Develop Reference

Specific if else statement after using a SQL call to retrieve data from a database. Visual Studios 2015 / C#

I'm attempting to add the finishing touch to a project I've been working on and am currently trying to modify a feature that I've created. The feature being that if a student has completed an examination, they are able to view the results. However, what I want to do is create an if else statement that is essentially: if the exam has been taken and completed, then they are redirected to the page that shows them the specific exam's results. Else, it returns a message at the top of the page stating "This examination has not been completed yet."
The current code I have (which is operated through a button on the page) is:
protected void btnViewPrevExam_Click(object sender, EventArgs e)
{
Session["intExaminationID"] = ddlExamination.SelectedValue;
Int32 int32StudentID = Convert.ToInt32(Session["StudentID"]);
Session["int32StudentID"] = Convert.ToInt32(int32StudentID);
// Define the ADO.NET connection object.
SqlConnection objSqlConnection = new SqlConnection(WebConfigurationManager.ConnectionStrings["OPT"].ConnectionString);
// Develop the SQL call.
// Develop the SQL call.
String strSQL = "";
strSQL = "SELECT AnswerID, Question, OptionA, OptionB, OptionC, OptionD, CorrectAnswer, Answer ";
strSQL += " FROM Question, Answer, Examination, Student ";
strSQL += " WHERE Examination.ExaminationID = " + ddlExamination.SelectedValue;
strSQL += " AND Student.StudentID = " + int32StudentID;
strSQL += " AND Answer.QuestionID = Question.QuestionID ";
strSQL += " AND Answer.StudentID = Student.StudentID ";
strSQL += " AND Examination.ExaminationID = Question.ExaminationID ";
// Create the SQL command object.
SqlCommand objSqlCommand = new SqlCommand(strSQL, objSqlConnection);
// Retrieve the row from the table.
objSqlConnection.Open();
SqlDataReader objSqlDataReader = objSqlCommand.ExecuteReader();
objSqlDataReader.Read();
if (strSQL != null)
{
objSqlDataReader.Close();
objSqlConnection.Close();
Response.Redirect("StudentExamResults.aspx");
}
else
{
this.Master.MessageForeColor = System.Drawing.Color.Red;
this.Master.Message = "The selected examination has not been completed.";
}
}
What this button does currently is that it will send the student to the examination results page regardless if the examination has been completed or not. This is due to the line "if (strSQL != null)" and it never being null because the SQL call has been made and filled. I've attempted other ideas, as well as performing a objSqlDataReader for the AnswerID but it didn't work properly. This is a small extra feature I'd like to add to this project that I thought of and would be very pleased if I could find some help on sorting out what I'm doing wrong. Thank you in advance!

Testing if strSQL is not null will always succeed because you are setting it to a non-null value earlier in the method.
To see if a record already exists for a previously-completed examination, you need to check the return value of the call to objSqlDataReader.Read(); it will return true as long as there are additional rows (or, in this case, a first row) to consume from your SELECT query. Thus, change this...
objSqlDataReader.Read();
if (strSQL != null)
{
...to this...
if (objSqlDataReader.Read())
{
As an additional note, consider wrapping objSqlConnection, objSqlCommand, and objSqlDataReader in using blocks to ensure they are properly closed/disposed. As it is now, you are not closing objSqlDataReader and objSqlConnection when the exam needs to be completed, and objSqlCommand is not disposed at all. objSqlDataReader would then be closed as follows, regardless of which branch of the if is taken...
using (SqlDataReader objSqlDataReader = objSqlCommand.ExecuteReader())
{
if (objSqlDataReader.Read())
{
//objSqlDataReader.Close();// No longer necessary - handled by using
objSqlConnection.Close();
Response.Redirect("StudentExamResults.aspx");
}
else
{
this.Master.MessageForeColor = System.Drawing.Color.Red;
this.Master.Message = "The selected examination has not been completed.";
}
}

Assuming you don't care about the contents, rather you just want to check if the row exists, you can do something like this:
string sql = "SELECT COUNT(AnswerID) FROM Question ........ WHERE ......";
using (var connection = CreateConnection()) {
using (var cmd = new SqlCommand(sql, connection)) {
bool exists = (int) cmd.ExecuteScalar() > 0;
if (exists) {
Response.Redirect("StudentExamResults.aspx");
} else {
// Do the other thing
}
}
}

