Develop Reference

Eliminating an option from a ComboBox (2), based on the input of the first ComboBox (1) c#

I am creating an airline booking system and I have 2 combo boxes. The first is for Departure City and the second is for Arrival City. I want to be able to eliminate the choice in the first combo box from the second, as I don't want the same city to be able to be submitted as both the departure and arrival city. I am querying the city names from a database.
Here is my code:
public partial class main : Form
{
public main()
{
InitializeComponent();
string connectionString = #"Base Schema Name=cyanair;data source=C:\Users\Client 0819\source\repos\Cyanair\cyanair.db";
//Departure ComboBox
SQLiteConnection conn = new SQLiteConnection(connectionString);
try
{
conn.Open();
SQLiteCommand cmd = new SQLiteCommand();
cmd.Connection = conn;
cmd.CommandType = System.Data.CommandType.Text;
cmd.CommandText = "SELECT * FROM CyanairAirports";
SQLiteDataAdapter da = new SQLiteDataAdapter(cmd);
DataTable dt = new DataTable();
da.Fill(dt);
comboDeparture.DataSource = dt;
comboDeparture.ValueMember = "Descriptions";
comboDeparture.DisplayMember = "Descriptions";
conn.Close();
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}
//Arrival ComboBox
private void comboDeparture_DisplayMemberChanged(object sender, EventArgs e)
{
string connectionString = #"Base Schema Name=cyanair;data source=C:\Users\Client 0819\source\repos\Cyanair\cyanair.db";
SQLiteConnection conn = new SQLiteConnection(connectionString);
**String city = comboDeparture.DisplayMember;**
try
{
conn.Open();
SQLiteCommand cmd = new SQLiteCommand();
cmd.Connection = conn;
cmd.CommandType = System.Data.CommandType.Text;
cmd.CommandText = "SELECT * FROM CyanairAirports WHERE Descriptions IS NOT '" + comboDeparture.SelectedValue.ToString() + "'";
richTextBox1.Text = "SELECT * FROM CyanairAirports WHERE Descriptions IS NOT '" + comboDeparture.SelectedValue + "'";
SQLiteDataAdapter da = new SQLiteDataAdapter(cmd);
DataTable dt = new DataTable();
da.Fill(dt);
comboArrival.DataSource = dt;
comboArrival.ValueMember = "Descriptions";
comboArrival.DisplayMember = "Descriptions";
conn.Close();
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}
Thanks :)

It looks like you're handling the DisplayMemberChanged event on comboDeparture, and trying to update the values of comboArrival in that handler. However, DisplayMemberChanged only triggers when the DisplayMember property changes.
DisplayMember only tells the control which property to display on a data bound control. It isn't tied to the index or value selected in the ComboBox. So, the only time the code to populate comboArrival runs is in the constructor when you set comboDepartarture.DisplayMember. Instead, handle either ComboBox.SelectedIndexChanged or ComboBox.SelectedValueChanged and set the items of comboArrival.
A few other important things to note about your code.
First, you should use a parameterized query when running Sql Statements, rather than concatenating strings. Concatenating strings as you're doing opens you up to SQL Injection Attacks. I'm not familiar with SqlLite and can't provide you with an example of how to modify your code, but perhaps this question can help.
Second, you don't need to re-run the query every time you change the selected value in comboDeparture. Just add comboArrival's data source as a field on the Form and you can filter it. For example...
public partial class main : Form
{
// Your constructors...
private void comboDepartures_SelectedIndexChanged(object sender, EventArgs e)
{
if (_arrivalsDataSource == null)
{
_arrivalsDataSource = new System.Data.DataTable();
// Load _arrivalsDataSource from the database, basically how you're doing it now.
comboArrival.DataSource = _arrivalsDataSource.DefaultView;
comboArrival.DisplayMember = "Descriptions"
comboArribal.ValueMember = "Descriptions"
}
if (comboDeparture.SelectedIndex == -1)
{
_arrivalsDataSource.DefaultView.RowFilter = null; // Clear the filter.
}
else
{
// Set the filter.
_arrivalsDataSource.DefaultView.RowFilter = $"Description <> '{comboDeparture.SelectedValue}'";
}
}
private System.Data.DataTable _arrivalsDataSource = null;
}

