I am having a problem with the sql datareader. Whenever I try to read data it gives me an error saying invalid attemp to call Read when the reader is closed. Please help me figure out the problem
private void button1_Click(object sender, EventArgs e)
{
string name = this.textBox1.Text;
string connstring = #"Data Source=(LocalDB)\v11.0;AttachDbFilename=C:\Users\Gardezi\Documents\Visual Studio 2012\Projects\homeWork2\homeWork2\Database1.mdf;Integrated Security=True";
SqlConnection con = new SqlConnection(connstring);
string query = "Select * from diaryDB";
SqlCommand com = new SqlCommand(query, con);
SqlParameter p = new SqlParameter("name", name);
con.Open();
SqlDataReader d = com.ExecuteReader();
con.Close();
deleteResult r = new deleteResult(d);
r.Show();
}
this is the constructor of deleteResult
public deleteResult(SqlDataReader d)
{
InitializeComponent();
while (d.Read())
{
this.comboBox1.Items.Add((d["Title"] +"-" +d["Description"]).ToString());
}
}
You can't read after closing the connection.
Just change this part of your code:
FROM
(...)
con.Close();
deleteResult r = new deleteResult(d);
(...)
TO
(...)
deleteResult r = new deleteResult(d);
con.Close();
(...)
Please try to use the using statement that correctly enclose the connection, the command and the reader in appropriate blocks.
private void button1_Click(object sender, EventArgs e)
{
string name = this.textBox1.Text;
string connstring = #"....";
string query = "Select * from diaryDB";
using(SqlConnection con = new SqlConnection(connstring))
using(SqlCommand com = new SqlCommand(query, con))
{
SqlParameter p = new SqlParameter("name", name);
con.Open();
using(SqlDataReader d = com.ExecuteReader())
{
deleteResult r = new deleteResult(d);
r.Show();
}
}
}
In this way the connection is kept open while you read from the reader. This is essential to avoid the error.
The more important point is that you don't have to worry to close and dispose the connection when it is no more needed. The exit from the using block close and dispose the connection, the command and the reader ALSO in case of exceptions.
Related
This is the code I'm working with right now, I don't get any errors so I can't pinpoint where it's not working:
private void btnAdd_Click(object sender, EventArgs e)
{
string constring = $"Data Source=(LocalDB)\\MSSQLLocalDB;AttachDbFilename=" +
Directory.GetCurrentDirectory().ToString() + "\\BarcodeDB.mdf;Integrated Security=True";
string query =
"INSERT INTO Products (Barcodes, Name, EDate, Quantity, Price) VALUES (#barcodeValue, #nameValue, #dateValue, #quantityValue, #priceValue) ;";
SqlConnection conDataBase = new SqlConnection(constring);
conDataBase.Open();
using (var cmd = new SqlCommand(query, conDataBase))
{
cmd.Parameters.AddWithValue("#barcodeValue", tbxBar.Text);
cmd.Parameters.AddWithValue("#nameValue", tbxName.Text);
cmd.Parameters.AddWithValue("#dateValue", dateDate.Value.Date);
cmd.Parameters.AddWithValue("#quantityeValue", tbxQua.Text);
cmd.Parameters.AddWithValue("#priceValue", tbxPrice.Text);
}
conDataBase.Close();
}
The code might just be wrongly build or I could be missing some part I'm not sure.
I figured out what was not working, was the connection string. So opening a new question for that.
What i had to do is to open the connection and then execute the command
You're not actually running the command. You need to call ExecuteNonQuery or ExecuteScalar:
using (var cmd = new SqlCommand(query, conDataBase))
{
// set parameters...
cmd.ExecuteNonQuery();
}
I am new to C# and I have connected it with Oracle11g and using Visual Studio 2013 as a tool. I am trying to display a 'name ' that is being returned by a query to a textbox but it is neither displaying an Error Message nor displaying the Output. Pleases help me to resolve this problem. Thanks... here is my code..
private void button1_Click(object sender, EventArgs e)
{
try
{
string oradb = "Data Source=ORCL;User Id=hr; Password=123;";
OracleConnection conn = new OracleConnection(oradb); // C#
conn.Open();
OracleCommand cmd = new OracleCommand();
cmd.Connection = conn;
cmd.CommandText = "select name from std where cgpa=2.82;";
cmd.CommandType = CommandType.Text;
OracleDataReader dr = cmd.ExecuteReader();
dr.Read();
textBox1.Text = dr.GetString(0);
conn.Dispose();
}
catch (Exception ex) { MessageBox.Show("\n"+ex); }
}
after setting textBox1.Text = dr.GetString(0);
it is giving me the attached exception.
