Develop Reference

Defensive opening of a database connection with option to retry

Occasionally I am getting a SqlException exception on the line marked below.
It occurs when we have network problems and the server cannot be found.
public void markComplete(int aTKey)
{
SqlCommand mySqlCommand = null;
SqlConnection myConnect = null;
mySqlCommand = new SqlCommand();
try
{
myConnect = new SqlConnection(ConfigurationManager.ConnectionStrings["foo"].ConnectionString);
mySqlCommand.Connection = myConnect;
mySqlCommand.Connection.Open(); //<<<<<<<< EXCEPTION HERE <<<<<<<<<<<<<<<<<<
mySqlCommand.CommandType = CommandType.Text;
mySqlCommand.CommandText =
" UPDATE dbo.tb_bar " +
" SET LastUpdateTime = CONVERT(Time,GETDATE()) " +
" WHERE AKey = " + aTKey;
mySqlCommand.ExecuteNonQuery();
mySqlCommand.Connection.Close();
}
finally
{
if(mySqlCommand != null)
{
mySqlCommand.Dispose();
}
}
}
I have two questions so maybe should split into 2 SO questions:
Is my finally statement sufficiently defensive ?
Rather than just failing how involved would it be to amend the method so instead of crashing it waits for say 10 minutes and then tries again to open the connection - tries a maximum of 3 times and if still no valid connection it moves on?

Use parameters. Do not concatenate strings to create SQL statements. Read about SQL Injection.
Instead of using try...finally you can simplify your code with the using statement. You need to dispose all the instances that implements the IDisposable interface, so you also need to use the using statement with the SqlConnection. In fact, it's even more important then disposing the SqlCommand, since it allows ADO.Net to use connection pooling.
You don't need to do the entire procedure again and again, just the connection.Open and the ExecuteNonQuery.
Using the constructor that accepts a string an SqlConnection saves you the need to set them via properties.
You don't need to specify CommandType.Text - it's the default value.
Here is a basic implementation of retry logic with some improvements to your code:
public void markComplete(int aTKey)
{
var sql = " UPDATE dbo.tb_bar " +
" SET LastUpdateTime = CONVERT(Time,GETDATE()) " +
" WHERE AKey = #aTKey";
using(var myConnect = new SqlConnection(ConfigurationManager.ConnectionStrings["foo"].ConnectionString))
{
using(var mySqlCommand = new SqlCommand(sql, myConnect))
{
mySqlCommand.Parameters.Add("#aTKey", SqlDbType.Int).Value = aTKey;
var success = false;
var attempts = 0;
do
{
attempts++;
try
{
mySqlCommand.Connection.Open();
mySqlCommand.ExecuteNonQuery();
success = true;
}
catch(Exception ex)
{
// Log exception here
Threading.Thread.Sleep(1000);
}
}while(attempts < 3 || !success);
}
}
}
Update:
Well, I've had some free time and I remember writing a general retry method a few years back. Couldn't find it but here is the general idea:
static void RetryOnException(Action action, int numberOfRetries, int timeoutBetweenRetries)
{
var success = false;
var exceptions = new List<Exception>();
var currentAttempt = 0;
do
{
currentAttempt++;
try
{
action();
success = true;
}
catch(Exception ex)
{
exceptions.Add(ex);
Threading.Thread.Sleep(timeoutBetweenRetries);
}
} while(!success || currentAttempt < numberOfRetries);
// Note: The Exception will only be thrown in case all retries fails.
// If the action completes without throwing an exception at any point, all exceptions before will be swallowed by this method. You might want to log them for future analysis.
if(!success && exceptions.Count > 0)
{
throw new AggregateException("Failed all {numberOfRetries} retries.", exceptions);
}
}
Using this method you can retry all sort of things, while keeping your methods simpler and cleaner.
So here is how it should be used:
public void markComplete(int aTKey)
{
var sql = " UPDATE dbo.tb_bar " +
" SET LastUpdateTime = CONVERT(Time,GETDATE()) " +
" WHERE AKey = #aTKey";
using(var myConnect = new SqlConnection(ConfigurationManager.ConnectionStrings["foo"].ConnectionString))
{
using(var mySqlCommand = new SqlCommand(sql, myConnect))
{
mySqlCommand.Parameters.Add("#aTKey", SqlDbType.Int).Value = aTKey;
// You can do this inside a `try...catch` block or let the AggregateException propagate to the calling method
RetryOnException(
() => {
mySqlCommand.Connection.Open();
mySqlCommand.ExecuteNonQuery();
}, 3, 1000);
}
}
}