Using SAME Textbox to search and insert data in dataGridView

I have a Textbox with which I want to be able to Search and Insert data into Table. Insert works fine with one exception: When I try to Insert data that isn't already in DB(it's searching while I'm typing) it gives me:
"Exception User-Unhandled System.NullReferenceException: 'Object
reference not set to an instance of an object.'
System.Windows.Forms.DataGridView.CurrentRow.get returned null.
I think I'm missing something in the Search code.
//UPDATE: All of the code.// This is my Insert and Search code:
namespace UDDKT
{
public partial class FrmGlavna : Form
{
DataSet ds = new DataSet();
SqlDataAdapter DaDavaoci = new SqlDataAdapter();
SqlDataAdapter DaAkcije = new SqlDataAdapter();
SqlConnection cs = new SqlConnection(#"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\UDDKT.mdf;Integrated Security=True");
public FrmGlavna()
{
InitializeComponent();
}
//Popunjava DataGridViews sa podacima iz baze
private void FrmGlavna_Load(object sender, EventArgs e)
{
SqlCommand SlctDavaoci = new SqlCommand("SELECT * FROM Davaoci ORDER BY DavaocID DESC", cs);
DaDavaoci.SelectCommand = SlctDavaoci;
DaDavaoci.Fill(ds, "TblDavaoci");
SqlCommand SlctAkcije = new SqlCommand("SELECT * FROM AkcijaDDK", cs);
DaAkcije.SelectCommand = SlctAkcije;
DaAkcije.Fill(ds, "TblAkcije");
DgDavaoci.DataSource = ds.Tables["TblDavaoci"];
}
//Povezuje DataGridViews Davaoca i Akcija
private void DgDavaoci_SelectionChanged(object sender, EventArgs e)
{
ds.Tables["TblAkcije"].DefaultView.RowFilter = "DavaocID =" + DgDavaoci.CurrentRow.Cells["DavaocID"].Value;
DgAkcije.DataSource = ds.Tables["TblAkcije"];
}
//Osvježava DataGridView nakon unosa/izmjene/brisanja podataka u bazu
private void RefreshTable()
{
SqlConnection cs = new SqlConnection(#"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\UDDKT.mdf;Integrated Security=True");
String query = "SELECT * FROM Davaoci ORDER BY DavaocID DESC";
SqlCommand cmd = new SqlCommand(query, cs);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
da.Fill(dt);
DgDavaoci.DataSource = dt;
}
//Čisti TextBox nakon upisa/izmjene/brisanja podataka u bazu
public void ClearTxtBx()
{
TxtIme.Clear();
TxtPrezime.Clear();
TxtTezina.Clear();
TxtAdresa.Clear();
TxtBrojTel.Clear();
TxtBrojLK.Clear();
}
//Upis podataka u Tabelu Davaoci
private void BtnDodajDavaoca_Click(object sender, EventArgs e)
{
String query = "INSERT INTO Davaoci (Ime,Prezime,Pol,DatumRodjenja,KrvnaGrupa,Tezina,Adresa,BrojTel,BrojLK) VALUES (#Ime, #Prezime, #Pol, #DatumRodjenja, #KrvnaGrupa, #Tezina, #Adresa, #BrojTel, #BrojLK)";
using (SqlConnection cs = new SqlConnection(#"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\UDDKT.mdf;Integrated Security=True"))
using (SqlCommand command = new SqlCommand(query, cs))
{
command.Parameters.Add("#Ime", SqlDbType.NVarChar).Value = TxtIme.Text;
command.Parameters.Add("#Prezime", SqlDbType.NVarChar).Value = TxtPrezime.Text;
command.Parameters.Add("#Pol", SqlDbType.NChar).Value = TxtPol.Text;
command.Parameters.Add("#DatumRodjenja", SqlDbType.Date).Value = TxtDatumRodjenja.Text;
command.Parameters.Add("#KrvnaGrupa", SqlDbType.VarChar).Value = TxtKrvnaGrupa.Text;
command.Parameters.Add("#Tezina", SqlDbType.Float).Value = TxtTezina.Text;
command.Parameters.Add("#Adresa", SqlDbType.NVarChar).Value = TxtAdresa.Text;
command.Parameters.Add("#BrojTel", SqlDbType.NVarChar).Value = TxtBrojTel.Text;
command.Parameters.Add("#BrojLK", SqlDbType.NVarChar).Value = TxtBrojLK.Text;
cs.Open();
command.ExecuteNonQuery();
cs.Close();
RefreshTable();
ClearTxtBx();
}
}
//Pretraga postojećih Davalaca
private void TxtIme_TextChanged(object sender, EventArgs e)
{
(DgDavaoci.DataSource as DataTable).DefaultView.RowFilter = string.Format("Ime LIKE '{0}%'", TxtIme.Text);
}
}
}
}
Here is the MockUp of the Form before I begin to type/search/insert Data that isn't already in the Table (First Textbox*).
And after I start typing Name(Име) that starts with an "A" (name that isn't already in the Table).
I want to Search DB for that Column, but if there aren't any existing names, I want to be able to continue typing (without interuption) so that I can Insert new data into table.

