how to check duplicate record before insert, using vb.net and sql? - sql

Question

can someone help me with my code, i need to check first if record exist. Well i actually passed that one, but when it comes to inserting new record. im getting the error "There is already an open DataReader associated with this Command which must be closed first." can some help me with this? thanks.
Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim reg_con As SqlConnection
Dim reg_cmd, chk_cmd As SqlCommand
Dim checker As SqlDataReader
Dim ID As Integer
Dim fname_, mname_, lname_, gender_, emailadd_, college_, password_ As String
ID = idnumber.Value
fname_ = fname.Value.ToString
mname_ = mname.Value.ToString
lname_ = lname.Value.ToString
gender_ = gender.Value.ToString
college_ = college.Value.ToString
emailadd_ = emailadd.Value.ToString
password_ = reg_password.Value.ToString
reg_con = New SqlConnection("Data Source=JOSH_FLYHEIGHT;Initial Catalog=QceandCceEvaluationSystemDatabase;Integrated Security=True")
reg_con.Open()
chk_cmd = New SqlCommand("SELECT IDnumber FROM UsersInfo WHERE IDnumber = '" & ID & "'", reg_con)
checker = chk_cmd.ExecuteReader(CommandBehavior.CloseConnection)
If checker.HasRows Then
MsgBox("Useralreadyexist")
Else
reg_cmd = New SqlCommand("INSERT INTO UsersInfo([IDnumber], [Fname], [Mname], [Lname], [Gender], [Emailadd], [College], [Password]) VALUES ('" & ID & "', '" & fname_ & "', '" & mname_ & "', '" & lname_ & "', '" & gender_ & "', '" & emailadd_ & "', '" & college_ & "', '" & password_ & "')", reg_con)
reg_cmd.ExecuteNonQuery()
End If
reg_con.Close()
End Sub

Answer 1

Add this string to your connection string
...MultipleActiveResultSets=True;";
Starting from Sql Server version 2005, this string allows an application to maintain multiple active statements on a single connection. Without it, until you close the SqlDataReader you cannot emit another command on the same connection used by the reader.
Apart from that, you insert statement is very dangerous because you use string concatenation. This is a well known code weakness that could result in an easy Sql Injection vulnerability
You should use a parameterized query (both for the insert and for the record check)
reg_cmd = New SqlCommand("INSERT INTO UsersInfo([IDnumber], ......) VALUES (" & _
"#id, ......)", reg_con)
reg_cmd.Parameters.AddWithValue("#id", ID)
.... add the other parameters required by the other field to insert.....
reg_cmd.ExecuteNonQuery()
In a parameterized query, you don't attach the user input to your sql command. Instead you put placeholders where the value should be placed (#id), then, before executing the query, you add, one by one, the parameters with the same name of the placeholder and its corresponding value.

Answer 2

You need to close your reader using checker.Close() as soon as you're done using it.

Answer 3

Quick and dirty solution - issue checker.Close() as a first command of both IF and ELSE block.
But (better) you don't need a full blown data reader to check for record existence. Instead you can do something like this:
chk_cmd = New SqlCommand("SELECT TOP (1) 1 FROM UsersInfo WHERE IDnumber = '" & ID & "'", reg_con)
Dim iExist as Integer = chk_cmd.ExecuteScalar()
If iExist = 1 Then
....
This approach uses ExecuteScalar method that returns a single value and doesn't tie the connection.
Side note: Instead of adding parameters like you do now - directly to the SQL String, a much better (and safer) approach is to use parametrized queries. Using this approach can save you a lot of pain in the future.