Array indexes start at index zero, not 1. The returned string (if any) is at
textBox1.Text = dr.GetString(0);
A more correct way to write your code is the following
private void button1_Click(object sender, EventArgs e)
{
try
{
string oradb = "Data Source=ORCL;User Id=hr; Password=123;";
// Use the using statements around disposable objects....
using(OracleConnection conn = new OracleConnection(oradb))
using(OracleCommand cmd = new OracleCommand())
{
conn.Open();
// These two parameters could be passed directly in the
// OracleCommand constructor....
cmd.Connection = conn;
cmd.CommandText = "select name from std where cgpa=2.82;";
// Again using statement around disposable objects
using(OracleDataReader dr = cmd.ExecuteReader())
{
// Check if you have a record or not
if(dr.Read())
textBox1.Text = dr.GetString(0);
}
}
}
catch (Exception ex) { MessageBox.Show("\n"+ex); }
}
And if your code is supposed to return just a single record with a single column then you can use the more performant ExecuteScalar without building an OracleDataReader
// Again using statement around disposable objects
object result = cmd.ExecuteScalar();
if(result != null)
textBox1.Text = result.ToString();
Can someone help me out?
I just get as result tb_localidade: System.Data.SqlClient.SqlDataReader
Why? Here is the code:
private void btn_normalizar_Click(object sender, EventArgs e)
{
//connection string - one or other doenst work
//SqlConnection conn = new SqlConnection("DataSource=FRANCISCO_GP;Initial Catalog=Normalizacao;Integrated Security=True;");
SqlConnection conn = new SqlConnection(Properties.Settings.Default.connString);
string sql = "SELECT ART_DESIG from Arterias where ART_COD = '10110'";
SqlCommand cmd = new SqlCommand(sql, conn);
conn.Open();
SqlDataReader leitor = cmd.ExecuteReader();
tb_localidade.Text = leitor.ToString();
conn.Close();
}
You can do this by calling Read() on your data reader and assigning the results:
private void btn_normalizar_Click(object sender, EventArgs e)
{
using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.connString))
{
conn.Open();
string sql = "SELECT ART_DESIG from Arterias where ART_COD = '10110'";
using (SqlCommand cmd = new SqlCommand(sql, conn))
{
SqlDataReader leitor = cmd.ExecuteReader();
while (leitor.Read())
{
tb_localidade.Text = leitor["ART_DESIG"].ToString();
}
}
}
}
Another note is that using a using block for your SqlConnection and SqlCommand objects is a good habit to get into.
Note: this is assigning the result to the tb_localidade.Text for every row in the resultset. If you are only intending for this to be one record, you might want to look into .ExecuteScalar() instead (see below).
private void btn_normalizar_Click(object sender, EventArgs e)
{
using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.connString))
{
conn.Open();
string sql = "SELECT ART_DESIG from Arterias where ART_COD = '10110'";
using (SqlCommand cmd = new SqlCommand(sql, conn))
{
tb_localidade.Text = cmd.ExecuteScalar().ToString();
}
}
}
before execute "executeReader()" then you must read to get results.
Improvement on Siyual's response. You're only looking for a single result, and this explicitly disposes both the connection and the datareader.
private void btn_normalizar_Click(object sender, EventArgs e)
{
using (SqlConnection conn = new SqlConnection(Properties.Settings.Default.connString))
{
conn.Open();
string sql = "SELECT ART_DESIG from Arterias where ART_COD = '10110'";
using(SqlCommand cmd = new SqlCommand(sql, conn)) {
using(SqlDataReader leitor = cmd.ExecuteReader())
{
if (leitor.Read())
{
tb_localidade.Text = leitor["ART_DESIG"].ToString();
}
}
}
}
}
you should just this
SqlDataReader leitor = cmd.ExecuteReader();
string res="";
while(leitor.Read())
{
res=leitor.GetValue(0).ToString()///////if in sql it is varchar or nvarshar
}
tb_localidade.Text = res;
actully datareader is a 1d table and we can access to this with GetValue or GetInt32 or ...
