I have below code and I am getting exception:
There is already an open DataReader associated with this Connection which must be closed first.
I am using Visual Studio 2010/.Net 4.0 and MySQL for this project. Basically I am trying to run another SQL statement while using data reader to do my other task. I am getting exception at line cmdInserttblProductFrance.ExecuteNonQuery();
SQL = "Select * from tblProduct";
//Create Connection/Command/MySQLDataReader
MySqlConnection myConnection = new MySqlConnection(cf.GetConnectionString());
myConnection.Open();
MySqlCommand myCommand = new MySqlCommand(SQL, myConnection);
MySqlDataReader myReader = myCommand.ExecuteReader();
myCommand.Dispose();
if (myReader.HasRows)
{
int i = 0;
// Always call Read before accessing data.
while (myReader.Read())
{
if (myReader["frProductid"].ToString() == "") //there is no productid exist for this item
{
strInsertSQL = "Insert Into tblProduct_temp (Productid) Values('this istest') ";
MySqlCommand cmdInserttblProductFrance = new MySqlCommand(strInsertSQL, myConnection);
cmdInserttblProductFrance.ExecuteNonQuery(); //<=====THIS LINE THROWS "C# mySQL There is already an open DataReader associated with this Connection which must be closed first."
}
}
}
You are using the same connection for the DataReader and the ExecuteNonQuery. This is not supported, according to MSDN:
Note that while a DataReader is open, the Connection is in use
exclusively by that DataReader. You cannot execute any commands for
the Connection, including creating another DataReader, until the
original DataReader is closed.
Updated 2018: link to MSDN
Always, always, always put disposable objects inside of using statements. I can't see how you've instantiated your DataReader but you should do it like this:
using (Connection c = ...)
{
using (DataReader dr = ...)
{
//Work with dr in here.
}
}
//Now the connection and reader have been closed and disposed.
Now, to answer your question, the reader is using the same connection as the command you're trying to ExecuteNonQuery on. You need to use a separate connection since the DataReader keeps the connection open and reads data as you need it.
Just use MultipleActiveResultSets=True in your connection string.
Add MultipleActiveResultSets=true to the provider part of your connection string
example in the file appsettings.json
"ConnectionStrings": {
"EmployeeDBConnection": "server=(localdb)\\MSSQLLocalDB;database=YourDatabasename;Trusted_Connection=true;MultipleActiveResultSets=true"}
You are trying to to an Insert (with ExecuteNonQuery()) on a SQL connection that is used by this reader already:
while (myReader.Read())
Either read all the values in a list first, close the reader and then do the insert, or use a new SQL connection.
The issue you are running into is that you are starting up a second MySqlCommand while still reading back data with the DataReader. The MySQL connector only allows one concurrent query. You need to read the data into some structure, then close the reader, then process the data. Unfortunately you can't process the data as it is read if your processing involves further SQL queries.
This exception also happens if you don't use transaction properly. In my case, I put transaction.Commit() right after command.ExecuteReaderAsync(), did not wait with the transaction commiting until reader.ReadAsync() was called. The proper order:
Create transaction.
Create reader.
Read the data.
Commit the transaction.
You have to close the reader on top of your else condition.
In my case, I was awaiting an async call, but in the calling scope, I was not awaiting that method that I was making the call in. So, the calling scope was continuing on while my connection was still open.
called scope:
protected override async Task AfterProcessing()
{
var result = await Stats.WriteAsync();
Log.Information("Stopping");
}
calling scope:
public virtual async Task Run()
{
BeforeProcessing();
try
{
Process();
}
finally
{
AfterProcessing(); // this line was missing an "await"
}
}
There is another potential reason for this - missing await keyword.
Related
I'm using SQLite3 database for a small level system. The problem is that when I try to rename the db file using IO file system, System.IO returns an exception that the file is being used by another process however I've already closed the connection using connection.Close() method. I searched on this topic but cold not find any thing useful for me. Someone please suggest me the right way to release an SQLite connection..
