there is value is no incremented in the text box

Question

This is my code for generating roll no in pattern of

txtrollno = dropdownlist1.selectedvalue+ "00" + 1

i.e in dropdown list we can select a course id and its is start of roll no means my roll no is 306001 i.e my course id is 306 

and there is no increment is done 

 

 

 

con.Open();

a = "select count(*),Course_ID from Student_Master where Course_ID="+DropDownList3.SelectedValue +"group by Course_ID";

cmd = new SqlCommand(a,con);

try

{

rdr = cmd.ExecuteReader();

while (rdr.Read())

{

str1=rdr.GetInt32(0).ToString();

cnt = rdr.GetInt32(0);

txtRollNo.Text = str1;

}

rdr.Close();

}

catch(Exception ex)

{

con.Close();

throw ex;

}

str1=DropDownList3.SelectedValue;

if (cnt > 0)

{

if (cnt <= 9)

newid = str1 + "00" + cnt;

else

newid = str1 + "0" + cnt;

}

else

{//--- if no records in database

str1 = DropDownList3.SelectedValue;

newid = str1 + "00" + 1;

}

if (newid != "")

txtRollNo.Text = newid;