public AppointmentBaseCollection Conflicts { get; }

Public ReadOnly Property Conflicts As AppointmentBaseCollection

using DevExpress.XtraScheduler;
// ...

// Appointment = Lecture
// Resource = Room
// appointment.CustomFields["Teacher"] = Teacher
// appointment.CustomFields["Group"] = Group of Students

// Start of the AllowAppointmentConflicts event handler.
// ==================================================

private void schedulerControl1_AllowAppointmentConflicts(object sender, 
    AppointmentConflictEventArgs e) {
   Appointment currentLecture = e.Appointment;
   object roomId = currentLecture.ResourceId;
   string teacher = GetTeacher(currentLecture);
   string group = GetGroup(currentLecture);
   DateTime start = currentLecture.Start;
   TimeSpan duration = currentLecture.Duration;

   // e.Conflicts contains all the lectures held at the same time.
   // All of them the SchedulerControl considers to be conflicting.
   AppointmentBaseCollection lecturesInTheSameTime = e.Conflicts;

   AppointmentBaseCollection conflictedLectures = new AppointmentBaseCollection();
   AppointmentBaseCollection lecturesInDifferentRoomWithDifferentTeacher = 
    new AppointmentBaseCollection();
   ArrayList groupsInDifferentRoomWithDifferentTeacher = new ArrayList();
   ArrayList groups = new ArrayList();

   int count = lecturesInTheSameTime.Count;
   for (int i = 0; i < count; i++) {
      Appointment lecture = lecturesInTheSameTime[i];

      // Check if the lecture is in the same room.
      if (Object.Equals(lecture.ResourceId, roomId)) { 
         // Check if the lecture is by the same teacher.
         if (String.Compare(teacher, GetTeacher(lecture), true) == 0) { 
            if (lecture.Start != start || lecture.Duration != duration) {
               // Conflict! Lecture with a bad time frame.
               conflictedLectures.Add(lecture);
            }
            else {
               // No conflict!
               groups.Add(GetGroup(lecture));
            }
         }
         // Lecture by a different teacher.
         else { 
            // Conflict! Lecture by a different teacher in the same room.
            conflictedLectures.Add(lecture);
         }
      }
      // Lecture in a different room.
      else { 
         // Lecture by the same teacher.
         if (String.Compare(teacher, GetTeacher(lecture), true) == 0) { 
            // Conflict! Lecture of the same teacher in the different room.
            conflictedLectures.Add(lecture);
         }
         // Lecture by a different teacher.
         else { 
            // No conflict! Lecture by a different teacher in a different room.
            groupsInDifferentRoomWithDifferentTeacher.Add(GetGroup(lecture));
            lecturesInDifferentRoomWithDifferentTeacher.Add(lecture);
         }
      }
   }

   // Search for the groups which should be in different rooms at the same time.
   count = groups.Count;
   for (int i = 0; i < count; i++) {
      int conflictIndex = groupsInDifferentRoomWithDifferentTeacher.IndexOf(groups[i]);
      if (conflictIndex >= 0)
         conflictedLectures.Add(lecturesInDifferentRoomWithDifferentTeacher[conflictIndex]);
   }

   e.Conflicts.Clear();
   // If the conflictedLectures will contain no lectures after this event has occured, 
   // then e.Conflicts will be empty and this will indicate that there are no conflicts.
   e.Conflicts.AddRange(conflictedLectures);
}

// End of the AllowAppointmentConflicts event handler.
// ==================================================

// Additional functions.
// ====================
// Determines the teacher for the specified lecture.
string GetTeacher(Appointment lecture) {
   object teacher = lecture.CustomFields["Teacher"];
   if (teacher == null)
      return String.Empty;

   return teacher.ToString();
}
// Determines the group for the specified lecture.
string GetGroup(Appointment lecture) {
   object group = lecture.CustomFields["Group"];
   if (group == null)
      return String.Empty;

   return group.ToString();
}

Imports DevExpress.XtraScheduler
' ...

