LINQ: why does this query not work on an ArrayList?

Question

public static  ArrayList   GetStudentAsArrayList()
{
    ArrayList  students = new ArrayList
    {
        new Student() { RollNumber = 1,Name ="Alex " , Section = 1 ,HostelNumber=1 },
        new Student() { RollNumber = 2,Name ="Jonty " , Section = 2 ,HostelNumber=2 }
    };
    return students;
}

The following code doesn't compile. The error is ArrayList is not IEnumerable

ArrayList lstStudents = GetStudentAsArrayList();
var res = from r in lstStudents select r;  

This compiles:

ArrayList lstStudents = GetStudentAsArrayList();
var res = from  Student   r in lstStudents select r;

Can anybody explain what the difference is between these two snippets? Why the second works?

Answer 1

Since ArrayList allows you to collect objects of different types, the compiler doesn't know what type it needs to operate on.

The second query explicitly casts each object in the ArrayList to type Student.

Consider using List<> instead of ArrayList.

Answer 2

In the second case, you're telling LINQ what the type of the collection is. ArrayList is weakly typed, so in order to use it effectively in LINQ you can use Cast<T>:

IEnumerable<Student> _set = lstStudents.Cast<Student>();