Thanks in Advance
One of the things I learned programming with SQLite is that closing the connection is simply not good enough. You need to wrap everything in a using block. Make sure your SQLiteConnection object is wrapped in a using block, your SQLiteCommand is wrapped in a using block and your SQLiteDataReader is wrapped in a using block. Your code should look similar to this:
using (var connection = new SQLiteConnection())
{
connection.ConnectionString = connectionString;
using (var command = new SQLiteCommand())
{
connection.Open();
command.CommandType = CommandType.Text;
command.CommandText = sql;
command.Connection = connection;
using (SQLiteDataReader reader = command.ExecuteReader())
{
if (reader.HasRows)
{
// Do something
}
}
}
}
This question already has answers here:
Do I have to Close() a SQLConnection before it gets disposed?
(8 answers)
Closed 9 years ago.
When using a using block for SQL connections in C#, does this also a close method? I'm asking as I need to explicitly use the con.Open() method. I found this example:
using (SqlConnection con = new SqlConnection(connectionString))
{
con.Open(); // open method
string queryString = "select * from db";
SqlCommand cmd = new SqlCommand(queryString, con);
SqlDataReader reader = cmd.ExecuteReader();
reader.Read();
??? // What about a close method?
}
Or does the using block close the connection itself?
using translates to:
SqlConnection con = new SqlConnection(connectionString)
try
{
con.Open(); <-- open method
string queryString = "select * from db";
SqlCommand cmd = new SqlCommand(queryString, con);
SqlDataReader reader = cmd.ExecuteReader();
reader.Read();
}
finally
{
if (con!= null)
((IDisposable)con).Dispose();
}
where ((IDisposable)con.Dispose(); closes what is to be closed.
You don't have to close it - using is enough. MSDN says:
To ensure that connections are always closed, open the connection inside of a using block, as shown in the following code fragment. Doing so ensures that the connection is automatically closed when the code exits the block.
No, you don't have to call the Close-Method of the SqlConnection.
When it gets disposed on the end of the using, it closes automatically.
(The Dispose-Method calls Close()) [1]
The other answers are correct, however, I want to be a bit more explicit.
The MSDN states the following:
Close and Dispose are functionally equivalent.
Because using using will call Dispose a separate call to Close is not needed.
when you use the 'using' keyword and create a connection in it .
the connection is open as far as there is the scope of the 'using' keyword when closing bracket is reached the connection is closed automatically as it was defined within a scope and the scope is not in exist
in other words treat it as the local variable which can be accessed within the scope only.
hope this will help.
I have my main form which has a datagrid view connected to a database. Then I have a button that opens a separate form and I've got a few buttons etc on that secondary form.
I need to query the database from the secondary form but I'm not sure how to do that without creating a whole new connection, which I don't think I need since the program is already connected to the database. I'm just not sure how to reference the oleDB connection I made in that first form (I didn't code it in, I used the little arrow on the datagridview to connect it to the database using visual studio)
Now instead of creating that new connection, how do I reference the first connection made in the primary form?
Here is my code:
//parameterized update query
string updateCommandString = "UPDATE RoomsTable SET [Date Checked]=#checkedDate WHERE ID = #id";
using (OleDbConnection conn = new OleDbConnection(#"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=c:\users\spreston\documents\visual studio 2012\Projects\roomChecksProgram\roomChecksProgram\roomsBase.accdb"))
{
using (OleDbCommand updateCommand = new OleDbCommand())
{
OleDbTransaction transaction = null;
updateCommand.Connection = conn;
updateCommand.Transaction = transaction;
updateCommand.CommandText = updateCommandString;
updateCommand.CommandType = CommandType.Text;
updateCommand.Parameters.AddWithValue("#checkedDate", this.dateTimePicker1.Value.ToShortDateString());
updateCommand.Parameters.AddWithValue("#id", row.roomID);
try
{
conn.Open();
transaction = conn.BeginTransaction();
updateCommand.Transaction = transaction;
updateCommand.ExecuteNonQuery();
transaction.Commit();
conn.Close();
conn.Dispose();
}
catch(OleDbException ex)
{
MessageBox.Show(ex.Message.ToString());
}
}
}
From a design standpoint, you should consider making a data access layer for your forms to utilize. You can create methods to retrieve those Db results for you so you consolidate that code and separate it from your form functionality. It may be just a small project, but it's good practice and if it's a project you want to grow, you'll want it laid out to be extendable.
Something like
class SomethingDA {
static DataTable GetMyStuff(your params) {
// establish connection, get your results
}
}
You can then call SomethingDa.GetMyStuff() to get what you need.
If you are Using the "Using" statement, your connection is closed after this code is run. You don't in fact need the conn.Close() and conn.Dispose() statements. Using does that for you.
