I am having a problem is getting a value from mysql using c#. The connection string is right, but it throws the following error: Invalid attempt to access a field before calling Read()
Can anyone tell me the problem which occurs in the code below
string strConnection = ConfigurationSettings.AppSettings["ConnectionString"];
MySqlConnection connection = new MySqlConnection(strConnection);
MySqlCommand command = connection.CreateCommand();
MySqlDataReader reader;
command.CommandText = "SELECT application_domain_name FROM `test`.`application_domains` WHERE idapplication_domains = " + reference;
connection.Open();
reader = command.ExecuteReader();
lblApplicationDomain.Text = reader.GetString(0);
connection.Close();
You must call reader.Read() before accessing the results.
Before you do that, the reader 'cursor' will be placed before the first element. Placing the cursor before the first element will make the behavior consistent even if the result set is empty.
You need to call reader.Read() at least once. Like a normal SqlDataReader, the pattern is like so:
while(reader.Read())
{
.. Do Stuff
}
while(sqlDataReader.MoveNext())
{
.. Do Stuff
}
Related
I'm trying to get the value of a Field (User Access level it's 1 or 2 in string format) after login
OleDbConnection connection = new OleDbConnection(#"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=dsms.accdb");
connection.Open();
OleDbDataReader reader = null;
OleDbCommand command = new OleDbCommand("SELECT AL From Users WHERE Username='" + textusername.text + "'", connection);
reader = command.ExecuteReader();
if( reader.HasRows)
{
MessageBox.Show("success","status");
label1.Text = reader.GetString(1);
}
else
MessageBox.Show("failur", "status");
connection.Close();
I did execute the code in Access and it's was totally fine
but in the program, it says "No data exist for the row/column"
The main problem in your code is the fact that you need to call reader.Read() to get anything out from a DataReader. Just calling HasRows doesn't position the reader on the first record of your query.
There are other problems in your code.
Disposable objects like connections, commands and readers should be created in a using statement to ensure proper disposition after use and because you have only one field in your query, you should use the index 0 to retrieve it not 1.
Finally the most important one. You should NEVER concatenate strings to build an sql query. In this way a malicious user could write anything in your textbox, even valid sql commands that could be executed against your database. It is called Sql Injection and if you search for these terms you will find very detailed discussions about it. However, to avoid this problem (and others like parsing input with apostrophes) you use a parameterized query like below.
using(OleDbConnection connection = new OleDbConnection(.....))
using(OleDbCommand command = new OleDbCommand("SELECT AL From Users WHERE Username=#name", connection);
{
connection.Open();
command.Parameters.Add("#name", OleDbType.VarWChar).Value = txtusername.text;
using(OleDbDataReader reader = command.ExecuteReader())
{
if( reader.Read())
{
MessageBox.Show("success","status");
label1.Text = reader.GetString(0);
}
else
MessageBox.Show("failur", "status");
}
}
I know that this probably has been answered before, but I have rewritten this single line 10 times it still won't work. I have assured myself that this is written properly, yet it won't work.
This is my last resort. Here's a screenshot:
For security reasons, and for the exact reason you are asking, you should not be setting raw T-SQL in theCommandText property of your SqlCommand.
In your case, your string likely has ' characters in it that are breaking your query making the syntax invalid.
Instead, you CommandText should be initialized with Parameters, for example:
findItForMe.CommandText = "SELECT Name, LicenseType, till FROM myTable WHERE SomeColumn = #SomeParameter"
Then in your findItForMe command add the Parameters.
findItForMe.Parameters.AddWithValue("#SomeParameter", Somevalue)
Building your findItForMe SqlCommand this way will fix your errors and prevent malicious actors from perform SQL injection hacks against your application.
A full example:
string name = "Jacob's Ladder";
string commandText = "SELECT Name, LicenseType, till FROM myTable WHERE Name = #Name";
using (SqlConnection connection = new SqlConnection(connectionString))
{
SqlCommand findItForMe = new SqlCommand(commandText, connection);
// Use AddWithValue to assign name
// The parameterized query will escape your strings and keep you safe from hackers.
command.Parameters.AddWithValue("#name", name);
try
{
connection.Open();
SqlDataReader dr = command.ExecuteReader();
while (dr.Read())
{
// do something here
}
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
}
i have this sql command
string myreg = "select registration_no from truck where truck_id ='" + truckID + "'";
MySqlCommand cmd = new MySqlCommand(myreg, conn);
i want to put the value of myreg to my RegistrationNo.Text label.
i have this RegistrationNo.Text = myreg; and it displays select registration_no from truck where truck_id on my page
You need to read something about the workings of ADO.NET and its providers.
To get the result of that query in your textbox you need
Open a connection to your MySql Server
Prepare a command to send to the Server
Get back the result
Write the result to your textbox
All these passages requires the use of specific classes and some code to glue everything together
// Prepare your command using a parameter placeholder
string myreg = "select registration_no from truck where truck_id =#id";
// Build the connection to the server and build the command to execute
using (MySqlConnection cnn = new MySqlConnection(.... the connection string that identifies your server and db ))
using (MySqlCommand cmd = new MySqlCommand(myreg, cnn))
{
// Open the connection
cnn.Open();
// Add the parameter expected
cmd.Parameters.Add("#id", MySqlDbType.VarChar).Value = truckID;
// Execute the command and get back the return value (if found)
object result = cmd.ExecuteScalar();
// Check if the ExecuteScalar has returned something
if(result != null)
RegistrationNo.Text = result.ToString();
else
... message to your user about the failed search ...
