I had to rewrite my problem. The blackquoted part below is this post before the edit.

I have two properties : `capacitiveCurrent` and `ShowProductSum`.
Before any changes in my MVVM application I used to bind `capacitiveCurrent` with a value that was set in the contructor - it was 15 always. 

`capacitiveCurrent` is placed in a different class `LineOut`, code :

**capacitiveCurrent property**

```
    private double _capacitiveCurrent;
    public double CapacitiveCurrent
    {
        get { return _capacitiveCurrent; }
        set
        {
            if (_capacitiveCurrent != value)
            {
                _capacitiveCurrent = value;
                OnPropertyChanged("_capacitiveCurrent");
            }
        }
    }

```

The second property is placed in `MainWindow.xaml.cs` code:

`ShowProductSum`

```
        public double _showProductSum;
    public double ShowProductSum
    {
        get { return _showProductSum; }
        set
        {
            if (_showProductSum != value)
            {
                _showProductSum = value;
                OnPropertyChanged("ProductSum");
            }
        }
    }

```

The logic behing setting and getting `ShowProductSum` is done.. almost. But what I expect from those two properties is that I want them to show up in the ComboBox. This is what I have:

```
    private ObservableCollection<double> _comboBoxCapacitiveCurrent = new ObservableCollection<double>();
    public ObservableCollection<double> ComboBoxCapacitiveCurrent
    {
        get
        {
            _comboBoxCapacitiveCurrent.Clear();
            _comboBoxCapacitiveCurrent.Add(lineWy.capacitiveCurrent);
            _comboBoxCapacitiveCurrent.Add(ShowProductSum);
            return _comboBoxCapacitiveCurrent;
        }
        set
        {
            if (_comboBoxCapacitiveCurrent != value)
            {
                _comboBoxPradPojemnosciowy = value;
                OnPropertyChanged("ComboBoxCapacitiveCurrent");
            }
        }
    }
    public double SelectedItem { get; set; }

```

**ComboBox**:

```
      <DataGridTemplateColumn Header="Capacitive Current " HeaderStyle="{StaticResource ZwarcioweHeaderStyle}">
      <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
         <ComboBox 
                  ItemsSource="{Binding Path=ComboBoxCapacitiveCurrent}"
                  SelectedValue="{Binding Path=SelectedItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
          >
                  </ComboBox>
         </DataTemplate></DataGridTemplateColumn.CellTemplate></DataGridTemplateColumn>

```

CombBox is present but nothing is in the dropdownlist of combobox.Some blank space just underneath this combobox.
Everything implements INotifyPropertyChanged interface. 

Thanks in advance.

> 
> I'm an absolute beginner in MVVM C#. I'm trying to bind two properties
>  which are located in two different classes. 
> 
> 
> First property is `public double capacitiveCurrent { get; set; }` from
>  one class and until then I used this property to View it with a normal
>  binding - the initialized value was shown.
> 
> 
> 
> ```
>  public double productLengthShortCircuitCurrent
>     {
>     get { return length* capacitiveShortCircuitCurrentwithGround; }
>     set { }
>     } is a combination of the two `length` and `capacitiveShortCircuitCurrentwithGround` both with getters and
> 
> ```
> 
> setters. 
> 
> 
> ComboBox is placed inside 
> 
> 
> 
> ```
> `<DataGridTemplateColumn Header"ABC">
>   <DataGridTemplateColumn.CellTemplate
>    <DataTemplate>
>     <ComboBox .../>`
> 
> ```
> 
> What I'm trying to do is having those two properties listed in a
>  ComboBox and:
> 
> 
> 1. I need `capacitiveCurrent` to be editable from the View,
> 
> 
> 
> > 
> > 2. The `productLengthShortCircuitCurrent` should be a sum of all the set values ( so a sum product).. should I call a function to do such
> >  and then bind it to the ComboBox?
> > 
> > 
> > 
> 
> 
> \*\*I've maged to set and get this property and achieved to get the `ProductSum` property below.. It is all about displaying the two
>  properties in a ComboBox.
> 
> 
> I'm really confused. Searched for an answer for two days, didn't come
>  up with an idea. Maybe me knowledge is to shallow but... Tried to bind
>  those two properties to an interface with something like `public
>  IConnectedProperties Categories` and a private `_category` list with
>  getter and setter.
> 
> 
>