update button is not working in my wpf editable datagrid.pls fix this problem.helpmeee

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Data;
using System.Data.SqlClient;

namespace testedit
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }



        public void ShowData()
        {

            SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\v11.0;AttachDbFilename=D:\MyPractice\testedit\testedit\Database1.mdf;Integrated Security=True");

            con.Open();

            SqlCommand comm = new SqlCommand("Select * from userdata", con);

            DataTable dt = new DataTable();

            SqlDataAdapter da = new SqlDataAdapter(comm);

            da.Fill(dt);

            mygd.ItemsSource = dt.DefaultView;

        }





        private void windwloaded(object sender, RoutedEventArgs e)
        {
            ShowData();
        }



        private void insert_btn_Click(object sender, RoutedEventArgs e)
        {
            int Id = Convert.ToInt32( txtid.Text);
            string Username = textBox1.Text;

            string password = textBox2.Text;

            SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\v11.0;AttachDbFilename=D:\MyPractice\testedit\testedit\Database1.mdf;Integrated Security=True");

            con.Open();

            SqlCommand comm = new SqlCommand("insert into userdata(Id,username,password) values(@Id,@username,@password)", con);
            comm.Parameters.AddWithValue("@Id", txtid.Text);

            comm.Parameters.AddWithValue("@username", textBox1.Text);

            comm.Parameters.AddWithValue("@password", textBox2.Text);

            comm.ExecuteNonQuery();

            con.Close();

            ShowData();
        }







        private void clrbtn_Click(object sender, RoutedEventArgs e)
        {
            textBox1.Text = "";

            textBox2.Text = "";
            txtid.Text = "";
        }





        private void dltbtn_Click(object sender, RoutedEventArgs e)
        {
            if (   mygd.SelectedItems.Count > 0)
            {

                DataRowView drv = (DataRowView)mygd.SelectedItem;

                string id = drv.Row[0].ToString();

                SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\v11.0;AttachDbFilename=D:\MyPractice\testedit\testedit\Database1.mdf;Integrated Security=True");

                con.Open();

                SqlCommand comm = new SqlCommand("delete from userdata where id=@id", con);

                comm.Parameters.AddWithValue("@id", id);

                comm.ExecuteNonQuery();

                ShowData();
            }
        }






        private void updtbtn_Click(object sender, RoutedEventArgs e)
        {
            if (mygd.SelectedItems.Count > 0)
            {

                DataRowView drv = (DataRowView)mygd.SelectedItem;

                string id = drv.Row[0].ToString();

                SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)\v11.0;AttachDbFilename=D:\MyPractice\testedit\testedit\Database1.mdf;Integrated Security=True");


                con.Open();

                SqlCommand comm = new SqlCommand("update userdata set username=@username,password=@password where id=@id", con);

                comm.Parameters.AddWithValue("@id", txtid.Text);

                comm.Parameters.AddWithValue("@username", textBox1.Text);

                comm.Parameters.AddWithValue("@password", textBox2.Text);

                comm.ExecuteNonQuery();

                con.Close();
                ShowData();
               
            }
        }









        
    }




<window x:class="testedit.MainWindow" xmlns:x="#unknown">
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="693.283" Width="625" Loaded="windwloaded">
    <grid margin="0,0,-8,-1">
        <textbox margin="129,548,207,81" name="textBox1" datacontext="{Binding ElementName=listView1,Path=SelectedItem}" text="{Binding Path=Username}" />
        <textbox height="27" margin="129,0,207,25" name="textBox2" verticalalignment="Bottom" datacontext="{Binding ElementName=listView1,Path=SelectedItem}" text="{Binding<br mode=" hold=" />	 Path=Password}" />
        <Label Margin="20,544,525,81" Name="label1">Username :</Label>
        <Label Height="25" Margin="20,0,535,27" Name="label2" VerticalAlignment="Bottom" RenderTransformOrigin="0.504,-1.301">Password :</Label>

        <Button x:Name="insert_btn" Margin="454,93,43,535" Content="Insert" Click="insert_btn_Click"></Button>
        <Button x:Name="updtbtn" Margin="454,196,43,433" Content="Update" Click="updtbtn_Click"/>
        <Button x:Name="dltbtn" Margin="454,308,43,321" Content="Delete" Click="dltbtn_Click"/>
        <Button x:Name="clrbtn" Margin="454,409,43,220" RenderTransformOrigin="0.631,4.197" Content="Clear" Click="clrbtn_Click"/>
        <textbox margin="129,494,207,135" x:name="txtid" datacontext="{Binding SelectedItem, ElementName=listView1}" text="{Binding Username}" />
        <Label Margin="20,490,525,135" x:Name="label1_Copy" Content="id"/>
        <datagrid x:name="mygd" autogeneratecolumns="False" margin="40,93,223,201">
            <datagrid.columns>


                <datagridtextcolumn header="Id" binding="{Binding Id}" />
                <datagridtextcolumn header="username" binding="{Binding username}" />
                <datagridtextcolumn header="password" binding="{Binding password}" />
               
            </datagrid.columns>



        </datagrid>



    </grid>
</window>