SQL ParameterDirection.InputOutput / SqlCommand.ExecuteNonQuery() error

I have a very strange problem that only occurs when the code in question is in a high load situations. My ASP.NET web client C# code calls a T-SQL stored procedure that has an OUTPUT parameter.
Under high loads, the data being returned sometimes does not make it back to the calling C# code. I have extracted all the relevant code into the example below;
Stored procedure:
CREATE PROCEDURE GetLoginName
#LoginId BIGINT,
#LoginName VARCHAR(50) OUTPUT
AS
SET NOCOUNT ON
SELECT #LoginName = LoginName
FROM Logins
WHERE Id = #LoginId
SET NOCOUNT OFF
GO
Database base class:
public class DatabaseContextBase : IDisposable
{
private SqlConnection _connection;
private string _connectionString;
private SqlInt32 _returnValue;
private int _commandTimeout;
private static int _maxDatabaseExecuteAttempts = 3;
private static int s_commandTimeout = 30;
protected DBContextBase()
{
// try and get the connection string off the identity first...
string connectionString = GetConnectionStringFromIdentity();
if(connectionString != null)
{
ConstructionHelper(connectionString, s_commandTimeout);
}
else
{
// use the initialised static connection string, and call the other overload
// of the constructor
ConstructionHelper(s_connectionString, s_commandTimeout);
}
}
private void ConstructionHelper( string connectionString, int commandTimeout )
{
// store the connection string in a member var.
_connectionString = connectionString;
// store the timeout in a member var.
_commandTimeout = commandTimeout;
}
public static string GetConnectionStringFromIdentity()
{
IIdentity identity = Thread.CurrentPrincipal.Identity as wxyz.Security.wxyzIdentityBase;
string connectionString = null;
if(identity != null)
{
connectionString = ((wxyz.Security.wxyzIdentityBase) identity ).ConnectionString;
}
return connectionString;
}
public void Dispose()
{
if (_connection.State != ConnectionState.Closed)
{
_connection.Close();
}
_connection.Dispose();
_connection = null;
}
protected void ExecuteNonQuery(SqlCommand command)
{
SqlConnection con = this.Connection;
lock (con)
{
if (con.State != ConnectionState.Open)
{
con.Open();
}
// don't need a try catch as this is only ever called from another method in this
// class which will wrap it.
command.Connection = con;
command.Transaction = _transaction;
command.CommandTimeout = _commandTimeout;
for (int currentAttempt = 1; currentAttempt == _maxDatabaseExecuteAttempts; currentAttempt++)
{
try
{
// do it
command.ExecuteNonQuery();
// done, exit loop
break;
}
catch (SqlException sex)
{
HandleDatabaseExceptions(currentAttempt, sex, command.CommandText);
}
}
}
}
protected void HandleDatabaseExceptions(int currentAttempt, SqlException sqlException, string sqlCommandName)
{
if (DataExceptionUtilities.IsDeadlockError(sqlException))
{
if (!this.IsInTransaction)
{
// Not in a transaction and a deadlock detected.
// If we have not exceeded our maximum number of attemps, then try to execute the SQL query again.
string deadlockMessage = string.Format("Deadlock occured in attempt {0} for '{1}':", currentAttempt, sqlCommandName);
Logging.Write(new ErrorLogEntry(deadlockMessage, sqlException));
if (currentAttempt == DBContextBase.MaxDatabaseExecuteAttempts)
{
// This was the last attempt so throw the exception
throw sqlException;
}
// Wait for a short time before trying again
WaitShortAmountOfTime();
}
else
{
// We're in a transaction, then the calling code needs to handle the deadlock
string message = string.Format("Deadlock occured in transaction for '{0}':", sqlCommandName);
throw new DataDeadlockException(message, sqlException);
}
}
else if (this.IsInTransaction && DataExceptionUtilities.IsTimeoutError(sqlException))
{
// We're in a transaction and the calling code needs to handle the timeout
string message = string.Format("Timeout occured in transaction for '{0}':", sqlCommandName);
// Raise a Deadlock exception and the calling code will rollback the transaction
throw new DataDeadlockException(message, sqlException);
}
else
{
// Something else has gone wrong
throw sqlException;
}
}
/// <summary>
/// get the SqlConnection object owned by this database (already connected to db)
/// </summary>
public SqlConnection Connection
{
get {
// check whether we've got a connection string (from either identity or static initialise)
if ( _connectionString == null )
{
throw new ArgumentNullException( "connectionString", "Connection string not set" );
}
if ( _connection != null )
{
return _connection;
