how to save multiple selected values of dropdownlist in mvc

hi I have a model class in which i have two properties as arrays like student and courses and on view i have multi select dropdown list to select multiple students and multiple courses , Now on post in my controller i am getting the selected values as arrays , but i am confused like when i am looping through all selected values i am saving them into a string or int variable , but i am unable to assign this string or int value to the model object properties as it is of type array below is the code can someone correct this code as i need to loop through the students first and then assign all courses to first student then second students and then so one .

 

public ActionResult Create([Bind(Include = "Id,Year_Id,Program_Id,Student_id,Module_Id,Course_Id")]Student_Assigned_courses SAC)

{

ViewBag.PopulatePrograms = _IEducation.PopulatePrograms();

if (SAC.Student_id != null)

{

foreach(var student in SAC.Student_id)

{

foreach(var course in SAC.Course_Id)

{

Student_Assigned_courses Sa = new Student_Assigned_courses();

for(int i=0;i

{

string s = SAC.Student_id[i].ToString();

//Sa.Student_id = Convert.ToInt32(SAC.Student_id[i].ToString());

Sa.Course_Id = SAC.Course_Id;

Sa.Module_Id = SAC.Module_Id;

Sa.Year_Id = SAC.Year_Id;

Sa.Program_Id = SAC.Program_Id;

db.StudentCoursesAssigned.Add(Sa);

// Sa.Student_id = SAC.Student_id[i];

}

if (db.SaveChanges() > 0)

{

TempData["MessageDesignation"] = "Department Saved Successfully!";

}

}

}

return RedirectToAction("Index");

}

return View(SAC);

}yellow highlighted line gives error