DgDavaoci.CurrentRow in your DgDavaoci_SelectionChanged method is null, so attempting to access DgDavaoci.CurrentRow.Cells["DavaocID"] throws the NullReferenceException. The reason, best I can tell, is as follows:
You begin to type a value into your text box, a value that happens not to be found in the data set. As you type, you cause the TxtIme_TextChanged method to execute. It filters according to your search, and since the value is not found, it filters out every row in the set. Here's the important part: whenever the data set is filtered, it has the possibility of causing DgDavaoci_SelectionChanged to execute. Since the selection changed from the first row to no row at all (since there are no filtered rows to display), this method does execute. Now, when the method attempts to access the current row, there is no current row, and so we get a null here. Attempting to access a field of null throws the exception you're getting.
How can you fix this behavior? A simple null-check in DgDavaoci_SelectionChanged should do the trick. It looks to me like you can simply return from that method if(DgDavaoci.CurrentRow == null), or you can code in additional behavior. Just perform a check so that you don't reference the null object.

Probably the filter inside TxtIme_TextChanged is causing the DataGridView's SelectionChanged event to fire and the code is entering DgDavaoci_SelectionChanged. The exception indicates that DgDavaoci.CurrentRow is null, so you'll need to handle the case where DgDavaoci.CurrentRow is null in DgDavaoci_SelectionChanged.
A simple way to deal with this would be to just check DgDavaoci.CurrentRow is null and return from the function if that evaluates to true.
private void DgDavaoci_SelectionChanged(object sender, EventArgs e)
{
if (DgDavaoci.CurrentRow is null)
{
return;
}
ds.Tables["TblAkcije"].DefaultView.RowFilter = "DavaocID =" +
DgDavaoci.CurrentRow.Cells["DavaocID"].Value;
DgAkcije.DataSource = ds.Tables["TblAkcije"];
}
It looks like you might have a second DataGridView (DgAkcije) that is designed to show the details of the currently selected row in DgDavaoci. So, another approach might be to just clear DgAkcije if DgDavaoci.CurrentRow is null.
private void DgDavaoci_SelectionChanged(object sender, EventArgs e)
{
if (DgDavaoci.CurrentRow is null)
{
DgAkcije.DataSource = null; //I'm not 100% sure this will work, I haven't tested it.
return;
}
ds.Tables["TblAkcije"].DefaultView.RowFilter = "DavaocID =" +
DgDavaoci.CurrentRow.Cells["DavaocID"].Value;
DgAkcije.DataSource = ds.Tables["TblAkcije"];
}
Ultimately, however, you'll have to decide what you want to happen when DgDavaoci_SelectionChanged is called but DgDavaoci.CurrentRow is null.

Solution if anyone else is interested:
//Povezuje DataGridViews Davaoca i Akcija
private void DgDavaoci_SelectionChanged(object sender, EventArgs e)
{
if (DgDavaoci.CurrentRow != null)
{
ds.Tables["TblAkcije"].DefaultView.RowFilter = "DavaocID =" + DgDavaoci.CurrentRow.Cells["DavaocID"].Value;
DgAkcije.DataSource = ds.Tables["TblAkcije"];
}
}

C#(loading search results in textbox)

I am trying to search the database and set the results in textboxes. I am
getting error, which says "invalid cast exception". I need your guide please.
private void btn_search_Click(object sender, EventArgs e)
{
con.Open();
string STR="select * from TICKETSALES where REFERENCE="+txtSearch.Text;
cmd = new SqlCommand(STR,con );
dr = cmd.ExecuteReader();
if(dr.Read())
{
txtTrans.Text = dr.GetInt32("TRANSACTIONNUMBER").ToString();
txtPax.Text = dr.GetString("PASSENGERNAME");
}
else
{
MessageBox.Show("Ticket Number not Found");
}
}

