How do I make it so everyone has a separate account when they login - vb.net

Question

I am trying to create a login system for a revision app. However, I was wondering if there was a way where everyone can have separate accounts.
Private Sub Btnlogin_Click(sender As Object, e As EventArgs) Handles Btnlogin.Click
Dim sqlstring As String
sqlstring = "select * FROM login where username = '" & txtusername.Text & "'"
connection.Open()
dataadapter = New OleDb.OleDbDataAdapter(sqlstring, connection)
dt.Clear()
dataadapter.Fill(dt)
connection.Close()
If dt.Rows.Count = 0 Then
MsgBox("no such user")
Exit Sub
End If
If dt.Rows(0)(2) = Txtpassword.Text Then
Flashcard.Show()
Else
Txtpassword.Text = ""
txtusername.Text = ""
MsgBox("Invalid username and password combination")
End If
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Register.Show()
End Sub
That's the code I have if a user already has an account.
Imports System.Data.OleDb
Public Class Register
Dim pro As String
Dim connstring As String
Dim command As String
Dim myconnection As OleDbConnection = New oledbconnection
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
pro = "provider=microsoft.ACE.OLEDB.12.0;Data Source=flashcard login.accdb"
connstring = pro
myconnection.ConnectionString = connstring
myconnection.Open()
command = " insert into login ([username],[password]) values ('" & TextBox1.Text & "','" & TextBox2.Text & "')"
Dim cmd As OleDbCommand = New OleDbCommand(command, myconnection)
cmd.Parameters.Add(New OleDbParameter("username", CType(TextBox1.Text, String)))
cmd.Parameters.Add(New OleDbParameter("password", CType(TextBox1.Text, String)))
MsgBox("You have successfully signed up!")
Form1.Show()
Try
cmd.ExecuteNonQuery()
cmd.Dispose()
myconnection.Close()
TextBox1.Clear()
TextBox2.Clear()
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
End Class
That's the code if the user presses the register button (does not have an account).
This code works where the user can succesfully login or register, but the problem is that everyone will have the same windows form. So, I was wondering if there was a way to make each windows form unique to each user?

Answer 1

You can declare and initialize a variable on the same line. The only information you need is if the userName and password exist in the database. Don't pull down data your don't need. You can get this with Count.
Always declare and dispose the connection and command it the method where they are used. Both of these objects need to be disposed so that their unmanaged resources can be released. Using... End Using blocks handle this.
Always use parameters to avoid sql injection. In OleDb the order that the parameters appear in the sql string must match the order that they are added to the parameters collection.
Don't open the connection until directly before the .Execute...
Private ConStr As String = "provider=microsoft.ACE.OLEDB.12.0;Data Source=flashcard login.accdb" '"Your connection string"
Private Sub Btnlogin_Click(sender As Object, e As EventArgs) Handles Btnlogin.Click
Dim rowCount As Integer
Dim sqlstring = "select Count(*) FROM login where [username] = #UserName ANd [password] = #password;"
Using connection As New OleDbConnection(ConStr),
cmd As New OleDbCommand(sqlstring, connection)
cmd.Parameters.Add("#UserName", OleDbType.VarChar).Value = txtusername.Text
connection.Open()
rowCount = CInt(cmd.ExecuteScalar)
End Using
If rowCount = 0 Then
MsgBox("no such user")
Else
Flashcard.Show()
End If
End Sub
Same ideas for the insert. A text box's Text property is always a String so you don't have to convert it.
You are not doing the parameters correctly. You are not using these parameters because the don't appear in the CommandText
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim Command = " insert into login ([username],[password]) values (#UserName, #Password);"
Try
Using myconnection As New OleDbConnection(ConStr),
cmd As OleDbCommand = New OleDbCommand(Command, myconnection)
cmd.Parameters.Add("#UserName", OleDbType.VarChar).Value = TextBox1.Text
cmd.Parameters.Add("#Password", OleDbType.VarChar).Value = TextBox2.Text
myconnection.Open()
cmd.ExecuteNonQuery()
End Using
Catch ex As Exception
MsgBox(ex.Message)
Exit Sub
End Try
MsgBox("You have successfully signed up!")
Form1.Show()
TextBox1.Clear()
TextBox2.Clear()
End Sub
Finally, and very important. You should never store passwords as plain text. Look into salting and encrypting.