}
PS. I have assumed that your variable truckID is a string because in your original code you have passed it between single quotes, but if it is an integer then you need to modify the parameter type to MySqlDbType.Int32
Also, I have used the ExecuteScalar method instead of ExecuteReader because I think that your query returns just a row with a single column and for this task it is better to use ExecuteScalar
You can use datareader also.See MSDN documentation here.
using (connection)
{
SqlCommand command = new SqlCommand(
"SQL Query",
connection);
connection.Open();
SqlDataReader reader = command.ExecuteReader();
if (reader.HasRows)
{
while (reader.Read())
{
Console.WriteLine("{0}\t{1}", reader.GetInt32(0),
reader.GetString(1));
}
}
else
{
Console.WriteLine("No rows found.");
}
reader.Close();
}
When I run the following code:
query = "select count(*) from table where name = '?name'";
MySqlConnection connection =
new MySqlConnection(ConfigurationManager.ConnectionStrings["mydb"].ToString());
connection.Open();
MySqlCommand command = new MySqlCommand(query,connection);
command.Parameters.Add("?name", name);
Int32 number = command.ExecuteScalar();
number is always zero, even when cast to an int.
I have tried converting it to int64, no dice. I have tried command.Prepare(). I have tried using Convert.ToInt32() and every other variation. I have tried just about everything under the sun including quoting verbatim what this suggests and I get no dice. Trying to cast the object as an integer, as a long, as an int32, none of this seems to work. These results are always 0 or cause a MySQL error.
EDIT: Stack overflow will not format that code properly in code tags, i apologize
The reason for that is because the parameter is enclose with single quote thus making it a string. Remove it and it will work,
query = "select count(*) from table where name = #name";
MySqlConnection connection = new MySqlConnection(ConfigurationManager.ConnectionStrings["mydb"].ToString());
connection.Open();
MySqlCommand command = new MySqlCommand(query,connection);
command.Parameters.Add("#name", name);
for better code,
use using for proper object disposal
using try-catch block for proper handling of exceptions
code snippet,
query = "select count(*) from table where name = #name";
string connString =ConfigurationManager.ConnectionStrings["mydb"].ToString();
using(MySqlConnection connection = new MySqlConnection(connString))
{
using(MySqlCommand command = new MySqlCommand(query, connection))
{
command.Parameters.Add("#name", name);
try
{
connection.Open();
// other codes
}
catch(MySqlException ex)
{
// do somthing with the exception
// don't hide it
}
}
}
The following code does not work. There is only 1 row in this table. How do I just get the statement that the sql would return?:
SqlConnection conn = new SqlConnection(connectionStringArg);
SqlCommand command = new SqlCommand("select applicationname from tbl_settings");
command.Connection = conn;
conn.Open();
string simpleValue = command.ExecuteReader()[0].ToString();
conn.Close();
return simpleValue;
Okay any help on how to achieve this relatively simple task would be great.
Since there's only a single value returned, you could do this:
string value = (string)command.ExecuteScalar();
If you need more than the first column from the first row returned, you'll need to use ExecuteReader(). See driis' answer.
The DataReader can't be indexed like that. You need something like:
using(conn)
using(var reader = command.ExecuteReader())
{
reader.Read();
string simpleValue = reader.GetString(0);
}
The general idea is to advance the reader for each record (using Read), then you can read values from the row. As pointed out in another answer, if you know there is only one single value in the result set, command.ExecuteScalar() gives you just that.
You have to call the Read method on a DataReader returned from ExecuteReader to get to the first row. Something like this:
SqlDataReader rdr = command.ExecuteReader();
if (rdr.Read())
...
You'd do e.g.
using(SqlConnection conn = new SqlConnection(connectionStringArg))
using(SqlCommand command = new SqlCommand("select applicationname from tbl_settings")) {
command.Connection = conn;
conn.Open();
SqlDataReader reader = command.ExecuteReader();
if(reader.read()) {
return reader.GetString(0);
}
return null;
}
For a single string you want to use ExecuteScalar to get a single value and then attempt to cast it to string.
From doc: Executes the query, and returns the first column of the first row in the result set returned by the query. Additional columns or rows are ignored.
Example:
string result = command.ExecuteScalar() as string;
For small stuff I often find LINQ2SQL the easiest to set up and use. This tutorial should get you up and running in a few minutes. For bigger projects LINQ2SQL is considered obsolete, see this question for discussion and alternatives.
Check DataReader.Read() you need to call it.
string applicationname = "";
using(var reader = command.ExecuteReader())
{
reader.Read();
applicationname = reader.GetString(reader.GetOrdinal("applicationname"));
}
var query_result = com.ExecuteScalar() as string;
if (!string.IsNullOrEmpty(query_result )) // do your thing here...