I wrote this code but I do not know why this line gives error!
String sname = dr.GetString ("name");
My code:
SqlConnection cn = new SqlConnection(
"Data Source=.;Initial Catalog=logindb;Integrated Security=True");
string query1 = "select * from tbllogin";
SqlCommand cmd = new SqlCommand(query1);
SqlDataReader dr;
try
{
cn.Open();
dr = cmd.ExecuteReader();
while (dr.Read())
{
String sname = dr.GetString("name");
comboBox1.Items.Add(sname);
}
}
catch (Exception e)
{
// do smth about exception
}
First of all you have to check this again:
SqlConnection cn = new SqlConnection(
"Data Source=.;Initial Catalog=logindb;Integrated Security=True");
Data Source=.; This is wrong and it will give you an error.
After that you can use the code below to achieve what you want. The code below also uses using statement to dispose the connection.
using (
SqlConnection connection = new SqlConnection(strCon)) // strCon is the string containing connection string
{
SqlCommand command = new SqlCommand("select * from tbllogin", connection);
connection.Open();
DataReader reader = command.ExecuteReader();
if (reader.HasRows)
{
while (reader.Read())
{
comboBox1.Items.Add(reader.GetString(int index)); // index of column you want, because this method takes only int
}
}
reader.Close();
}
(Amazingly) GetString only take an index as parameter.
See MSDN
public override string GetString(int i)
With no other signatures :-(
However you could write:
String sname = dr.Item["name"].ToString();
MSDN says that SqlDataReader.GetString method accepts an int as a parameter, which is the index of the column.
What you need is this:
while (dr.Read())
{
String sname = (string)dr["name"];
comboBox1.Items.Add(sname);
}
Here's your code:
SqlConnection cn = new SqlConnection("Data Source=.;Initial Catalog=logindb;Integrated Security=True");
string query1 = "select * from tbllogin"; SqlCommand cmd = new SqlCommand(query1); SqlDataReader dr;
try {
cn.Open(); dr = cmd.ExecuteReader();
while (dr.Read())
{ String sname = (string)dr["name"];
comboBox1.Items.Add(sname);
}
}
catch (Exception e) { MessageBox.Show(e.Message, "An error occurred!"); }
The catch block wasn't written correctly, you missed the (Exception e) part.
Can someone tell me how to fix this error?
SqlCommand cmd = new SqlCommand(sqlCmd, conn)
--> conn: Aurgument type 'System.Data.OleDb.OleDbConnection' is not assignable to parameter type 'System.Data.SqlClient.SqlConnection'.
private void Form1_Load(object sender, EventArgs e)
{
string connString = "Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source=C:\\Users\\KevinDW\\Desktop\\dotNET\\Week 5\\Prak1\\demo1.accdb";
OleDbConnection conn = new OleDbConnection(connString);
conn.Open();
string sqlCmd = "SELECT CursusNaam FROM tblCursus";
SqlCommand cmd = new SqlCommand(sqlCmd, conn);
using (SqlDataReader reader = cmd.ExecuteReader())
{
listBox1.Items.Add(reader);
}
conn.Close();
}
}
You're mixing up Sql and OleDb
Use OleDbCommand instead of SqlCommand
and use OleDBDataReader instead of SqlDataReader
For example:
private void Form1_Load(object sender, EventArgs e)
{
string connString = "Provider=Microsoft.ACE.OLEDB.12.0;" + "Data Source=C:\\Users\\KevinDW\\Desktop\\dotNET\\Week 5\\Prak1\\demo1.accdb";
OleDbConnection conn = new OleDbConnection(connString);
conn.Open();
string sqlCmd = "SELECT CursusNaam FROM tblCursus";
OleDbCommand cmd = new OleDbCommand(sqlCmd, conn);
using (OleDBDataReader reader = cmd.ExecuteReader())
{
listBox1.Items.Add(reader);
}
conn.Close();
}
}
You are using SqlCommand/etc which requires the use of SqlConnection object instead of OleDbConnection.
Is it a SQL database you are connecting to? If so use SqlConnection instead
Edit: Obviously not, reading the connection string ... :D