Modify your select statement to get required column details.
While assigning values to test box, use column index to get value from dr
Convert value to string assign it to respective textbox
Here is sample implementation
con.Open();
//Use of camelCasing. transactionNumber instead of TRANSACTIONNUMBER
string STR="select transactionNumber,passengerNumber from TICKETSALES where REFERENCE=#search";
cmd = new SqlCommand(STR,con );
cmd.Parameters.Add("#search", txtSearch.Text);
dr = cmd.ExecuteReader();
if(dr.Read())
{
txtTrans.Text = Convert.ToString(dr[0]);
txtPax.Text = Convert.ToString(dr[1]);
}
Few tips for best coding practices (Credits: #tsahi-asher)
Don't pass values inside query, use parameters in query and use Paramere.Add() function to replace parameter with its value
Don't put your sql statements in presentation layer. Have some dedicated layer of SQL.
Don't use select *, use specific column name.
Don't use all-caps for identifier names, use camelCase.

How about something like this:
Note the sql injection protection by paramtising the sql query.
private void btn_search_Click(object sender, EventArgs e)
{
using (SqlConnection con = new SqlConnection(connectionString))
{
string query = "select top 1 TRANSACTIONNUMBER, PASSENGERNAME from ticketsales where reference=#ref";
using (SqlDataAdapter adap = new SqlDataAdapter(query, con))
{
con.Open();
DataTable dt = new DataTable();
adap.SelectCommand.Parameters.AddWithValue("#ref", txtSearch.Text.Trim());
adap.Fill(dt);
if (dt.Rows.Count > 0)
{
txtTrans.Text = dt.Rows[0]["TRANSACTIONNUMBER"].ToString().Trim();
txtPax.Text = dt.Rows[0]["PASSENGERNAME"].ToString().Trim();
}
else
{
MessageBox.Show("Ticket Number not Found");
}
}
}
}

There are few issue already mentioned in comments and posts. I will chip in my remarks - you don't dispose of unmanaged resources, one answer covers that, but it violates your code. So here is alternative solution:
SqlConnection con;
SqlCommand cmd;
SqlDataReader dr;
//some methods, fields
private void btn_search_Click(object sender, EventArgs e)
{
con.Open();
// as it has benn already said, you have to prevent yourself from SQL injection!
cmd = (new SqlCommand("select * from TICKETSALES where REFERENCE=#ref", con)).Parameters.AddWithValue("#res", txtSearch.Text.Trim());
dr = cmd.ExecuteReader();
if (dr.Read())
{
txtTrans.Text = dr.GetInt32("TRANSACTIONNUMBER").ToString();
txtPax.Text = dr.GetString("PASSENGERNAME");
}
else
{
MessageBox.Show("Ticket Number not Found");
}
}
// it looks like you have unamanaged resources held by fields in your form,
// so to release them you have to call their Dispose() method!
// normally you should use using keyword if they were used locally in a method, as other answer states
public void Dispose()
{
base.Dispose();
if (con != null) con.Dispose();
if (cmd != null) cmd.Dispose();
if (dr != null) dr.Dispose();
}

Database MS Access 2007 connection to C# ASP.net , login page

I have a database that contains a table named "User(login,password,firstname,lastname)" . And I need to make login page . I've watched some tutorials , but it didn't help . I need to check if login and password exist in the database . and then redirect(if correct) to other page . This is what I already did:
OleDbConnection con = new OleDbConnection();
public bool check()
{
con.ConnectionString = #"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Volodia\Documents\WebSiteDatabase.accdb";
con.Open();
string commandstring = "SELECT login,password FROM User";
//objadapter = new SqlDataAdapter(CommandString, sqlconn.ConnectionString);
OleDbDataAdapter objadapter = new OleDbDataAdapter(commandstring, con.ConnectionString);
DataSet dataset = new DataSet();
objadapter.Fill(dataset, "User");// it shows "Syntax error in FROM clause." here
DataTable datatable = dataset.Tables[0];
for (int i = 0; i < datatable.Rows.Count; i++)
{
string unam = datatable.Rows[i]["login"].ToString();
string upwd = datatable.Rows[i]["password"].ToString();
if ((unam == TextBox1.Text)&&(upwd==TextBox2.Text))
{
return true;
}
}
return false;
}
protected void Button1_Click(object sender, EventArgs e)
{
if (check() == true)
{
Response.Redirect("WebForm2.aspx");
}
}

