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 <Grid>
                <DataGrid 
                    Name="Grid" 
                    ItemsSource="{Binding Path=Item, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                    SelectedItem="{Binding Path=SelectedRow, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                    IsReadOnly="True"
                    CanUserAddRows="False"
                    CanUserDeleteRows="False"
                    AutoGenerateColumns="True"
                    AutoGeneratingColumn="Grid_OnAutoGeneratingColumn">
                    <DataGrid.ContextMenu>
                        <ContextMenu>
                             <MenuItem Header="Update" Command="{Binding Path=BtnUpdate}"/>
                        </ContextMenu>
                    </DataGrid.ContextMenu>
           
                </DataGrid>
 </Grid>					

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  public RelayCommand BtnUpdate { get; set; }

   public ObservableCollection<User> Item
   {
       get
        { return _user; }
       set
         {
          _user = value;
           OnPropertyChanged(nameof(Item));
          }
   }

  public User SelectedRow
   {
       get { return _selectedRow;}
       set
       {
          _selectedRow = value;
          OnPropertyChanged(nameof(SelectedRow));
        }
    }

    public void AddColumn()
    {
            BtnUpdate = new RelayCommand(Select);
            Item.Add(new User() { Name = "Jane", Age=26, Mail = "hello@gmail.com"});
            
    }

    private void Select(object obj)
        {
            if (SelectedRow is null)
                return;
           //TODO edit Name to Tom for example _selectedRow
        }