Добавьте файлы проекта.

musicschoolapp/Pages/QuickEnroll.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.Data;
+using System.Linq;
+using System.Text;
+using System.Threading;
+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;
+
+namespace musicschoolapp.Pages
+{
+    /// <summary>
+    /// Логика взаимодействия для QuickEnroll.xaml
+    /// </summary>
+    public partial class QuickEnroll : Page
+    {
+        musicschoolEntities1 mse = new musicschoolEntities1();
+        public QuickEnroll()
+        {
+            InitializeComponent();
+            comboboxCourse.ItemsSource = mse.Course.ToList();
+            comboboxCourse.DisplayMemberPath = "Name";
+            dGridStudent.ItemsSource = mse.Student.ToList();
+        }
+
+        private void dGridStudent_SelectionChanged(object sender, SelectionChangedEventArgs e)
+        {
+            
+            if (dGridStudent.SelectedCells[3] != null)
+            {
+
+            }
+            foreach (var student in dGridStudent.ItemsSource)
+            {
+                var student_ = student as Student;
+                if (true)
+                {
+
+                    if (student_.toEnroll)
+                    {
+                        Enrollment enrollment = new Enrollment();
+                        enrollment.StudentID = student_.StudentID;
+                        enrollment.CourseID = (comboboxCourse.SelectedItem as Course).CourseID;
+                        enrollment.EnrollmentDate = DateTime.Now;
+                        enrollment.Grade = student_.grade;
+                        mse.Enrollment.Add(enrollment);
+                        student_.toEnroll = false;
+                    }
+                    else
+                    {
+                    }
+                }
+                else
+                {
+                }
+            }
+        }
+
+        private void buttonStart_Click(object sender, RoutedEventArgs e)
+        {
+            MessageBox.Show("Добавлено: " + mse.SaveChanges());
+        }
+
+        private void comboboxCourse_SelectionChanged(object sender, SelectionChangedEventArgs e)
+        {
+            dGridStudent.IsEnabled = true;
+        }
+    }
+}