Your best bet is to open up the connection again. It is generally a good practice to open and close connections as quickly as possible, although likely less important if your Access DB local. This generally does not impact performance too much as the OLE DB driver behind the scene will pool the connection and keep it open for a period of time.
I have a method where I insert some data in a local MDB-Acces file. With this code it works fine:
try
{
foreach (DataRow rowS99 in dtS99.Rows)
{
con.Open();
da.InsertCommand = con.CreateCommand();
da.InsertCommand.Parameters.AddWithValue("#wertListName", rowS99["WertListName"]);
da.InsertCommand.Parameters.AddWithValue("#key", rowS99["Key"]);
da.InsertCommand.Parameters.AddWithValue("#bezeichner", rowS99["Bezeichner"]);
da.InsertCommand.Parameters.AddWithValue("#keyAufbereitet", rowS99["KeyAufbereitet"]);
da.InsertCommand.CommandText = sql;
da.InsertCommand.ExecuteNonQuery();
con.Close();
}
check = true;
}
When i put the Open and Close method outside the foreach loop the code works, but the local file, which was used with the connection, is already opened with Visual Studio so that i can't delete or move it. The Status of the connection says after con.Close() that it is closed.
What is the problem here?
You don't need to have the Connection Opened and Closed within the loop. If there are 500 rows in your table then you will be opening & closing the connection 500 times, its not best practice..
Your connection should open in the Try statement before the loop and close once all Rows have been processed.
I think you would be best using the Update() function rather than the method your doing above.
You can take out the need to loop through each and every row.
http://msdn.microsoft.com/en-us/library/system.data.common.dataadapter.update.aspx
Using this functionality allows you to add, edit and delete rows simply by calling the Update() method. You will need all 3 Commands setting up, UPDATE, INSERT, DELETE.
Hope this helps
IDbConnection.Close method by default just returns connection back to the connection pool. It doesn't close the physical connection (either network connection or file). You may disable connection pooling (but this will decrease productivity of your application), or clear connection pool (see OleDbConnection.ReleaseObjectPool).
Try to put con.close in finally().
try
{
con.Open();
foreach (DataRow rowS99 in dtS99.Rows)
{
da.InsertCommand = con.CreateCommand();
da.InsertCommand.Parameters.AddWithValue("#wertListName", rowS99["WertListName"]);
da.InsertCommand.Parameters.AddWithValue("#key", rowS99["Key"]);
da.InsertCommand.Parameters.AddWithValue("#bezeichner", rowS99["Bezeichner"]);
da.InsertCommand.Parameters.AddWithValue("#keyAufbereitet", rowS99["KeyAufbereitet"]);
da.InsertCommand.CommandText = sql;
da.InsertCommand.ExecuteNonQuery();
}
con.Close();
check = true;
OleDb.OleDbConnection.ReleaseObjectPool();
GC.Collect(); // I know attation
}
Works for me
Basically, I would like a brief explanation of how I can access a SQL database in C# code. I gather that a connection and a command is required, but what's going on? I guess what I'm asking is for someone to de-mystify the process a bit. Thanks.
For clarity, in my case I'm doing web apps, e-commerce stuff. It's all ASP.NET, C#, and SQL databases.
I'm going to go ahead and close this thread. It's a little to general and I am going to post some more pointed and tutorial-esque questions and answers on the subject.
MSDN has a pretty good writeup here:
http://msdn.microsoft.com/en-us/library/s7ee2dwt(VS.71).aspx
You should take a look at the data-reader for simple select-statements. Sample from the MSDN page:
private static void ReadOrderData(string connectionString)
{
string queryString =
"SELECT OrderID, CustomerID FROM dbo.Orders;";
using (SqlConnection connection = new SqlConnection(
connectionString))
{
SqlCommand command = new SqlCommand(
queryString, connection);
connection.Open();
SqlDataReader reader = command.ExecuteReader();
try
{
while (reader.Read())
{
Console.WriteLine(String.Format("{0}, {1}",
reader[0], reader[1]));
}
}
finally
{
// Always call Close when done reading.
reader.Close();
}
}
}
It basicly first creates a SqlConnection object and then creates the SqlCommand-object that holds the actual select you are going to do, and a reference to the connection we just created. Then it opens the connection and on the next line, executes your statements and returns a SqlDataReader object.