Access Database - Operation must use an updateable query. (Works fine on local)

I checked other questions when i typed the title in, i can't understand anymore. i need to cool down a bit :)
I started asp.net 1,5 week ago everything was working fine on local and i uploaded my site files to ftp server and i'm getting this error:
Operation must use an updateable query.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Data.OleDb.OleDbException: Operation must use an updateable query.
Source Error:
//* Goruntulenme sayısını güncelleme //translation updating view count *
// This was the old code after checking other same titled questions i changed to this one and still no luck
//sorgu.Append(" UPDATE sorular SET [goruntulenme] = [goruntulenme] + 1 ");
//sorgu.Append(" WHERE soruid = #soruid ");
//komut = new OleDbCommand(sorgu.ToString(), baglanti);
//komut.Parameters.Add("#soruid", OleDbType.VarChar).Value = sorgusoruid.ToString();
//komut.ExecuteNonQuery();
string SqlString = "UPDATE sorular SET [goruntulenme] = [goruntulenme] + 1 WHERE soruid = ?";
using (OleDbCommand cmd = new OleDbCommand(SqlString, baglanti))
{
cmd.CommandType = CommandType.Text;
cmd.Parameters.AddWithValue("soruid", sorgusoruid.ToString());
cmd.ExecuteNonQuery();
}
}
Error points to that cmd.ExecuteNonQuery(); line. The things i checked:
*Table has a primary key
*Code works on local just like i need it
*This is a forum part of my project the simplest way i have 3 tables "users" "questions" and "answers"
*i can log in and log out on server. it works fine but can't update. i guess i'm suspecting permissions because this logging in code works (if you want i can translate variable parts of code).
OleDbConnection baglan = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + Server.MapPath("App_Data\\db.accdb"));
baglan.Open();
string k_ad = Kullanici.Text;
string k_sifre = sifre.Text;
OleDbCommand kontrol = new OleDbCommand("SELECT * FROM Kullanicilar WHERE K_Ad='" + k_ad + "' and K_Sfr='" + k_sifre + "'", baglan);
OleDbDataReader oku = kontrol.ExecuteReader();
if (oku.Read())
{
Session.Add("Kullanici", k_ad);
Response.Redirect("Forum.aspx");
lbl.Visible = false;
}
else
{
lbl.Text = "Kullanıcı adı veya şifre yanlış";
lbl.Visible = true;
}
baglan.Close();
baglan.Dispose();
}
I don't know what else i can give here as info. pls let me know if any more info you need about that and how can i find that information :))
Ohh and i'm using ms-access database, using sql database server seems more complicated to me.

Problem solved, i changed database folder permissions on my hosting service provider's website i logged in a panel with same id pw with the ones i use at filezilla there was settings for folders and they were read only i changed them to full access minus deleting and changing ownership

Database connection closure location in try catch/using

Which is better to make sure that the db connection is closed if the execution fails?
try
{
using (var mySqlConnection = new MySqlConnection(DatabaseManagement.DatabaseConnectionString))
{
DatabaseManagement.DatabaseCommand = mySqlConnection.CreateCommand();
DatabaseManagement.DatabaseCommand.CommandText = "INSERT INTO lastlaptimes Values ('" + UserObject.UserName + "','" + _lastLapTime + "')";
mySqlConnection.Open();
DatabaseManagement.DatabaseCommand.ExecuteNonQuery();
}
}
catch (MySqlException exception)
{
Logger.Error(exception);
}
Or this:
using (var mySqlConnection = new MySqlConnection(DatabaseManagement.DatabaseConnectionString))
{
try
{
DatabaseManagement.DatabaseCommand = mySqlConnection.CreateCommand();
DatabaseManagement.DatabaseCommand.CommandText = "INSERT INTO lastlaptimes Values ('" + UserObject.UserName + "','" + _lastLapTime + "')";
mySqlConnection.Open();
DatabaseManagement.DatabaseCommand.ExecuteNonQuery();
}
catch (MySqlException exception)
{
mySqlConnection.Close();
Logger.Error(exception);
}
}
I'm having issue with too many connections against the db, and I'm wondering if my first approach is leading to the problem with the connections, as the code is called numerous times and a new connection is opened and fails again and increasing the connections by 1.
Thanks for any help.

