I have this problem: Invalid attempt to call HasRows when reader is closed.
I have tried alot; removing connection close line, closing the connection in the end. but having same issue. I can't get whats wrong with my code.
try
{
con = new SqlConnection(ConfigurationManager.ConnectionStrings["TextItConnectionString"].ConnectionString);
using (con)
{
con.Open();
Library.writeErrorLog("connection build and open");
SqlCommand cmd = con.CreateCommand();
using (cmd)
{
cmd.CommandText = "Select [name] From [dbo].[Users]";
SqlDataReader reader = cmd.ExecuteReader();
using (reader)
{
user.dt.Load(reader);
if (reader.HasRows)
{
while (reader.Read())
{
Library.writeErrorLog(reader.GetString(0));
}
}
else
Library.writeErrorLog("no rows");
reader.Close();
con.Close();
}
}
}
//SqlDataAdapter adap = new SqlDataAdapter("Select [name] From [dbo].[Users]", con);
//adap.Fill(user.dt);
}
catch (Exception ex)
{
Library.writeErrorLog(ex);
}
Thanks for the help!
I assume that user.dt returns a DataTable. You know that DataTable.Load(reader) will consume all records of the resultset and advances the reader to the next set? I'm asking because you are using HasRows after you've already used DataTable.Load.
As Steve has commented
Looking at the reference source of DataTable.Load you could clearly
see that the DataReader is closed before exiting from the method.
So if there is no other resultset(f.e. SELECT * FROM T1;SELECT* from T2) the reader will be closed at the end of Load which will cause the exception if you try to use SqlDataReader.HasRows.
I'd call this a lack of documentation since it's mentioned nowhere on MSDN.
So either use
reader.Read and reader.GetString in a loop and add it to the DataTable manually,
use DataTable.Load and loop the table afterwards or
use SqlDataAdapter.Fill(table):
1) while loop and manually filling the table
using (SqlDataReader reader = cmd.ExecuteReader())
{
if(reader.HasRows)
{
while (reader.Read())
{
string name = reader.GetString(0);
user.dt.Rows.Add(name);
Library.writeErrorLog(name);
}
}
else
Library.writeErrorLog("no rows");
}
2) requires two loops, one in DataTable.Load and the foreach
using (SqlDataReader reader = cmd.ExecuteReader())
{
if(reader.HasRows)
{
user.dt.Load(reader); // all records added
foreach(DataRow row in user.dt.Rows)
{
string name = row.Field<string>(0);
Library.writeErrorLog(name);
}
}
else
Library.writeErrorLog("no rows");
}
3) another option is to use a SqlDataAdapter and it's Fill(dataTable) method:
using (var da = new SqlDataAdapter(cmd))
{
da.Fill(user.dt);
if (user.dt.Rows.Count > 0)
{
foreach (DataRow row in user.dt.Rows)
{
string name = row.Field<string>(0);
Library.writeErrorLog(name);
}
}
else
Library.writeErrorLog("no rows");
}
Side-note: you don't need to use reader.Close or con.Close if you use the using-statement.
Related
I have a problem with my MySQL Reader - I am running the reader in a loop, checking if a configured entry exists in my database. After the reader is applied, the reader is getting closed and set to null again.
Anyhow, I always get this error message when I am running the "CheckExistEntry" - function in my code.
"07.04.2021 14:28:05 ERROR: There is already an open Data Reader associated with this connection which must be closed first."
The error does not occur in the following situations:
If I set a breakpoint at the relevant position in the code, the error does not occur.
If I set a sleep for 1000 MS before the reader is executed, the error does not occur.
Is it possible that C# is running loops via multithreading without me knowing it?
Here's the code:
public bool CheckExistEntry(string iColumnName)
{
MySqlDataReader reader = null;
string query2 = "SHOW COLUMNS FROM defectdetection.defects_main";
MySqlCommand cmd = new MySqlCommand(query2, MySqlConn);
try
{
// SELECT column_name FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_SCHEMA =[Database Name] AND TABLE_NAME =[Table Name];#
// write_Log()
write_Log(log_directory, log_file, "BEFORE ExecuteReader");
//COMMENT FOR STACKOVERFLOW: cmd.ExecuteReader() triggers the Error.
reader = cmd.ExecuteReader();
write_Log(log_directory, log_file, "AFTER ExecuteReader");
//now, communication with MySQL is finished. ..