}
else
{
_connection = new SqlConnection( _connectionString );
return _connection;
}
}
}
/// <summary>
/// Return value from executed stored procedure
/// </summary>
public SqlInt32 ReturnValue
{
get { return _returnValue; }
set { _returnValue = value; }
}
}
Database access class:
public class AuthenticationDBCommands
{
public static SqlCommand GetLoginName()
{
System.Data.SqlClient.SqlCommand cmd = new System.Data.SqlClient.SqlCommand("GetLoginName");
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add(new SqlParameter("#RETURN_VALUE", System.Data.SqlDbType.Int, 0, System.Data.ParameterDirection.ReturnValue, false, 0, 0, "RETURN_VALUE", System.Data.DataRowVersion.Current, SqlInt32.Null));
cmd.Parameters.Add(new SqlParameter("#LoginId", SqlDbType.BigInt, 8, ParameterDirection.Input, false, 0, 0, "LoginId", DataRowVersion.Current, SqlInt64.Null));
cmd.Parameters.Add(new SqlParameter("#LoginName", SqlDbType.VarChar, 50, ParameterDirection.InputOutput,false, 0, 0, "LoginName", DataRowVersion.Current, SqlString.Null));
return cmd;
}
}
public class AuthenticationDBContext : DatabaseContextBase
{
public AuthenticationDBContext() : base()
{
}
public void GetLoginName(SqlInt64 LoginId, ref SqlString LoginName)
{
SqlCommand cmd = AuthenticationDBCommands.GetLoginName();
cmd.Parameters[1].Value = LoginId;
cmd.Parameters[2].Value = LoginName;
base.ExecuteNonQuery(cmd);
base.ReturnValue = (SqlInt32) cmd.Parameters[0].Value;
LoginName = (SqlString)(cmd.Parameters[2].Value);
}
}
So when it's used inside the ASP.NET web client it look like this:
protected string GetLoginName(long loginId)
{
SqlString loginName = SqlString.Null;
using (AuthenticationDBContext dbc = new AuthenticationDBContext())
{
dbc.GetLoginName(loginId, ref loginName);
}
return loginName.Value;
}
As you can see this is fairly standard stuff. But when the AuthenticationDBContext.GetLoginName() method is called by many different users in quick succession the loginName object is sometimes null.
When the SqlCommand.ExecuteNonQuery() fails to return any data it does not throw an exception.
I have tested the SQL and it always finds a value (I've inserted #LoginName into a table and it's never null). So the problem is happening after or in SqlCommand.ExecuteNonQuery();
As I said, this is an example of what it happening. In reality, data is not being returned for lots of different stored procedures.
It's also worth stating that other web clients that share the app pool in IIS are not affected when the web client in question is under a heavy load.
I'm using .NET 4.5 and my database is on SQL Server 2008.
Has anyone seen anything like this before?
Can anyone recommend any changes?
Thanks in advance,
Matt
UPDATE. Thanks for all your comments. I have made the following change to the DatabaseContextBase class.
private void ExecuteNonQueryImpl(SqlCommand command)
{
object _lockObject = new object();
lock (_lockObject)
{
SqlConnection con = this.GetConnection();
if (con.State != ConnectionState.Open)
{
con.Open();
}
// don't need a try catch as this is only ever called from another method in this
// class which will wrap it.
command.Connection = con;
command.Transaction = _transaction;
command.CommandTimeout = _commandTimeout;
for (int currentAttempt = 1; currentAttempt <= _maxDatabaseExecuteAttempts; currentAttempt++)
{
try
{
// do it
command.ExecuteNonQuery();
// done, exit loop
break;
}
catch (SqlException sex)
{
HandleDatabaseExceptions(currentAttempt, sex, command.CommandText);
}
}
if (!this.IsInTransaction)
{
con.Close();
}
}
}
public SqlConnection GetConnection()
{
if (this.IsInTransaction)
{
return this.Connection;
}
else
{
// check whether we've got a connection string (from either identity or static initialise)
if ( _connectionString == null )
{
string exceptionMessage = Language.Translate("DbContextNotInitialized");
throw new ArgumentNullException( "connectionString", exceptionMessage );
}
return new SqlConnection(_connectionString);
}
}
However, in a load test the data still sometimes comes back as null. The web client is not working in a transaction so a new SqlConnection object is created, opened and closed every time a call is made. (there are other areas of code which share the DatabaseContextBase class that do work in a transaction so the original connection property is needed)
I would like to mention that again that I'm confident that the store procedure is working correctly as I have inserted the #LoginName value into a table and it's never null.
Thanks,
Matt