The word PASSWORD is a reserved keyword for MS-Access Jet SQL. If you want to use it you need to enclose it in square brackets, the same for USER
string commandstring = "SELECT login, [password] FROM [User]";
This will resolve the immediate problem of the Syntax Error but let me add some other code to show a different approach
public bool check()
{
string conString = #"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Volodia\Documents\WebSiteDatabase.accdb";
using(OleDbConnection con = new OleDbConnection(conString)
{
con.Open();
string commandstring = "SELECT count(*) as cntUser FROM [User] " +
"WHERE login = ? AND [password] = ?";
using(OleDbCommand cmd = new OleDbCommand(commandstring, con))
{
cmd.Parameters.AddWithValue("#p1", TextBox1.Text);
cmd.Parameters.AddWithValue("#p2", TextBox2.Text);
int result = (int)cmd.ExecuteScalar();
if(result > 0)
return true;
}
}
return false;
}
First, do not use a global connection object but create and use the
connection only when needed.
Second, encapsulate the disposable objects like the connection and
the command with the using statement that will ensure a correct close
and dispose,
Third, pass the login and the password as conditions for the where
clause (more on this later)
Fourth, use the parametrized query to avoid syntax errors and sql
injection
Usually is not a good practice to store a password in clear text inside the database. You need to store only the hash of the password and recalculate this hash every time you need to check the user authenticity

C#: help return dataset and assign values to controls

I'm not really used to C# sharp but have used VB.NET before.
I'm needing to set the value of text fields, dropdowns etc from data from a query. To enter the data I have been using a class Computer with the method saveComputer() that takes values from user controls. Now I want an edit page that uses the id from url & uses getComputer(id) from Computer class and returns the values to be set to the user controls. I'm unsure about using this method to set the control values.
Edit.aspx.cs
protected void btnSave_Click(object sender, EventArgs e)
{
int id = 3; //will be replaced to GET value
Computer comp = new Computer();
//comp.updateComputer(ref id);
}
My Computer class
public getComputer(ref int id)
{
DataSet data = new DataSet();
using (SqlConnection conn = new SqlConnection(
"Server=JURA;Database=ReadyForSeven;User id=;Password="))
{
String sql = "SELECT * FROM computers WHERE id=#id";
//replace contatenation of variable with parameter name
SqlCommand cmd = new SqlCommand();
cmd.Connection = conn;
cmd.CommandText = sql.ToString();
cmd.CommandType = CommandType.Text;
//Define SqlParameter object and assign value to the parameter
cmd.Parameters.Add("#id", SqlDbType.Int);
cmd.Parameters["#id"].Value = id;
try
{
using (SqlDataAdapter da = new SqlDataAdapter(cmd))
{
da.Fill(data);
// return data here
}
}
catch (SqlException ex)
{
//send user to error page and log message
}
}
}
So what I'm wanting to achieve is using the getcomputer method of Computer to set the values of the controls on Edit.aspx
Can anyone help me out?

You'll need to modify your getComputer method to return a DataSet like:
public DataSet getComputer(int id) {
Once that's done we can call it and populate the form controls on the page load with something like:
protected void Page_Load(object sender, EventArgs e) {
if (!IsPostBack) {
int id = 3; // get from querystring
DataSet ds = getComputer(id);
DataRow dr = ds.Tables[0].Rows[0]; // get the first row returned
// populate form controls
txtFirstName.Text = dr["FirstName"].ToString();
ddlState.SelectedValue = dr["State"].ToString();
}
}
Below is an updated version of getComputer that will always return a value and is a little tighter:
public DataSet getComputer(int id) // you don't need to pass int by ref unless you're planning on changing it inside this method
{
DataSet data = new DataSet();
using (SqlConnection conn = new SqlConnection("Server=JURA;Database=ReadyForSeven;User id=;Password=")) {
using (SqlCommand cmd = new SqlCommand("SELECT * FROM computers WHERE id = #id", conn)) {
cmd.Parameters.AddWithValue("id", id);
using (SqlDataAdapter da = new SqlDataAdapter(cmd)) {
da.Fill(data);
return data;
}
}
}
}
I had to remove the try/catch blog to ensure the method always returned a value. If you absolutely need the try/catch block you'll need to return an empty DataSet at the end of the method to get to compile correctly.