The only difference between these is whether or not you are explicitly calling the Close statement (both have a using statement, which runs Dispose automatically).
So the real question here is - does Dispose close the connection or do you need to call it explicitly? I believe the answer is that Dispose will call this for you (see this question). What that means is either works well - pick whichever you favor (I suppose the first is technically one line less code...)

ExecuteNonQuery() not saving any record

I'm working a WinForms based C# tool which has an attached MDF file based database. I'm trying to use the SqlCommand.ExecuteNonQuery() method to save a record to this attached MDF database, but the record is not saved. No error or exception occurs; only problem is that the record is not actually saved.
I have a Console.WriteLine at the top which shows the query I'm trying to run. Its correct syntax-wise, and if I copy-paste it from the output windows and run it separately, it works.
I have correctly defined the connection string as the following, and it works fine for fetching records:
public static String connectionString = #"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\TestBuildDB.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True";
Here the function I'm using to save records:
public static void PerformDBWriteTransaction(string inputSQLStatement)
{
Console.WriteLine(inputSQLStatement);
DataTable returnDataTable = new DataTable();
SqlConnection sqlConnection = new SqlConnection();
sqlConnection.ConnectionString = connectionString;
SqlCommand cmd = new SqlCommand();
cmd.Connection = sqlConnection;
cmd.CommandType = CommandType.Text;
cmd.CommandText = inputSQLStatement;
cmd.Connection.Open();
try
{
cmd.ExecuteNonQuery();
}
catch (SqlException ex)
{
errorMessages.Clear();
errorMessages.Append("The following errors were found in the SQL statement:\n\n");
for (int i = 0; i < ex.Errors.Count; i++)
{
errorMessages.Append("Index #" + i + "\n" +
"Message: " + ex.Errors[i].Message + "\n" +
"LineNumber: " + ex.Errors[i].LineNumber + "\n" +
"Source: " + ex.Errors[i].Source + "\n" +
"Procedure: " + ex.Errors[i].Procedure + "\n");
}
MessageBox.Show(errorMessages.ToString());
}
finally
{
cmd.Connection.Close();
}
}
Can someone tell me what might be the problem ? Do I need to perform a 'commit' somehow ?
EDIT:
I have found the problem, and have written up a solution below .. Thanks to all who helped me out though :)

I found the problem ! It was very simple, and it was stupid really :) .. The code above is all correct .. Yes, people pointed out optimizations, etc, but still the code above was correct.
The problem was that when I imported the TestDB.MDF file into my Visual 2010 project, a copy of that was made inside the project's folder. When you run/debug the program, another copy of the this file is made and is put in the \bin\Debug\ folder. In the connection string I was using, I had mentioned: AttachDbFilename=|DataDirectory|\TestBuildDB.mdf .. This meant that all reads/writes were done to the copy in the bin\Debug folder. However, the TestDB.MDF file I was looking into to verify if records were inserted or not, was in the project's folder ! So basically, there were two MDF files, and I was writing the records into one file, but was trying to find them in the other :)
When you add an MDF file into your VS2010 Project, VS2010 by default makes a connection to that MDF file, from where you can browse the stuff in that MDF file .. The MDF file used for this purpose was the one placed in the project's folder, NOT the one in bin\Debug\ folder. And like I said earlier, my code used the one in the bin\Debug folder :)
So what I've done now is that I've removed the Test.MDF file reference from my project, which removes the copy present in the project's folder. However, I DO have a copy of TestDB.MDF file in the bin\Debug\ folder, which I connect to from within my application. And if I want to browse the MDf file outside my project, I use SQL Management Studio. The only problem here is that an MDF file can only be used by one program at a given time. So if I have to use it with my application, I have to take it offline from SQL Management studio, and vica versa !
I hope this explanation helps someone out there :)

The solution to this problem is very simple just give the full path of the original MDF file in the connection String like this:
connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=**C:\VISUAL STUDIO 2012\PROJECTS\ENGLISHTOHINDIDICTIONARY\ENGLISHTOHINDIDICTIONARY\DICTIONARY.MDF**;Initial Catalog=Dictionary;Integrated Security=false"
providerName="System.Data.SqlClient"
That's it, your problem is solved.