Your "for loop" definition is not correct.
for (int currentAttempt = 1; currentAttempt == _maxDatabaseExecuteAttempts; currentAttempt++)
This will initialize currentAttempt to 1, run the loop, increment currentAttempt, and then check to see if currentAttempt is equal to 3, which it isn't, and exit the loop. I think what you want is
for (int currentAttempt = 1; currentAttempt <= _maxDatabaseExecuteAttempts; currentAttempt++)

C# method that returns a SQL string without passing any variables

I am trying to create a method that runs a query then reads the 2 values it returns then place them in the global variables so I can access them in another page. My question is what method should I use because I have two variables to set. Typically I pass the variables that I will be using but in this case I'm not. This main seem simple but I can't think of a way to get these values. I am not sure how to look this problem up to research it either. I have included the code below what I have attempted so far. Thank you for you help.
public string getTotals3()
{
WorkerData workerData = new WorkerData();
StringBuilder sqlString = new StringBuilder();
sqlString.Append("SELECT DISTINCT DataWin8Data, DataWin7Data ");
sqlString.Append("FROM Data ");
sqlString.Append("WHERE Number = 4");
SqlDataReader reader = null;
SqlConnection dbConn = App_Code.DBHelper.getConnection();
try
{
reader = App_Code.DBHelper.executeQuery(dbConn, sqlString.ToString(), null);
if (reader != null)
{
while (reader.Read())
{
workerData.TotalCases4 = reader["DataWin8Data"] != DBNull.Value ? reader["DataWin8Data"].ToString() : string.Empty;
workerData.TotalPercentage4 = reader["DataWin7Data"] != DBNull.Value ? reader["DataWin7Data"].ToString() : string.Empty;
}
}
else
throw new Exception("No records returned");
}
catch (Exception ex)
{
throw ex;
}
finally
{
if (dbConn != null)
{
try { dbConn.Close(); dbConn.Dispose(); }
catch { }
}
if (reader != null)
{
try { reader.Close(); reader.Dispose(); }
catch { }
}
}
return workerData.ToString();
}