In the while-loop it then outputs the values from the first row in the reader. Every time "reader.Read()" is called the reader will contain a new row.
Then the reader is then closed, and because we are exiting the "using"-secret, the connection is also closed.
EDIT: If you are looking for info on selecting/updating data in ASP.NET, 4GuysFromRolla has a very nice Multipart Series on ASP.NET 2.0's Data Source Controls
EDIT2: As others have pointed out, if you are using a newer version of .NET i would recommend looking into LINQ. An introduction, samples and writeup can be found on this MSDN page.
The old ADO.Net (sqlConnection, etc.) is a dinosaur with the advent of LINQ. LINQ requires .Net 3.5, but is backwards compatible with all .Net 2.0+ and Visual Studio 2005, etc.
To start with linq is ridiculously easy.
Add a new item to your project, a linq-to-sql file, this will be placed in your App_Code folder (for this example, we'll call it example.dbml)
from your server explorer, drag a table from your database into the dbml (the table will be named items in this example)
save the dbml file
You now have built a few classes. You built the exampleDataContext class, which is your linq initializer, and you built the item class which is a class for objects in the items table. This is all done automatically and you don't need to worry about it. Now say I want to get record with the itemID of 3, this is all I need to do:
exampleDataContext db = new exampleDataContext(); // initializes your linq-to-sql
item item_I_want = (from i in db.items where i.itemID == 3 select i).First(); // using the 'item' class your dbml made
And that's all it takes. Now you have a new item named item_I_want... now, if you want some information from the item you just call it like this:
int intID = item_I_want.itemID;
string itemName = item_I_want.name;
Linq is very simple to use! And this is just the tip of the iceberg.
No need to learn antiquated ADO when you have a more powerful, easier tool at your disposal :)
Reads like a beginner question. That calls for beginner video demos.
http://www.asp.net/learn/data-videos/
They are ASP.NET focused, but pay attention to the database aspects.
topics to look at:
ADO.NET basics
LINQ to SQL
Managed database providers
If it is a web application here are some good resources for getting started with data access in .NET:
http://weblogs.asp.net/scottgu/archive/2007/04/14/working-with-data-in-asp-net-2-0.aspx
To connect/perform operations on an SQL server db:
using System.Data;
using System.Data.SqlClient;
string connString = "Data Source=...";
SqlConnection conn = new SqlConnection(connString); // you can also use ConnectionStringBuilder
connection.Open();
string sql = "..."; // your SQL query
SqlCommand command = new SqlCommand(sql, conn);
// if you're interested in reading from a database use one of the following methods
// method 1
SqlDataReader reader = command.ExecuteReader();
while (reader.Read()) {
object someValue = reader.GetValue(0); // GetValue takes one parameter -- the column index
}
// make sure you close the reader when you're done
reader.Close();
// method 2
DataTable table;
SqlDataAdapter adapter = new SqlDataAdapter(command);
adapter.Fill(table);
// then work with the table as you would normally
// when you're done
connection.Close();
Most other database servers like MySQL and PostgreSQL have similar interfaces for connection and manipulation.
If what you are looking for is an easy to follow tutorial, then you should head over to the www.ASP.net website.
Here is a link to the starter video page: http://www.asp.net/learn/videos/video-49.aspx
Here is the video if you want to download it: video download
and here is a link to the C# project from the video: download project
Good luck.
I would also recommend using DataSets. They are really easy to use, just few mouse clicks, without writing any code and good enough for small apps.
If you have Visual Studio 2008 I would recommend skipping ADO.NET and leaping right in to LINQ to SQL
#J D OConal is basically right, but you need to make sure that you dispose of your connections:
string connString = "Data Source=...";
string sql = "..."; // your SQL query
//this using block
using( SqlConnection conn = new SqlConnection(connString) )
using( SqlCommand command = new SqlCommand(sql, conn) )
{
connection.Open();
// if you're interested in reading from a database use one of the following methods
// method 1
SqlDataReader reader = command.ExecuteReader();
while (reader.Read()) {
object someValue = reader.GetValue(0); // GetValue takes one parameter -- the column index
}
// make sure you close the reader when you're done
reader.Close();
// method 2
DataTable table;
SqlDataAdapter adapter = new SqlDataAdapter(command);
adapter.Fill(table);
// then work with the table as you would normally
// when you're done
connection.Close();
}