How to retrieve Distinct strings from comparing two Arrays?

Question

I want to retrieve Absent student names from sql database. I have a Listbox in which i have Present student Rfid data (Something like 42 39 A0 11), I have stored the student details along with Rfid in database as nvarchar(1500) datatype.

 

Using the present stud id in list box i want to retrieve absent students name in an array.

 

I used NOT IN condition in where but i was not able to retrieve

 

Then i thought of using for loops to match the get the students who's id was not in the Listbox

 

But still I am losing the values and not getting proper absent students Id.

private void checkabst_Click(object sender, EventArgs e)
{
string[] present = new string[3];
string[] absent = new string[3];
string[] allstd = new string[3];
List<string> total = new List<string>();
using (SqlConnection con = new SqlConnection(cs))
{
SqlCommand cmd = new SqlCommand("Select Rfid_Uid From Studetails", con);
con.Open();
SqlDataReader sdr = cmd.ExecuteReader();
while (sdr.Read())
{
total.add(sdr[0].ToString());
}
allstd=total.toArray();
for(int i=0;i
{
present[i] = listbox_present.Items[i].ToString();
}
bool isfound = false;
for (int i = 0; i < 3; i++)
{
isfound = false;
for (int j = 0; j < present.Length; j++)
{
if (allstd[i] == present[j])
{
isfound = true;
}
}
if (!isfound)
{
MessageBox.Show(allstd[i]);
}
}

Stuck here from past few days.

I am getting Present as well as the absent student Ids in messagebox.