I had the same challenge, I simply changed the database property "Copy to Output Directory" from "Copy always" to "Do not copy" then moved my database.mdf (drag & drop from my IDE) into the bin\debug folder.
Tip:
The bin directory is normally hidden, use the "Show All Files" to display it

Provide a catch clause for all Exceptions. If there something wrong other than SqlException, you will never see what is it and your db will neved be updated. Imagine there is a FormatException...
Also check the return of ExecuteNonQuery : it's the number of rows affected by the query.

First, you should always wrap up your IDisposable objects in a using to ensure they're closed and disposed of properly (and that connection pooling can do its thing). Second, when modifying data wrap up your sql in a transaction to maintain data integrity.
Try the following code and see if any exceptions are raised. I wouldn't normally recommend catching Exception as it's too general, I'd let that bubble up to the calling mechanism and handle it there - but for this instance it will show you any and all issues. I think your particular issue is at the .Open stage of the connection, so try stepping through.
public static void PerformDBWriteTransaction(string inputSQLStatement)
{
DataTable returnDataTable = new DataTable();
try
{
using (SqlConnection sqlConnection = new SqlConnection(connectionString))
{
sqlConnection.Open();
using (SqlTransaction sqlTrans = sqlConnection.BeginTransaction())
{
try
{
using (SqlCommand cmd = new SqlCommand(inputSQLStatement, sqlConnection))
{
cmd.CommandType = CommandType.Text;
cmd.Transaction = sqlTrans;
cmd.ExecuteNonQuery();
}
}
catch (SqlException sqlEx)
{
sqlTrans.Rollback();
throw sqlEx;
}
sqlTrans.Commit();
}
}
}
catch (Exception ex)
{
errorMessages.Clear();
errorMessages.Append("The following errors were found in the SQL statement:\n\n");
for (int i = 0; i < ex.Errors.Count; i++)
{
errorMessages.Append("Index #" + i + "\n" +
"Message: " + ex.Errors[i].Message + "\n" +
"LineNumber: " + ex.Errors[i].LineNumber + "\n" +
"Source: " + ex.Errors[i].Source + "\n" +
"Procedure: " + ex.Errors[i].Procedure + "\n");
}
MessageBox.Show(errorMessages.ToString());
}
}

Hi I am working on library database when I add student record executionNonQuery
shows error like invalid column name page opens but saving data from not happening.
Here I have given the code statement
public partial class add_student_info : Form
{
SqlConnection con = new SqlConnection(#"Data Source=DESKTOP-SPT6GLG\SQLEXPRESS;Initial Catalog=library_managment;Integrated Security=True;Pooling=False");
public add_student_info()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
con.Open();
SqlCommand cmd = con.CreateCommand();
cmd.CommandType = CommandType.Text;
cmd.CommandText = "insert into student_info values('" + textBox1.Text + "','" + textBox2.Text + "','" + textBox3.Text + "','" + textBox4.Text + "'," + textBox5.Text + "," + textBox6.Text + "," + textBox7.Text + ")";
cmd.ExecuteNonQuery();
con.Close();
MessageBox.Show("Student recorc addedd sussfully");
}
}
}

button sometimes redirects to "This page cannot be displayed"