' Appointment = Lecture
' Resource = Room
' appointment.CustomFields["Teacher"] = Teacher
' appointment.CustomFields["Group"] = Group of Students

' Start of the AllowAppointmentConflicts event handler.
' ==================================================

Private Sub OnAllowAppointmentConflicts(sender As Object, e As AppointmentConflictEventArgs) _
Handles SchedulerControl1.AllowAppointmentConflicts

   Dim currentLecture As Appointment = e.Appointment
   Dim roomId As Object = currentLecture.ResourceId
   Dim teacher As String = GetTeacher(currentLecture)
   Dim group As String = GetGroup(currentLecture)
   Dim start As DateTime = currentLecture.Start
   Dim duration As TimeSpan = currentLecture.Duration

   ' e.Conflicts contains all the lectures held at the same time.
   ' All of them the SchedulerControl considers to be conflicting.
   Dim lecturesInTheSameTime As AppointmentBaseCollection = e.Conflicts

   Dim conflictedLectures As New AppointmentBaseCollection()
   Dim lecturesInDifferentRoomWithDifferentTeacher As New AppointmentBaseCollection()
   Dim groupsInDifferentRoomWithDifferentTeacher As New ArrayList()
   Dim groups As New ArrayList()

   Dim count As Integer = lecturesInTheSameTime.Count
   Dim i As Integer
   For i = 0 To count - 1
      Dim lecture As Appointment = lecturesInTheSameTime(i)

      ' Check if the lecture is in the same room.
      If Object.Equals(lecture.ResourceId, roomId) Then
         ' Check if the lecture is by the same teacher.
         If String.Compare(teacher, GetTeacher(lecture), True) = 0 Then
            If (lecture.Start <> start) Or (lecture.End <> start.AddTicks(duration.Ticks)) Then
               ' Conflict! Lecture with a bad time frame.
               conflictedLectures.Add(lecture)
            Else
               ' No conflict!
               groups.Add(GetGroup(lecture))
            End If
         ' Lecture by a different teacher.
         Else
            ' Conflict! Lecture by a different teacher in the same room.
            conflictedLectures.Add(lecture)
         End If
      ' Lecture in a different room.
      Else
         ' Lecture by the same teacher.
         If String.Compare(teacher, GetTeacher(lecture), True) = 0 Then
            ' Conflict! Lecture of the same teacher in the different room.
            conflictedLectures.Add(lecture)
         ' Lecture by a different teacher.
         Else
            ' No conflict! Lecture by different teacher in a different room.
            groupsInDifferentRoomWithDifferentTeacher.Add(GetGroup(lecture))
            lecturesInDifferentRoomWithDifferentTeacher.Add(lecture)
         End If
      End If
   Next i

   ' Search for the groups which should be in different rooms at the same time.
   count = groups.Count
   For i = 0 To count - 1
      Dim conflictIndex As Integer = groupsInDifferentRoomWithDifferentTeacher.IndexOf(groups(i))
      If conflictIndex >= 0 Then
         conflictedLectures.Add(lecturesInDifferentRoomWithDifferentTeacher(conflictIndex))
      End If
   Next i 

   e.Conflicts.Clear()

   ' If the conflictedLectures will contain no lectures after this event occured, then e.Conflicts 
   ' will be empty and this will indicate that there are no conflicts.
   e.Conflicts.AddRange(conflictedLectures)

End Sub

' End of the AllowAppointmentConflicts event handler.
' ==================================================

' Additional functions.
' --------------------

' Determines the teacher for the specified lecture.
Function GetTeacher(lecture As Appointment) As String
   Dim teacher As Object = lecture.CustomFields("Teacher")
   If teacher Is Nothing Then
      Return String.Empty
   End If 

   Return teacher.ToString()
End Function

' Determines the group for the specified lecture.
Function GetGroup(lecture As Appointment) As String
   Dim group As Object = lecture.CustomFields("Group")
   If group Is Nothing Then
      Return String.Empty
   End If

   Return group.ToString()
End Function

e.Conflicts.Clear();
    FillConflictedAppointmentsCollection(e.Conflicts, e.Interval, ((SchedulerControl)sender).DataStorage.Appointments.Items, e.AppointmentClone);
}

e.Conflicts.Clear()
    FillConflictedAppointmentsCollection(e.Conflicts, e.Interval, CType(sender, SchedulerControl).DataStorage.Appointments.Items, e.AppointmentClone)
End Sub