List<string> ColumnNames = new List<string>();
while (reader.Read())
{
ColumnNames.Add((string)reader[0]);
}
if (ColumnNames.Contains(iColumnName))
{
reader.Close();
reader.Dispose();
reader = null;
return true;
}
else
{
reader.Close();
reader.Dispose();
reader = null;
return false;
}
}
catch (Exception ex)
{
if (reader != null)
{
reader.Close();
reader.Dispose();
reader = null;
}
exception = new ArgumentException(ex.Message);
MessageBox.Show(ex.Message);
//TODO handle exception
write_Log(log_directory, log_file,"ERROR: There is already an open Data Reader associated with this connection which must be closed first.");
progressBarForm.Invoke(new updatebar(progressBarForm.Close));
return false;
}
}
i tried it with a triple-using statement now (using MySqlConnection, using MySqlCommand, using MySqlDatareader). But it still does not work, i get another error now: "Connection must be valid and open."
using (MySqlConnection MySqlConnLocal = new MySqlConnection()) {
using (MySqlCommand cmd = new MySqlCommand(query2, MySqlConnLocal)) {
using (MySqlDataReader reader = cmd.ExecuteReader())
{
while (reader.Read())
{
ColumnNames.Add((string)reader[0]);
}
}
if (ColumnNames.Contains(iColumnName))
{
return true;
}
else
{
return false;
}
}
}
FIXED IT NOW: This Code here was the solution:
//System.Threading.Thread.Sleep(1);
using (MySqlConnection MySqlConnLocal = new MySqlConnection(ConnString))
{
MySqlConnLocal.Open();
using (MySqlCommand cmd2 = new MySqlCommand(query2, MySqlConnLocal))
{
reader = cmd2.ExecuteReader();
while (reader.Read())
{
ColumnNames.Add((string)reader[0]);
}
}
}
You haven't dispose of "cmd". Try the following:
...
public bool CheckExistEntry(string iColumnName)
{
string query2 = "SHOW COLUMNS FROM defectdetection.defects_main";
try
{
using (MySqlConnection mySqlConn = new MySqlConnection(ConnectionStr))
{
mySqlConn.Open();
using (MySqlCommand cmd = new MySqlCommand(query2, mySqlConn))
{
using (MySqlDataReader reader = cmd.ExecuteReader())
{
while (reader.Read())
{
if ((string)reader[0] == iColumnName)
{
return true;
}
}
}
}
}
}
catch (Exception ex)
{
var exception = new ArgumentException(ex.Message);
MessageBox.Show(ex.Message);
//TODO handle exception
write_Log(log_directory, log_file, "ERROR: There is already an open Data Reader associated with this connection which must be closed first.");
//ToDo: add any additional desired code
}
return false;
}
I have the following code and I was wondering if anyone knew the correct way to handle this.
SqlConnection objConn = new SqlConnection(ConfigurationManager.ConnectionStrings["someConnectionString"].ConnectionString);
SqlCommand objComm = new SqlCommand("usp_someStoredProcedure", objConn);
objComm.CommandType = CommandType.StoredProcedure;
objComm.Parameters.AddWithValue("#Variable1", VOne);
objComm.Parameters.AddWithValue("#Variable2", VTwo);
objConn.Open();
using (IDataReader dr = objComm.ExecuteReader(CommandBehavior.CloseConnection))
{
//do stuff
}
Now, let's say the stored procedure returns nothing, is there a method to handle this?
Normally the section you have marked with //do stuff would contain a
if (dr.Read())
{
// do stuff
}
or a
while (dr.Read())
{
// do stuff
}
The .Read() check ensures that you're only acting if it returned data.
while (dr.Read())
{
Console.WriteLine(String.Format("{0}", reader[0]));
}
you can refer from http://msdn.microsoft.com/en-us/library/y6wy5a0f.aspx
I normally use DataSet because It is very flexible. Recently I am assigned code optimization task , To reduce hits to the database I am changing two queries in a procedure. one Query returns the count and the other returns the actual data. That is , My stored procedure returns two tables. Now, I know how to read both tables using DataSets, But I need to read both tables using DataReader. In search of that I found This.
I follow the article and wrote my code like this:
dr = cmd.ExecuteReader();
while (dr.Read())
{
}
if (dr.NextResult()) // this line throws exception
{
while (dr.Read())
{
But I am getting an exception at dt.NextResult. Exception is :
Invalid attempt to call NextResult when reader is closed.