I have an ASP.NET c# web application published to a server of ours. It consists of three pages, each with a form on them, sort of like a 3-page login. After the third page's form is validated, it sends the user to a different site. Here's a little diagram so I can explain it better:
NamePage ---> DateOfBirthPage ---> IDNumberPage ---> OtherSite
This has been working great in all of our development tests and stress tests. However, after we put it into production, occasionally when the "Next" button on the IDNumberPage is clicked, the user sees "This page cannot be displayed" with a "diagnose connection problems" button. When this occurs for one user, the same problem occurs for all users (meaning once it happens, nobody can fully authenticate). NamePage and DateOfBirthPage always work, and when the crash occurs, the IDNumberPage link doesn't change, suggesting that the crash is occurring on this side of the application, not after it redirects to OtherSite. I have friendly HTTP errors turned off but it's not showing any errors on the page. If we go into the server and restart the application, it works again.
The frustrating part is that we can't replicate this error to see how/why it's occurring.
Some things that are noteworthy:
Each page uses one query on a MS SQL server database
Each page passes up to 4 Session variables (only small Strings containing what was entered into the textbox form on the previous page(s))
The session is abandoned when the final "next" button is clicked.
All resultsets/connections/commands are closed before redirect.
Redirects use the overloaded version using Response.Redirect(siteName, false)
Sorry if all of this is very vague, but the problem itself has done an oddly good job of hiding from us. We have tried hammering the server with test requests (many at once, many over a period of time, etc) and different combinations of logging in/trying to break the page in general, to no avail. Can anyone suggest some things to try to diagnose/fix/replicate this problem?
Edit: The click function on IDNumberPage's code-behind that is causing the problem:
{ SqlConnection dbconn = new SqlConnection(Application["dbconn"].ToString());
SqlCommand sqlValidate = dbconn.CreateCommand();
dbconn.Open();
sqlValidate.CommandText = "SELECT lastName, csn FROM Demographics WHERE lastName = '" + Session["lastName"].ToString() + "' " +
"AND dob = '" + Session["dobCheck"].ToString() + "' AND mrn = " + strMRN;
SqlDataReader results = sqlValidate.ExecuteReader();
if (results.HasRows)
{
string csn = "";
while (results.Read())
{
if (!String.IsNullOrEmpty(results["csn"].ToString()))
{
csn = results["csn"].ToString();
break;
}
}
string url = Application["surveyUrlString"] + "&lastname=" + Session["lastName"].ToString() + "&mrn=" + strMRN + "&dobday=" + Session["dobday"].ToString()
+ "&dobmonth=" + Session["dobmonth"].ToString() + "&dobyear=" + Session["dobyear"].ToString() + "&csn=" + csn;
results.Close();
dbconn.Close();
Response.Redirect(url, false);
}

The problem is due to leaking sql connections.
You aren't properly disposing of your resources. Over time these are going to stack up in the connection pool until you reach a point where the pool overflows and your app dies. Resetting will obviously fix the issue.
Also this issue might not show up in "stress" tests depending on how, exactly you are testing the application.
The solution is to reformat that code to handle your database call better.
{
string url = string.empty;
using (SqlConnection dbconn = new SqlConnection(Application["dbconn"].ToString())) {
using (SqlCommand sqlValidate = dbconn.CreateCommand()) {
dbconn.Open();
sqlValidate.CommandText = "SELECT lastName, csn FROM Demographics WHERE lastName = '" + Session["lastName"].ToString() + "' " +
"AND dob = '" + Session["dobCheck"].ToString() + "' AND mrn = " + strMRN;
using (SqlDataReader results = sqlValidate.ExecuteReader()) {
if (results.HasRows) {
string csn = "";
while (results.Read())
{
if (!String.IsNullOrEmpty(results["csn"].ToString()))
{
csn = results["csn"].ToString();
break;
}
}
url = Application["surveyUrlString"] + "&lastname=" + Session["lastName"].ToString() + "&mrn=" + strMRN + "&dobday=" + Session["dobday"].ToString()
+ "&dobmonth=" + Session["dobmonth"].ToString() + "&dobyear=" + Session["dobyear"].ToString() + "&csn=" + csn;
}
} // sqldatareader
} // using sqlcommand
} // using sqlconnection
if (!String.IsNullOrEmpty(url)) {
Response.Redirect(url, false);
}
}
notice that you aren't redirecting until after everything is cleaned up.
SqlConnection, SqlCommand and SqlDataReader all implement IDisposable. You have to properly clean up after use otherwise the resources will be left hanging. The "best" way of doing this is to wrap them in a using clause. This ensures they are properly removed once the code block is exited as they aren't garbage collected like other objects.
Also note that the above code has a good side benefit. Namely, in case of error it STILL cleans up after you. Whereas the original code that was posted would clearly leak in the event the DB server didn't respond or threw some type of error when running the query.
The query could error out depending on the values contained in dboCheck, lastname and mrn parameters. For example, what if "BOB" was passed in for the dobCheck field, or Nothing was passed in for mrn... If dob is a datetime field in your database then the query will throw an error that will result in a leaked connection. Do that enough times and your site is down.
Upon further review, I'm guessing that's probably what is happening: people are putting in garbage data that your app is allowing to get to this point and the query is failing. Most likely this isn't something that you've handled in your test cases.
Side note: Please don't create your sql statements by using concatentation. That is a complete security no no. At the very least, parameterize those queries.

Nice answer Chris, one question aren't the .Close() statements missing in the Using statements?. Both for the connection and the datareader:
results.Close();
} // using sqldatareader
} // using sqlcommand
dbconn.Close();
} // using sqlconnection