为什么我的组合框项目没有被发现和选择？

Question

Inspired by the answer here , I refactored my code to add a record to my generic list after its initil population via LINQ, and then attempt to select that added item in the combobox that is assigned the contents of the list.

For some context/conceptualization, the combo box gets populated with candidate students; if the week being displayed has already been scheduled, though, the student already scheduled in that slot for that week is inserted into the list that serves as the combobox's DataSource after the others. Finally, if such a student does exist in the combobox, my intent is to select that one (but the others remain available in the list in the event a change of assigned student needs to be made).

Here is the code:

private void PopulateBibleReadingComboBox()
{
    int BIBLE_READING_TALK_TYPE = 1;
    if (!System.IO.File.Exists(AYttFMConstsAndUtils.STUDENTS_FILENAME)) return;
    if (null == studentsList) return;
    string assignedStudentFirstname = string.Empty;
    string assignedStudentLastname = string.Empty;
    Student assignedStudent = null;

    if (currentWeekSaved)
    {
        DateTime currentWeek = Convert.ToDateTime(comboBoxWeekToSchedule.SelectedValue);
        AssignmentHistory ah = AYttFMConstsAndUtils.AssignmentHistList
            .FirstOrDefault(i => i.WeekOfAssignment == currentWeek && i.TalkType == 1);
        assignedStudentFirstname = AYttFMConstsAndUtils.GetStudentFirstNameForID(ah.StudentID_FK);
        assignedStudentLastname = AYttFMConstsAndUtils.GetStudentLastNameForID(ah.StudentID_FK);
        assignedStudent = new Student() {FirstName = assignedStudentFirstname, LastName = assignedStudentLastname, StudentID = ah.StudentID_FK};
    }
    List<Student> BRStudents =
    studentsList.Where(h => h.EnrolledInAYttFM)
        .Where(i => i.RecommendedNextTalkTypeID.Equals(BIBLE_READING_TALK_TYPE))
        .OrderBy(j => j.WeekOfLastAssignment)
        .ToList();
    if (null != assignedStudent)
    {
        BRStudents.Add(assignedStudent);
    }
    comboBoxBR.DataSource = BRStudents;
    comboBoxBR.DisplayMember = "FullName";
    comboBoxBR.ValueMember = "StudentID";
    if (null != assignedStudent))
    {
        comboBoxBR.SelectedIndex = comboBoxBR.Items.IndexOf(assignedStudent.FullName);
    }
}

The problem is that although the conditional "SelectedIndex/IndexOf" line is reached, and assignedStudent.FullName is what it should be, and is now added to the list and then the combobox, that item is not selected with that line:

comboBoxBR.SelectedIndex = comboBoxBR.Items.IndexOf(assignedStudent.FullName);

Rather, comboBoxBR.SelectedIndex is -1 (although, again, that fullName does exist in the combobox at that point).

Note: The "FullName" member of the Student class is calculated:

public class Student
{
    public int StudentID { get; set; }
    . . .
    public string FirstName { get; set; }
    public string LastName { get; set; }
    . . .
    public string FullName
    {
        get
        {
            return string.Format("{0} {1}", FirstName, LastName);
        }
        set { ; } 
    }
}

Answer 1

The list is made up of Student objects, but FullName is a string. You need to look for the entire Student object. You'll want to change the line in question to:

comboBoxBR.SelectedIndex = comboBoxBR.Items.IndexOf(assignedStudent);

Answer 2

When you use a DataSource it is as if the objects have been added to the collection (and in some form or fashion that may well be the case under the hood). So, searching for just the name in the items collection will fail. A more MCVE:

Students = new List<Student>();

Students.Add(new Student(7, "Zowie", "Halston"));
Students.Add(new Student(6, "Ziggy", "Watson"));
Students.Add(new Student(18, "Zalgo", "d'Artagnan"));
Students.Add(new Student(67, "Tabitha", "Black"));

Student luckyStudent = Students.First(w => w.FirstName == "Ziggy");

cbo1.DataSource = Students;
cbo1.DisplayMember = "FullName";
cbo1.ValueMember = "Id";

Then, setting the selection:

if (luckyStudent != null)
{ 
    // set selected: (WORKS):
    //cbo1.SelectedItem = luckyStudent;

    // set Index of item (WORKS):
    //cbo1.SelectedIndex = cbo1.Items.IndexOf(luckyStudent);

    // set Index of item name (FAILS):
    cbo1.SelectedIndex = cbo1.Items.IndexOf(luckyStudent.FullName);
}   

Generally, when using a DataSource , I try to avoid fiddling with the items collection at all. If you try to add or remove from items you get yelled at. So, for this, I would use

cbo1.SelectedItem = luckyStudent;

You can find them in the collection, but it can help you lose track that the control is bound.