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MySqlDataReader object not receiving any data from function

I have a post action method which takes in a username and password from a form. In this action method I am using a function to execute a mysql command to a database
This is the function to read from the database
public static MySqlDataReader GetDataFromDB(string command)
{
using (MySqlConnection con = new MySqlConnection(ConStr))
{
com = new MySqlCommand(command, con);
con.Open();
MySqlDataReader rdr1 = com.ExecuteReader();
while (rdr1.Read())
{
}
return rdr1;
}
}
This is how i tried to use this function in my action method
MySqlDataReader rdr2 = Helper.GetDataFromDB("select * from library.Accounts where username = '" + username + "' and password = '" + password + "'");
I put a breakpoint in the helper method and could see that it was reading from the database fine with it storing all the expected row data in the internal 'rdr1' object however it isnt returning anything at the end as the 'rdr2' object is empty once it's finished. I don't know if something is wrong or if im just being an idiot and so i would appretiate if anyone can tell me where im going wrong.

A Data Reader such as MySqlDataReader is Forward Only. This means that once the data has been read and moved on to the next record, or the end of the data, it cannot go back to the beginning.
If you remove this block of code:
while (rdr1.Read())
{
}
You should find it will return the Data Reader at the start, and therefore be able to read the data you loaded from the database.
In addition, you should know that when the using () {...} block finishes the connection will be closed, along with the Data Readers ability to read the data.
If you do want to return the Data Reader, remove the using() {...} block to be a simple statement like:
MySqlConnection con = new MySqlConnection(ConStr);
If you do want to navigate backwards and forwards, filter or sort the data after it has been loaded, you should LOAD the data in to a DataTable from the Data Reader.
Example:
DataTable dt = new DataTable();
dt.Load(rdr1);

Close SqlDataReader if there are no more rows left to read - C#

This is causing me a headache. I know this question (or atleast variants of it) has been asked many times but before one flags it as a duplicate please consider the following code:
string myConnectionString = myConnectionString = ConfigurationManager.ConnectionStrings["DBCS"].ToString();
SqlConnection mySQLConnection;
SqlCommand mySQLCommand;
SqlDataReader mySQLDataReader;
using (mySQLConnection = new SqlConnection(myConnectionString))
{
mySQLCommand = new SqlCommand("SELECT TOP 1 * FROM Table ORDER BY Id DESC", mySQLConnection);
mySQLCommand.Connection = mySQLConnection;
mySQLCommand.Connection.Open();
using(mySQLDataReader = mySQLCommand.ExecuteReader())
{
if (mySQLDataReader.HasRows)
{
if (mySQLConnection.State == ConnectionState.Open)
{
while (mySQLDataReader.Read())
{
//Perform Logic : If the last record being returned meets some condition then call the below method
MethodCalled();
}
}
}
}
MessageBox.Show("Connection state: " + mySQLConnection.State);
}
I would like to find a way to either:
Close the reader after it has finished reading
Break out of the while-loop when it has finished reading and there are no more rows left
But I just keep on getting a SqlException stating the following:
invalid attempt to call read when reader is closed
Just from broad observation, I can trace that error is due to me returning data that contains one row only. The problem is that after it has read that row, the compiler goes back to While(mySQLDataReader.Read()){} and attempts to read through a table that does not contain any rows.
I attempted the following:
Wrapping the ExecuteReader() from the command object in a using block so that it automatically closes the reader and the connection respectively once it has done reading like so:
using(mySQLDataReader = mySQLCommand.ExecuteReader())
{
//Logic performed
}
Just before the closing brace of the while-loop, I tried checking if there are any more rows left/returned from the sql command and breaking out the loop once that condition is satisfied:
if(mySQLDataReader.HasRows == false) //No more rows left to read
{
break; //break out of loop
}
Both attempts were unsuccessful. How can I get around this?

It must be one of the following 3 things:
You're using Read() OUTSIDE the using block. Remember that using block will implicitly call Close and Dispose on your reader. Thus any Read() calls must be placed inside the using block.
The body of your using block is explicitly closing the reader. This seems improbable.
Apparently you have declared your mySQLDataReader at a higher level. It could be that some other (async) code is closing the reader. This also is unlikely. You shouldn't, in most cases, define a DataReader at global level.
Edit
Reading the full code block that you have posted now, I'd suggest a few changes. Can you run the following and tell us if it runs:
using (var mySQLConnection = new SqlConnection(myConnectionString))
{
mySQLCommand = new SqlCommand("SELECT TOP 1 * FROM Table ORDER BY Id DESC", mySQLConnection, mySQLConnection);
mySQLCommand.Connection.Open();
using(mySQLDataReader = mySQLCommand.ExecuteReader())
{
while (mySQLDataReader.Read())
{
//Perform Logic : If the last record being returned meets some condition then call the below method
MethodCalled();
}
}
}
If this version runs fine, we can then dig the problem better.