Don't use global variables. Return the values out of the method. The calling code should be in charge of placing those values wherever it needs. I recommend reading about Dependency Inversion Principle.
public WorkerData GetWorkerData()
{
...
using (SqlDataReader reader = ...)
{
if (reader.Read())
{
return new WorkerData
{
TotalCases4 = reader["DataWin8Data"] != DBNull.Value ? reader["DataWin8Data"].ToString() : string.Empty,
TotalCases3 = workerData.TotalPercentage4 = reader["DataWin7Data"] != DBNull.Value ? reader["DataWin7Data"].ToString() : string.Empty;
}
}
}
throw new ApplicationException("Could not retrieve worker data.");
}
From your calling class, simply do whatever you want with the return value:
WorkerData workerData = someClass.GetWorkerData();

How to fix a boolean method that returns true everytime asp.net

I designed my webpage to read a data string then display the results on labels in an html table. I am attempting to highlight the row that my database reads as a current order. My only problem is only one record is set to be active but they all highlight as if they were active. I use an array to set my data and I also use the label to get the ID I need (all is in code below). I have posted my method and where I use it in the asp page load. How can I fix my method to return correctly?
The implementing of the method in page load
if (lineData.IsCurrentOrderFind(L68.Text))
{
myTable.Rows[1].Cells[0].BgColor = "#FE2E2E";
myTable.Rows[1].Cells[1].BgColor = "#FE2E2E";
myTable.Rows[1].Cells[2].BgColor = "#FE2E2E";
myTable.Rows[1].Cells[3].BgColor = "#FE2E2E";
myTable.Rows[1].Cells[4].BgColor = "#FE2E2E";
}
Here is method that label above gets passed to
public bool IsCurrentOrderFind(string itemNumber)
{
StringBuilder sqlString = new StringBuilder();
sqlString.Append("SELECT * ");
sqlString.Append("FROM WorkOrder ");
sqlString.Append("WHERE LineNumber = " + ConfigurationManager.AppSettings["Line"] + " AND LineCompleted = 0 AND (ScaleGroup LIKE '%1' OR ScaleGroup LIKE '%3') ");
sqlString.Append(" AND CaseGenNum6 = #CaseGenNum6");
SqlDataReader reader = null;
SqlConnection dbConn = App_Code.DBHelper.getConnection();
SqlParameter[] parameters = new SqlParameter[] { new SqlParameter("#CaseGenNum6", itemNumber) };
try
{
reader = App_Code.DBHelper.executeQuery(dbConn, sqlString.ToString(), parameters);
while (reader.Read())
{
IsCurrentOrder = (reader["IsCurrentOrder"] != DBNull.Value && !string.IsNullOrEmpty(reader["IsCurrentOrder"].ToString())) ? true : false;
}
reader.Close();
reader.Dispose();
dbConn.Close();
dbConn.Dispose();
}
catch (Exception ex)
{
throw ex;
}
finally
{
if (dbConn != null)
{
try { dbConn.Close(); dbConn.Dispose(); }
catch { }
}
if (reader != null)
{
try { reader.Close(); reader.Dispose(); }
catch { }
}
}
if (IsCurrentOrder == true) I realize this is not necessary
{
return true;
}
else
{
return false;
}
}

The problem could be with this expression:
!string.IsNullOrEmpty(reader["IsCurrentOrder"].ToString())
Instead of calling ToString(), try simply casting it to a string:
!string.IsNullOrEmpty((string)reader["IsCurrentOrder"])
Possibly even better (the previous line might throw an exception if it's not really a string):
!string.IsNullOrEmpty(reader["IsCurrentOrder"] as string)
The reason being is that if the string is really null, calling ToString() will return a non-null string "null".

