
        }
    }

    // Implement IDataErrorInfo for validation
    public string Error => null;

    public string this[string columnName]
    {
        get
        {
            string error = null;
            if (columnName == nameof(TopSeal))
            {
                if (string.IsNullOrEmpty(TopSeal))
                {
                    error = "Top Seal is required.";
                }
            }
            else if (columnName == nameof(BottomSeal))
            {
                if (string.IsNullOrEmpty(BottomSeal))
                {
                    error = "Bottom Seal is required.";
                }
            }
            return error;
        }
    }


    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    }
}