如何通过比较两个数组来检索不同的字符串值？

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 List.

Then i thought of using foreach loop to remove the students who's id was in the Listbox

But when i defined the two list like total and present with values and tried to remove the string from total which are in present the nthe output was successful

private void checkabst_Click(object sender, EventArgs e)
        {
            string[] present1 = new string[listbox_present.Items.Count];
            List<string> present = new List<string> { ""};
            List<string> absent = new List<string>();
            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());
                }
            }
           present = listbox_present.Items.Cast<string>().ToList();

            foreach(string temp in present)
            {
                total.Remove(temp);
            }

            foreach (string temp in total)
            {
                listbox_absent.Items.Add(temp);
            }
        }

Stuck here from past few days.

The problem i think the listbox values are giving trouble while removing the string from total list.

Answer 1

You can achieve this simply by an Except method. No need to for loops

Remove following code

 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]);
            }

        }

and add

absent = allstd.Except(present).ToArray();

and iterate over these to show in MessageBox .

 foreach (var stud in absent)
        {
             MessageBox.Show(stud);
        }

Just to clarify after OP comment , Give a working example

 string[] present =  { "A" , "C" , "D" ,"P"};
 string[] absent = new string[3];
 string[] allstd = { "A", "B", "C" ,"D" , "E" , "P"};           

 absent=   allstd.Except(present).ToArray();

absent will contain "B" and "E"

Update after OP's latest code update.

Try replacing your method with following method with removed tow foreach loops

private void checkabst_Click(object sender, EventArgs e)
    {
        string[] present1 = new string[listbox_present.Items.Count];
        List<string> present = new List<string> { ""};
        List<string> absent = new List<string>();
        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());
            }
        }
       present = listbox_present.Items.Cast<string>().ToList();

       absent = total.Except(present).ToArray();
       foreach (var stud in absent)
       {
           MessageBox.Show(stud);
       }
    }

Answer 2

根据您在上述Ehsan回答中的评论，我不确定您是否知道：

System.Linq.Enumerable.Distinct