Добавьте файлы проекта.

musicschoolapp/Class1.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace musicschoolapp
+{
+    public partial class Student
+    {
+        public override string ToString()
+        {
+            return LastName;
+        }
+    }
+    public partial class Course
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+    public partial class Instruments
+    {
+        public override string ToString()
+        {
+            return Name_instrument;
+        }
+    }
+}