How to make button datacontext of id row in datagrid view

Question

Hi I have some method which send me data from sql table and I want that when I click button in datagridview - that send parameter to the method and parameter will have ID value(like on picture 107 or 106). Below on the picture is datagridview with 2 buttons and ID column.

 

My method:

public ObservableCollection<MyClass> ReadUpdate(int id_update)
{
ObservableCollection<MyClass> result = new ObservableCollection<MyClass>();
string nwConn = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
SqlDataReader dr;
SqlConnection conn = new SqlConnection(nwConn);
try
{
SqlCommand cmd = new SqlCommand();
cmd.CommandType = CommandType.StoredProcedure;
cmd.Connection = conn;
cmd.CommandText = "Insert_Update";
cmd.Parameters.AddWithValue("@id_update", id_update);
conn.Open();
dr = cmd.ExecuteReader();
while (dr.Read())
{
MyClass lin = new MyClass();

lin.id = dr.GetInt32(1);
if (!dr.IsDBNull(2)) lin.other = dr.GetString(2);
if (!dr.IsDBNull(3)) lin.barkod = dr.GetString(3);
if (!dr.IsDBNull(4)) lin.pw = dr.GetInt32(4);

result.Add(lin);
}
dr.Close();
return result;

}
catch (SqlException e)
{
MyClass lin = new MyClass();
lin.other = e.Message;

result.Add(lin);
return result;

}
finally
{
conn.Close();

};
}

My class:

 

public class PIS
{
public int ID { get; set; } 

 }

 

And my button:

private void btnUpdate_Click(object sender, System.Windows.RoutedEventArgs e)
{
pis_update = (PIS)((Button)sender).DataContext;
ChildWindow_Update childWindow_update = new ChildWindow_Update();
childWindow_update.DataContext = ((Pismo)((Button)sender).DataContext).Id_Pismo;
childWindow_update.Closed += ChildWindow_Update_Closed;
childWindow_update.Show();
} 

 

public ChildWindow_Update()
{
InitializeComponent();
ServiceReference1.Service1Client webService = new ServiceReference1.Service1Client();

webService.ReadUpdateAsync((int)this.DataContext);
webService.ReadUpdateCompleted += WebService_ReadUpdateCompleted; 

 

private void WebService_ReadUpdateCompleted(object sender, ServiceReference1.ReadUpdateCompletedEventArgs e)
{
if (e.Result != null)
{
//do something

}
} 

I have error in webService.ReadUpdateAsync((int)this.DataContext);
"Null REference Exception".