public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }
    int focus = 0;
    TextBox txt = new TextBox();
    List<Res> unitListForTemp = new List<Res>();
    List<TextBox> textBoxListForTemp = new List<TextBox>();
    Res unit = new Res();
    private void validateTextDouble(object sender)
    {
        Exception X = new Exception();
        TextBox T = (TextBox)sender;
        try
        {
            double x = double.Parse(T.Text.Replace(',', '.'), CultureInfo.InvariantCulture);

            if (x < 0 || T.Text.Contains(','))
                throw X;
        }
        catch (Exception)
        {
            try
            {
                int CursorIndex = T.SelectionStart - 1;
                T.Text = T.Text.Remove(CursorIndex, 1);

                T.SelectionStart = CursorIndex;
                T.SelectionStart = 0;
            }
            catch (Exception) { }
        }
    }
    public List<TextBox> AllTextBoxes(DependencyObject parent)
    {
        var list = new List<TextBox>();
        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
        {
            var child = VisualTreeHelper.GetChild(parent, i);
            if (child is TextBox)
                list.Add(child as TextBox);
            list.AddRange(AllTextBoxes(child));
        }
        return list;
    }
    public static List<Res> UnitConversion(Res from, double value, List<Res> unitList)
    {
        var list = unitList;
        double v = 0.0;

        foreach (var unit in list)
        {
            v = (value - from.Shift) / from.Scale;   // ТУТ ОШИБКА
            unit.Value = v * unit.Scale + unit.Shift;
        }
        return list;
    }
    public void Calculate(Res unit, List<Res> unitList, string text, List<TextBox> textBoxList, TextBox txt)
    {
        double value = 0.0;

        if (txt.Text != string.Empty)
        {
            validateTextDouble(txt);

            if (double.TryParse(text, out value))
            {
                value = Convert.ToDouble(text);
                TextBox textbox = new TextBox();
                var list = UnitConversion(unit, value, unitList);
                var itemToRemove = list.Where(x => x.Name == unit.Name).FirstOrDefault();
                list.Remove(itemToRemove);
                foreach (var item in list)
                {
                    textbox = textBoxList.Where(x => x.Name == item.TextBoxName).FirstOrDefault();
                    textbox.Text = item.Value.ToString();
                }
                list.Add(itemToRemove);
            }
        }
        else if (txt.Text == string.Empty)
        {
            textBoxList.Remove(txt);
            foreach (var textBox in textBoxList)
            {
                textBox.Text = string.Empty;
            }
        }
    }
    private void txtBox_GotFocus(object sender, RoutedEventArgs e)
    {
        switch (((TextBox)sender).Name)
        {
            case "txtCelcius":
                focus = 1;
                break;
            case "txtFahrenheit":
                focus = 2;
                break;
            case "txtKelvin":
                focus = 3;
                break;
        }
    }
    private void txtBox_TextChanged(object sender, TextChangedEventArgs e)
    {
        switch (focus)
        {
            case 1:
                txt = txtCelcius;
                unit = TemperatureUnit.list.Where(x => x.Name == "Celcius").FirstOrDefault();
                Calculate(unit, unitListForTemp, txt.Text, textBoxListForTemp, txt);
                break;
            case 2:
                txt = txtFahrenheit;
                unit = TemperatureUnit.list.Where(x => x.Name == "Fahrenheit").FirstOrDefault();
                Calculate(unit, unitListForTemp, txt.Text, textBoxListForTemp, txt);
                break;
            case 3:
                txt = txtKelvin;
                unit = TemperatureUnit.list.Where(x => x.Name == "Kelvin").FirstOrDefault();
                Calculate(unit, unitListForTemp, txt.Text, textBoxListForTemp, txt);
                break;

        }
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        unitListForTemp = TemperatureUnit.list;
        textBoxListForTemp = AllTextBoxes(grdTemp);
    }
}
public class Res
{
    public string TextBoxName { get; set; }
    public string Name { get; set; }
    public double Scale { get; set; }
    public double Shift { get; set; }
    public string Type { get; set; }
    public double Value { get; set; }
}
public class TemperatureUnit
{
    public static List<Res> list = new List<Res>()
    {
        new Res() { Name="Celcius",TextBoxName="txtCelcius",Scale = 1.0,Shift=0.0,Type="Temperature",Value=0 },
        new Res() { Name="Fahrenheit",TextBoxName="txtFahrenheit",Scale = 1.8,Shift=32,Type="Temperature",Value=0 },
        new Res() { Name="Kelvin",TextBoxName="txtKelvin",Scale = 1.0,Shift=273.15,Type="Temperature",Value=0 }
    };
}