How to handle violating primary key constraints in asp.net? - asp.net

Question

I have a login page, users have ID's and ID is the primary key in the table. I also have an admin account, and admin can create users. But when i create a user account with an existing ID, the web page crashes. I want to handle this situation and give a warning indicating that this ID exists and cannot be created. Here is my code:
public void CreateStudent(int ID, String status, String email, String firstName, String lastName, String password, String level, String program)
{
SqlConnection con = new SqlConnection(GetConnectionString());
string query1 = "insert into StudentTable(Name,Surname,ID,email,level,program,status,password,Type) values(#firstName,#lastName,#ID,#email,#level,#program,#status,#password,'Student')";
SqlCommand command = new SqlCommand(query1,con);
command.Parameters.AddWithValue("#firstName", firstName);
command.Parameters.AddWithValue("#lastName", lastName);
command.Parameters.AddWithValue("#ID", ID);
command.Parameters.AddWithValue("#email", email);
command.Parameters.AddWithValue("#level", level);
command.Parameters.AddWithValue("#program", program);
command.Parameters.AddWithValue("#status", status);
command.Parameters.AddWithValue("#password", password);
int result;
con.Open();
result = command.ExecuteNonQuery();
con.Close();
}
Can anyone help me with this? Thanls

Answer 1

There are multiple ways. You could catch an Exception and display an error message. That will also help in other error scenario's, like a lost connection.
However, if it's a situation you expect to occur during normal operation, you should handle the situation without an Exception. One way to do that is to have your insert only insert a row with a new id:
insert YourTable
(id, col1, col2, ...)
select #id
, #col1
, #col2
, ...
where not exists
(
select *
from YourTable
where id = #id
)
Pass parameters to your query like:
command.Parameters.AddWithValue("#id", 42);
command.Parameters.AddWithValue("#col1", "value1");
command.Parameters.AddWithValue("#col2", "value2");
Now ExecuteNonQuery() returns the number of affected rows. You can use that to check if the insert actually added a new row to the table:
var result = command.ExecuteNonQuery();
if (result == 1)
{
lblResult.Text = "New row inserted!";
lblResult.Color = Color.Green;
}
else
{
lblResult.Text = "Failed to insert new row.";
lblResult.Color = Color.Red;
}

Answer 2

Rather than tying to handle the error that comes back from attempting to add a user with the same ID, it would be better to check for the user's existence first and only create the new account if you can. Though you still need to handle the exception to cater for the case where two people try to create the same record.
So you'd have the following SQL:
select * from StudentTable where ID = #newID
and if this found a record you could report the error. If it doesn't find anything then you can go ahead and create the new record.
Though if you are using the ID as the primary key it would be better to have that as an Identity column on the table and have it auto-increment when you create a new row. You'd still have to check to see if the student already exists - use the e-mail as the human readable uniqueness condition.
If you add uniqueness constraint on the e-mail column then you'll be able to trap the case of two people trying to create the same record.

Answer 3

I have made the e-mail attribute as a primary key and then checked the database whether same e-mail exists and that worked