Структура в List<>, "Не удалось изменить возвращаемое значение"

namespace
{
   struct  STRUCT
            {
                 internal float a;
                 internal float b;
            }
   class Storage
           {
                 List<STRUCT> _Array;
           }
           void Function(float b)
           {
                 //всё внимание на присваивание
                 _Array[0].a=b;
           }
}

var str = _Array[0];
str.a=b;
_Array[0] = str;

public static List<studentInfo> students = new List<studentInfo>();
 
static public void setStudentInfo(studentInfo student, string fieldName, object value)
        {
            studentInfo StudentWithNewInfo = student;
            Debug.WriteLine("1. "+StudentWithNewInfo.firstName + " " + StudentWithNewInfo.middleName + " " + StudentWithNewInfo.secondName);
            typeof(studentInfo).GetField(fieldName).SetValue(StudentWithNewInfo, value);
            Debug.WriteLine("2. " + StudentWithNewInfo.firstName + " " + StudentWithNewInfo.middleName + " " + StudentWithNewInfo.secondName);
            students[students.IndexOf(student)] = StudentWithNewInfo;
            Debug.WriteLine("3. " + students[students.IndexOf(student)].firstName + " " + students[students.IndexOf(student)].middleName + " " + students[students.IndexOf(student)].secondName);
           
}

studentInfo StudentWithNewInfo = student;
Debug.WriteLine("1. "+StudentWithNewInfo.firstName + " " + StudentWithNewInfo.middleName + " " + StudentWithNewInfo.secondName);
 
object temp = StudentWithNewInfo;
typeof(studentInfo).GetField(fieldName).SetValue(temp, value);
StudentWithNewInfo = (studentInfo)temp;
 
Debug.WriteLine("2. " + StudentWithNewInfo.firstName + " " + StudentWithNewInfo.middleName + " " + StudentWithNewInfo.secondName);
students[students.IndexOf(student)] = StudentWithNewInfo;

interface IChangeStructValue{void Change(int i, int j);}

IChangeStructValue x = (MyStruct)myList[0]; x.Change(1,1);

struct myStruct:IChangeStructValue{ public int x{get;set;} public int y{get;set;}public void Change(int x, int y){this.x=x;this.y=y}}

Object o = myList[0];((IChangeStructValue) o).Change(5, 5);

studentInfo[] students = new studentInfo[10];
students[0].x = 32; // OK