I also googled above error , but still not able to solve the issue.
Any help will be much appreciated. I need to read multiple tables using datareader, is this possible?
Try this because this will close connection ,data reader and command once task get over , so that this will not give datareader close exception
Also do check like this if(reader.NextResult()) to check there is next result,
using (SqlConnection connection = new SqlConnection("connection string here"))
{
using (SqlCommand command = new SqlCommand
("SELECT Column1 FROM Table1; SELECT Column2 FROM Table2", connection))
{
connection.Open();
using (SqlDataReader reader = command.ExecuteReader())
{
while (reader.Read())
{
MessageBox.Show(reader.GetString(0), "Table1.Column1");
}
if(reader.NextResult())
{
while (reader.Read())
{
MessageBox.Show(reader.GetString(0), "Table2.Column2");
}
}
}
}
}
I have tried to reproduce this issue (also because i haven't used multiple tables in a reader before). But it works as expected, hence i assume that you've omitted the related code.
Here's my test code:
using (var con = new SqlConnection(Properties.Settings.Default.ConnectionString))
{
using (var cmd = new SqlCommand("SELECT TOP 10 * FROM tabData; SELECT TOP 10 * FROM tabDataDetail;", con))
{
int rowCount = 0;
con.Open();
using (IDataReader rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
String object1 = String.Format("Object 1 in Row {0}: '{1}'", ++rowCount, rdr[0]);
}
if (rdr.NextResult())
{
rowCount = 0;
while (rdr.Read())
{
String object1 = String.Format("Object 1 in Row {0}: '{1}'", ++rowCount, rdr[0]);
}
}
}
}
}
I built on Pranay Rana's answer because I like keeping it as small as possible.
string rslt = "";
using (SqlDataReader dr = cmd.ExecuteReader())
{
do
{
while (dr.Read())
{
rslt += $"ReqID: {dr["REQ_NR"]}, Shpr: {dr["SHPR_NR"]}, MultiLoc: {dr["MULTI_LOC"]}\r\n";
}
} while (dr.NextResult());
}
The question is old but I find the answers are not correct.
Here's how I do it:
List<DataTable> dataTables = new();
using IDataReader dataReader = command.ExecuteReader();
do
{
DataTable dataTable = new();
dataTable.Load(dataReader);
dataTables.Add(dataTable);
}
while (!dataReader.IsClosed);
This question is continuation of my previous one. Without going into too much details, I'm filling dataset with 2 related 1-to-many tables.
So, my question now is - why this code works good
public DataAgencyR_DataSet SelectOne(int id)
{
DataAgencyR_DataSet result = new DataAgencyR_DataSet();
using (DbCommand command = Connection.CreateCommand())
{
try
{
command.CommandText = SqlStrings.SelectDataAgencyR_SelectOne();
var param = ParametersBuilder.CreateByKey(command, "ID_DeclAgenc", id, "ID_DeclAgenc");
command.Parameters.Add(param);
Connection.Open();
using (DbDataReader reader = command.ExecuteReader())
{
while (reader.Read())
{
System.Diagnostics.Trace.WriteLine(String.Format("{0}-{1}", reader[0], reader[1]));
}
System.Diagnostics.Trace.WriteLine("-------------");
reader.NextResult();
while (reader.Read())
{
System.Diagnostics.Trace.WriteLine(String.Format("{0}-{1}", reader[0], reader[1]));
}
}
}
catch (DbException e)
{
Logger.Error(e.Message, e);
throw new DataAccessException("Error occurs while SelectOne method porcessed", e);
}
finally
{
if (Connection.State != ConnectionState.Closed) Connection.Close();
}
}
return result;
}
public static string SelectDataAgencyR_SelectOne()
{
return "SELECT a.* FROM t0_DataAgency_R a WHERE a.SetToPartners = 1 AND a.ID_DeclAgenc = #ID_DeclAgenc;" +
"SELECT c.* FROM t01_ChoiceParam_R c JOIN t0_DataAgency_R a on a.ID_DeclAgenc = c.ID_DeclAgenc WHERE SetToPartners = 1 AND a.ID_DeclAgenc = #ID_DeclAgenc";
}
and this is not
public DataAgencyR_DataSet SelectOne(int id)
{