If there is no data to iterate, while loop will not execute at all. Do you need to check for HasRows as such? Also, you should use CommandBehavior.CloseConnection when you are creating data reader. This will make sure that underlying connection is closed once you have read through it.
Should if call SqlDataReader.HasRows if I am calling SqlReader.Read
SQLDataReader Source Code
using (SqlConnection mySQLConnection = new SqlConnection(myConnectionString))
{
using (SqlCommand mySQLCommand = new SqlCommand("SELECT TOP 1 * FROM Table ORDER BY Id DESC", mySQLConnection))
{
mySQLConnection.Open();
SqlDataReader mySQLDataReader = mySQLCommand.ExecuteReader(CommandBehavior.CloseConnection);
while (mySQLDataReader.Read())
{
//Code logic here
}
// this call to mySQLDataReader.Close(); will close the underlying connection
mySQLDataReader.Close();
}
MessageBox.Show("Connection state: " + mySQLConnection.State);
}

Multiple access to a single SQLite database file via System.Data.SQLite and c#

As I can read from SQLite FAQ it supports multiple processes reading (SELECT) and only one process writing (INSERT, UPDATE, DELETE) database at any moment in time:
SQLite uses reader/writer locks to control access to the database.
When any process wants to write, it must lock the entire database file
for the duration of its update. But that normally only takes a few
milliseconds. Other processes just wait on the writer to finish then
continue about their business
I'm using System.Data.SQLite adapter via c#.
Could someone expalin me plz, how exactly this process is going on?
Will this process work automatically and writing SQLiteCommand will simply wait if there is another writing SQLiteCommand already executing over the same database?
Or maybe it will throw an exception? What kind of it?
Sorry but I found no information about this mechanics :)
Thank you.
UPDATE:
I've found post saying that exception will be raised with a specific errorcode
Is that statement correct?

I've investigated it by myself:
I created a sample SQLite database c:\123.db with one table Categories containing two fields: ID (uniqueidentifier) and Name (nvarchar).
I then wrote some multi-thread code to emulate multiple write access to the database (don't forget to add a System.Data.SQLite reference to your project if you use this code):
using System;
using System.Data.SQLite;
using System.Threading.Tasks;
namespace SQLiteTest
{
class Program
{
static void Main(string[] args)
{
var tasks = new Task[100];
for (int i = 0; i < 100; i++)
{
tasks[i] = new Task(new Program().WriteToDB);
tasks[i].Start();
}
foreach (var task in tasks)
task.Wait();
}
public void WriteToDB()
{
try
{
using (SQLiteConnection myconnection = new SQLiteConnection(#"Data Source=c:\123.db"))
{
myconnection.Open();
using (SQLiteTransaction mytransaction = myconnection.BeginTransaction())
{
using (SQLiteCommand mycommand = new SQLiteCommand(myconnection))
{
Guid id = Guid.NewGuid();
mycommand.CommandText = "INSERT INTO Categories(ID, Name) VALUES ('" + id.ToString() + "', '111')";
mycommand.ExecuteNonQuery();
mycommand.CommandText = "UPDATE Categories SET Name='222' WHERE ID='" + id.ToString() + "'";
mycommand.ExecuteNonQuery();
mycommand.CommandText = "DELETE FROM Categories WHERE ID='" + id.ToString() + "'";
mycommand.ExecuteNonQuery();
}
mytransaction.Commit();
}
}
}
catch (SQLiteException ex)
{
if (ex.ReturnCode == SQLiteErrorCode.Busy)
Console.WriteLine("Database is locked by another process!");
}
}
}
}
The result on my Core2Duo E7500 is that Exception is never raised!
Looks like SQLite is optimised enough for my needs (locking/unlocking is really fast and normally only takes a few milliseconds as SQLite FAQ tells us) - Great!
Note that there is no need to retrieve an integer ErrorCode for an SQLiteException - you can use a special enum ReturnCode field instead. All codes are described here.
Hope this information will help somebody.

