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.
Related
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);
}
I am try to display data from a database. However even though data exists in the database no records are being returned.
If run the following query:
select Id, Movie_Name from [MovieTable] where Movie_Name like '10,000 BC'
I get being returned:
However when running a similar query in c# nothing seems to be being returned. My code is as follows:
try
{
string query = "select * from [MovieTable] where Movie_Name like #MovieName";
string movieName = "10,000 BC"
using (SqlConnection sconnection = new SqlConnection(#"Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename=E:\Application\ApplicationDatabase.mdf;Integrated Security=True");)
using (SqlCommand command = new SqlCommand(query, sconnection))
{
sconnection.Open();
command.Parameters.AddWithValue("#MovieName", movieName);
using (SqlDataReader oReader = command.ExecuteReader())
{
if (oReader != null)
{
while (oReader.Read())
{
MessageBox.Show(oReader["Movie_Name"].ToString());
}
}
}
}
}
catch (Exception e)
{
MessageBox.Show(e.ToString());
}
The message box never appears. Adding a third message box just above oReader.Read() displays the message "Invalid attempt to read when no data is present". Is there something i am missing?
Your code runs fine for me, with little adaptions though.
You query the value of the attribute 'Year' which is not present in the table and in your first query:
oReader.GetOrdinal("Year")
That causes an exception in the sample.
Single Quotes are not needed since you are using a parametrized query.
For debugging calls to System.Diagnostics.Debug.WriteLine() are more useful than MessageBoxes
Issue was with the data itself. Field was incorrectly set as type "Text". I altered the fields data type to "nvarchar(MAX)". It also mean't the query could be altered to:
"select * from [MovieTable] where Movie_Name = #MovieName"
I want to build a simple loop to check incoming data from SQL server, compare it to a textfield, and execute non query if there are no duplicates.
I wrote this code:
try
{
bool exists = false;
conn = new SqlConnection(DBConnectionString);
SqlCommand check_user = new SqlCommand("SELECT usrEmail FROM tblUsers", conn);
SqlCommand add_user = new SqlCommand("INSERT INTO tblUsers (usrEmail, usrPassword, usrRealname, usrIsowner) VALUES (#email, #pass, #name, #owner)", conn);
// (I have removed all the paramaters from this code as they are working and irrelevant)
conn.Open();
SqlDataReader check = check_user.ExecuteReader();
while (check.Read())
{
if (Convert.ToString(check[0]) == UserEmail.Text)
{
MessageBox.Show("The email you entered already exists in the system.");
exists = true;
break;
}
}
if (exists == false)
{
add_user.ExecuteNonQuery();
}
else
{
return;
}
}
catch (Exception ex)
{
MessageBox.Show("There was a problem uploading data to the database. Please review the seller's details and try again. " + ex.Message);
return;
}
finally
{
conn.Close();
}
I used breakpoints and saw that the code runs the while loop fine, but when it reaches the ExecuteNonQuery command, it returns an error message:
there is already an open datareader associated with this command which
must be closed first
I tried to use a check.Close(); command, but when I do, it suddenly gets stuck with the duplicate email error message for reasons passing understanding.
Additionally, there was a fix I tried in which the data actually WAS sent to the database (I saw it in SQL Server Management Studio), but still gave an error message... That was even stranger, since the nonquery command is the LAST in this function. If it worked, why did it go to the catch?
I have searched the site for answers, but the most common answers are MARS (I have no idea what that is) or a dataset, which I do not want to use in this case.
Is there a simple solution here? Did I miss something in the code?
The simples way out would be:
using(SqlDataReader check = check_user.ExecuteReader())
{
while (check.Read())
{
if (Convert.ToString(check[0]) == UserEmail.Text)
{
MessageBox.Show("The email you entered already exists in the system.");
exists = true;
break;
}
}
}
That said, there are some serious problems with this code.
First of all, you don't really want to read all users just to check that an email address is already taken. select count(*) from tblUsers where usrEmail = #email is fine...
...or not, because there's a possibility of a race condition. What you should do is add a unique constraint on a usrEmail column and just insert into tblUsers, catching violations. Or you can use merge if you feel like it.
Next, you don't really want to have your data access code all over the place. Factor it out into separate classes/methods at least.
I have written that gets data from a sensor, I am trying to write this data to a local SQL Server database file.
Code doesn't produce any errors but the data is not being written to the data table.
I have used the following code: (any suggestions?)
static void Insert(string date, double value, string deviceName)
{
string path = Directory.GetCurrentDirectory();
string filename = path + "\\Database1.mdf";
Console.WriteLine(filename);
string connectionString = "Data Source=(LocalDB)\\v11.0;AttachDbFilename=" + filename + ";Database=Database1";
using (SqlConnection conn = new SqlConnection(connectionString))
try
{
{
conn.Open();
SqlDataAdapter adapter = new SqlDataAdapter();
using (SqlCommand cmd = new SqlCommand("INSERT INTO DataTable VALUES(#Id, #Date, #Value, #Device Name)", conn))
{
num11 += 1;
cmd.Parameters.AddWithValue("#Id", num11);
cmd.Parameters.AddWithValue("#Date", date);
cmd.Parameters.AddWithValue("#Value", value);
cmd.Parameters.AddWithValue("#Device Name", deviceName);
cmd.ExecuteNonQueryAsync();
//rows number of record got inserted
}
conn.Close();
}
}
catch (SqlException es)
{
Console.WriteLine(es);
//Display Error message
}
}
You're calling ExecuteNonQueryAsync to asynchronously insert the record - but you're then closing the connection immediately, while the insert has almost certainly not yet completed.
You should either use the synchronous call instead, or use asynchrony properly - probably making the method asynchronous, and awaiting the result of ExecuteNonQueryAsync. You need to wait for the operation to complete before you close the connection, basically.
You don't need to call Close explicitly at all, by the way - you've already got a using statement, so the connection will be disposed as execution exits the block. (It's not clear why you've got an extra block inside the using statement either, by the way.)
Im using the code below, to update a record in an MS Access database, to store some information related to a property grid, however, im receiving a syntax error when the query tries to execute, and i cannot figure out why. ConnCheck simply looks to see if the connection is open, and if not, it opens it.
Thanks in advance
Main_Class.ConnCheck();
OleDbCommand cmd = new OleDbCommand("UPDATE [CALCULATION_RUN_TBL] SET [CAP_INPUTS]=?, [RA_INPUTS]=?, WHERE [CALCULATION_RUN_ID]=?", Main_Class.con);
try
{
cmd.Parameters.Add("#CAP_INPUTS", OleDbType.LongVarBinary).Value = SaveCAPSettings();
cmd.Parameters.Add("#RA_INPUTS", OleDbType.LongVarBinary).Value = eig.SaveSettings();
cmd.Parameters.Add("#CALCULATION_RUN_ID", OleDbType.Integer).Value = Main_Class.Calculation_Run_ID;
//Main_Class.con.Open();
cmd.ExecuteNonQuery();
Main_Class.con.Close();
}
catch (OleDbException ex)
{
//get the error message if connection failed
MessageBox.Show("Error in connection ..." + ex.Message);
Main_Class.con.Close();
}
Well a quick look says you have an extra comma
This: [RA_INPUTS]=?, WHERE should be [RA_INPUTS]=? WHERE