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?