丢弃参数C＃中的对象类型

Question

This is the portion of code I am having the problem with;

List<People> people = new List<People>();

and this is being populated with 3 different types of objects, all derived from the People class;

people.Add(new Student(constructorarguments);
people.Add(new AcademicStaff(constructorarguments);
people.Add(new AdministrativeStaff(constructorarguments);

and this is the code which is giving me problems;

private void studentCheckbox_CheckedChanged(object sender, EventArgs e)
{
    if (studentCheckbox.CheckState == CheckState.Checked)
    {
        foreach (Student student in people)
        {
            if (student.Compare(SearchTextBox.Text) == 0)
            {
                resultsListBox.Items.Add(student.Forename);
            }
        }
    }
    else
    {}
}

It's using a windows form, as you can see from the CheckState code. But the problem I'm having is that it's not restricting the compare to students. It tries to carry on past the students and tries to cast the AcademicStaff as Students, which is where my program faults. I've been struggling for the past couple of hours on this and have made no leeway, any help would be much appreciated!

I don't believe the problem to be with the form or the classes themselves, but I don't know why it's trying to move on to the AcademicStaff when I've restricted it to the Student type

Answer 1

It is expected behaviour, compiler enumerates your collection and implicitly cast each item to Student type.
If you want to loop only Students you should filter your people collection first with OfType<>() :

foreach(var student in people.OfType<Student>())
{
  ..
}

Answer 2

You can't iterate using "Student" Type on a list that has people, this code shouldn't compile. You need to iterate something like that :

foreach (People person in people)

and if you want to cast the current person as student you should write :

Student s = person as Student;

and then you will be able the access the student methods.

Answer 3

You could use the linq-method OfType():

private void studentCheckbox_CheckedChanged(object sender, EventArgs e)
{
    if (studentCheckbox.CheckState == CheckState.Checked)
    {
        foreach (Student student in people.OfType<Student>())
        {
            if (student.Compare(SearchTextBox.Text) == 0)
            {
                resultsListBox.Items.Add(student.Forename);
            }
        }
    }
    else
    {}
}

Hope this will help you in your quest.