Retrieve & update 20000 data records stops working

I am using the following code do get all data records from a MS SQL database and I try to update every single record. The code is used in a WebService. The issue is, that the code runs fine if I have 1000 data records but now I have 20000 data records an the code first returned with an timeout. Then I set the cmd.CommandTimeout to zero to have no timeout. Now when I invoke the function in the IE WebSvc the IE window is still blank and still try to load something but nothing happens. Only 150 datarecords are updated.
Do you have any idea where the issue might be ? Is the code not the best, so what should I change ?
Thank you very much!
WorldSignia
MyCode:
private string AddNewOrgBez()
{
try
{
SqlConnection sqlconn = new SqlConnection(this.connectionString);
SqlCommand cmd;
SqlDataReader reader;
sqlconn.Open();
cmd = new SqlCommand("SELECT * FROM dbo.mydata", sqlconn);
cmd.CommandTimeout = 0;
reader = cmd.ExecuteReader();
while (reader.Read())
{
// Felder holen
string okuerzel = reader["O_KURZ"].ToString();
string bezeichnung = reader["O_BEZ"].ToString();
string[] lines = CreateNewOrgBez(bezeichnung);
string sqlcmd = "UPDATE dbo.mydata SET WEB_OBEZ1 = '" + lines[0] + "', WEB_OBEZ2 = '" + lines[1] + "', WEB_OBEZ3 = '" + lines[2] + "' WHERE O_KURZ = '" + okuerzel + "'";
SqlConnection sqlconn2 = new SqlConnection(this.connectionString);
sqlconn2.Open();
SqlCommand cmd2 = new SqlCommand(sqlcmd, sqlconn2);
cmd2.CommandTimeout = 0;
cmd2.ExecuteNonQuery();
sqlconn2.Close();
}
reader.Close();
sqlconn.Close();
return "OK";
}
catch (Exception ex)
{
return ex.Message;
}
}

You are leaking every SqlCommand here - I suggest you review your use of SqlClient classes to find the ones that are IDisposable and restructure your code to ensure they are always freed, using the using construct.
For example, this ensures Dispose gets called even if there is an exception in the bracketed code:
using (SqlCommand cmd2 = new SqlCommand(sqlcmd, sqlconn2))
{
cmd2.CommandTimeout = 0;
cmd2.ExecuteNonQuery();
}
Using a new SqlConnection for every UPDATE is expensive too, this should be done outside the loop. Redundant connection establishment is likely the explanation for your timeout.
Take note of #ck's comment that for efficiency this type of piecemeal client-side operation is not as good as doing the heavy lifting server-side. You should be able to get this code working better, but that does not mean it's the ideal/fastest solution.

I found the issue.
You need first to get all the data records, for example in a new DataTable. The structure I used does not work, because it reads data from the database and also updates the database. After changing it to a new structure it works.

You were using two different connections to read and to update, and one of them was blocking another. This is why when you read all your data first, it began to work.

I doubt if your running into OutOfMemoryException. Can you profile your application and check the memory usage?
Since you are just overwriting the variables in While loop, why don't you try taking them out of the loop.

SQL Reading from a DB problem using a DataReader

I have a table of Users (tblUsers) which contains details of University staff. I am trying to populate a text box with the names of lecturers associated with a selected module.
I am getting all UserIDs associated with a particular module, testing if the User is a lecturer, if so then I add the ID to an ArrayList.
I then iterate through this array and call the method below during each iteration passing through the current ID.
However, if you look at the method below I am using a SqlDataReader and am getting an error while reading from it on this line:
txtLecturerName.Text += myReader["First_Name"].ToString();
The error message is:
'myReader["First_Name"]' threw an exception of type 'System.IndexOutOfRangeException'
The table layout I am using is below the method code. Any help with this would be greatly appreciated, I am one cup of coffee away from putting my head through the screen.
public void outputLecturerNames(string lecturerID)
{
// Create a new Connection object using the connection string
SqlConnection myConnection = new SqlConnection(conStr);
// If the connection is already open - close it
if (myConnection.State == ConnectionState.Open)
{
myConnection.Close();
}
// 'using' block allows the database connection to be closed
// first and then the exception handling code is triggered.
// This is a better approach than using a 'finally' block which
// would close the connection after the exception has been handled.
using (myConnection)
{
try
{
// Open connection to DB
myConnection.Open();
SqlCommand selectCommand = new SqlCommand(selectQuery, myConnection);
// Declare a new DataReader
SqlDataReader myReader;
selectQuery = "SELECT * FROM tblUsers WHERE User_ID='";
selectQuery += lecturerID + "'";
myReader = selectCommand.ExecuteReader();
while (myReader.Read())
{
txtLecturerName.Text += myReader["First_Name"].ToString();
txtLecturerName.Text += " ";
txtLecturerName.Text += myReader["Last_Name"].ToString();
txtLecturerName.Text += " , ";
}
myReader.Close();
}
catch (Exception err)
{
Console.WriteLine("Error: " + err);
}
}
}
tblUsers:
[User_ID][First_Name][Last_Name][Email_Address]

In your method, the variable selectQuery is not declared, and it is used as parameter to SqlCommand before it is assigned the query string on tblUsers.

You've probably misspelled a column name.
In general, you should never write SELECT * FROM ....
Instead, you should select only the columns you need.
This will make your program run faster by only querying the information that you need, and can produce better error messages.

This error is created when the column name given is not found. If you are anything like me, you've probably checked it several times, but is the table name correct (correct database, correct schema) and is the column name correct?
http://msdn.microsoft.com/en-us/library/f01t4cfy.aspx
You might try fully qualifying the name of the table (database.dbo.tblUsers). This would ensure that you are hitting the table you think you are. Also, try and put the names of the columns into the SQL statement. If they are not correct, your SQL statement will not execute properly.