IsCurrentOrder is not declared locally. It seems to be declared at a higher scope. When you enter this function, nothing is initializing the variable (back to false). So, it is remaining at its last setting. Try this code instead:
public bool IsCurrentOrderFind(string itemNumber)
{
bool IsCurrentOrder = false;
//and the rest of your source code

the line
IsCurrentOrder = (reader["IsCurrentOrder"] != DBNull.Value && !string.IsNullOrEmpty(reader["IsCurrentOrder"].ToString())) ? true : false;
}
It's not actually checking the value of the field, only that it's not null or empty.
Try
if(
(reader["IsCurrentOrder"] != DBNull.Value
&&
!string.IsNullOrEmpty(reader["IsCurrentOrder"].ToString()))
)
{
IsCurrentOrder = reader["IsCurrentOrder"];
}
else
IsCurrentOrder = false;
I think there is a lot of refactoring you could do to this method though that will simplify the logic.

How to move to next row in datatable if one row catches an error? C# 2.0

I am getting data from a mySql database and I am inserting it into another system. If a column has data in an incorrect format I log this and go to next row in the datatable. It works as expected but now if I have a search function in my method that gets some additional data and this function fails I want to immediately log this and go to next row. As it is now I just log it but it still gets inserted (without the value that didn't meet the search criteria).
My code:
private void updateCustomer()
{
MySqlConnection connection = new MySqlConnection("server=myServer;database=myDatabase;uid=myID;password=myPass");
MySqlCommand command = new MySqlCommand(#"mySelectCommand", connection);
DataTable customerTbl = new DataTable();
MySqlDataReader reader;
try
{
connection.Open();
reader = command.ExecuteReader();
if (reader.HasRows)
{
customerTbl.Load(reader);
}
reader.Close();
}
catch (Exception ex)
{
_out.error("Could not connect to mySql database");
}
finally
{
connection.Close();
}
foreach (DataRow row in customerTbl.Rows)
{
// Declare the customer variables
string customerID = Encoding.ASCII.GetString((byte[])row["Customer ID"]);
string ChildOf = row["Child of"].ToString();
// Create the customer object
Customer customer = new Customer();
customer.entityId = customerID;
if (ChildOf != "")
{
RecordRef parentRef = new RecordRef();
try
{
parentRef.internalId = searchCustomer(ChildOf);
}
catch
{
// If it fails here I want to log the customerID and then go to the next row in the datatable (could not find the internalid for ChildOf
_out.error(customerID + " was not updated. Error: invalid format Parent string");
}
finally
{
parentRef.typeSpecified = false;
customer.parent = parentRef;
}
}
// Invoke update() operation
WriteResponse response = _service.update(customer);
// Process the response
if (response.status.isSuccess)
{
}
else
{
_out.error(customerID + " was not updated. Error: " + getStatusDetails(response.status));
}
}
}

You need to remove the row in the catch block, and change the foreach loop to a backwards for loop to handle the removals.

I realized that I want to log the other failed fields as well. Maybe it's an inefficient way but I did something like:
bool findParent = true;
if (ChildOf != "")
{
try
{
RecordRef parentRef = new RecordRef();
parentRef.internalId = searchCustomer(ChildOf);
parentRef.typeSpecified = false;
customer.parent = parentRef;
}
catch
{
findParent = false;
_out.error(customerID + " was not inserted. Error: invalid format Parent string");
}
}
And then an if statement before trying to insert:
if (findPartner == true && findParent == true)
{
response = _service.add(customer);
// Process the response
if (response.status.isSuccess)
{
}
else
{
_out.error(customerID + " was not inserted. Error: " + getStatusDetails(response.status));
}
}
else
{
//_out.error(customerID + " was not updated. Error: " + getStatusDetails(response.status));
}

Use the row.HasError property.