DataAgencyR_DataSet result = new DataAgencyR_DataSet();
using (DbCommand command = Connection.CreateCommand())
{
try
{
command.CommandText = SqlStrings.SelectDataAgencyR_SelectOne();
var param = ParametersBuilder.CreateByKey(command, "ID_DeclAgenc", id, "ID_DeclAgenc");
command.Parameters.Add(param);
Connection.Open();
using (DbDataReader reader = command.ExecuteReader())
{
result.t0_DataAgency_R.Load(reader);
reader.NextResult();
result.t01_ChoiceParam_R.Load(reader);
}
}
catch (DbException e)
{
Logger.Error(e.Message, e);
throw new DataAccessException("Error occurs while SelectOne method porcessed", e);
}
finally
{
if (Connection.State != ConnectionState.Closed) Connection.Close();
}
}
return result;
}
public static string SelectDataAgencyR_SelectOne()
{
return "SELECT a.* FROM t0_DataAgency_R a WHERE a.SetToPartners = 1 AND a.ID_DeclAgenc = #ID_DeclAgenc;" +
"SELECT c.* FROM t01_ChoiceParam_R c JOIN t0_DataAgency_R a on a.ID_DeclAgenc = c.ID_DeclAgenc WHERE SetToPartners = 1 AND a.ID_DeclAgenc = #ID_DeclAgenc";
}
After second example, I have filled only result.t0_DataAgency_R table - but not result.t01_ChoiceParam_R. Why can it be so?
Thanks in advance
DataTable.Load automatically advances the reader to the next result. So you should remove your explicit call to NextResult.
Meaning:
using (DbDataReader reader = command.ExecuteReader())
{
result.t0_DataAgency_R.Load(reader);
result.t01_ChoiceParam_R.Load(reader);
}
Adding a DataSet to the mix... we used to use SqlDataAdapter and returned a DataSet but didn't take advantage of any of the offline features, etc., so a SqlDataReader is a better fit. Here's code to fill a DataSet. Found this was about 10% faster overall.
Dim s As DataSet = New DataSet()
Using reader As SqlDataReader = command.ExecuteReader()
Dim tables As New List(Of DataTable)
Do
Dim table As New DataTable()
table.Load(reader)
tables.Add(table)
s.Tables.Add(table)
Loop While Not reader.IsClosed
s.Load(reader, LoadOption.OverwriteChanges, tables.ToArray())
End Using
Hello I'm in need of a code to read columns and rows for C#.
I've come this far:
obj.MysqlQUERY("SELECT * FROM `players` WHERE name = "+name+";"); // my query function
Grateful for help;]
Here is a standard block of code that I use with MySql a lot. Note that you must be using the MySql connector available here.
string myName = "Foo Bar";
using (MySqlConnection conn = new MySqlConnection("your connection string here"))
{
using (MySqlCommand cmd = conn.CreateCommand())
{
conn.Open();
cmd.CommandText = #"SELECT * FROM players WHERE name = ?Name;";
cmd.Parameters.AddWithValue("Name", myName);
MySqlDataReader Reader = cmd.ExecuteReader();
if (!Reader.HasRows) return;
while (Reader.Read())
{
Console.WriteLine(GetDBString("column1", Reader);
Console.WriteLine(GetDBString("column2", Reader);
}
Reader.Close();
conn.Close();
}
}
private string GetDBString(string SqlFieldName, MySqlDataReader Reader)
{
return Reader[SqlFieldName].Equals(DBNull.Value) ? String.Empty : Reader.GetString(SqlFieldName);
}
Note that I am using a method to return a specific value if the database value is null. You can get creative and provide various return values or incorporate nullable types, etc.
Also you can use:
Create your own dataTable. When reader reaches end, you will have datatable which is custom created, and custom filled by yourself.
DataTable dt = new DataTable();
dt.Columns.Add("Id",typeof(int));
dt.Columns.Add("Name",typeof(string));
dt.Columns.Add("BlaBla",typeof(string));
dt.AcceptChanges();
// Your DB Connection codes.
while(dr.Read())
{
object[] row = new object[]()
{
dr[0].ToString(),// ROW 1 COLUMN 0
dr[1].ToString(),// ROW 1 COLUMN 1
dr[2].ToString(),// ROW 1 COLUMN 2
}
dt.Rows.Add(row);
}