how to add one row in datagrid in c# on class constructor

It is not so difficult you have to create a list or observeable collection and then have to bind it with your datagrid.Then when you wanna add new row just add item in your list.
As shown bellow

In XAMP:

<window x:class="WpfApplication2.MainWindow" 
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <grid>
        <datagrid autogeneratecolumns="True" height="233" horizontalalignment="Left" margin="66,21,0,0" name="dataGrid1" verticalalignment="Top" width="354"></datagrid>
        <button content="Add Row" height="27" horizontalalignment="Left" margin="66,260,0,0" name="button1" verticalalignment="Top" width="72" click="button1_Click" />
    </grid>
</window>

In code behind(C#)

public partial class MainWindow : Window
{
    List<employee> lstEmployee = new List<employee>();
    public MainWindow()
    {
        InitializeComponent();

       dataGrid1.CanUserAddRows=true;

    }

    void AddItemAndBind(int id)
    {
        Employee emp = new Employee();
        emp.ID = id;
        lstEmployee.Add(emp);

        dataGrid1.ItemsSource = lstEmployee;


    }



    public class Employee
    {
        public int ID { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
        public string City { get; set; }
        public string Phone { get; set; }
    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        AddItemAndBind(lstEmployee.Count+1);
    }
}