Managing and Closing Dynamically created SQL connections in .net

Question

I have a c# windows form application that connects to databases dynamically where each user may connect to different databases.

The current implementation is as follows:

Connection Repository that contains a dynamically populated list of connections (per user).

When a user initiates a request that requires a database connection the respective connection is looked up from the connection repository ,opened , and then used in the user request .

Code Sample from the connection repository

public class RepoItem
{
    public string databasename;
    public SqlConnection sqlcnn;
}

public class ConnectionRepository
{
    private List<RepoItem> connectionrepositroylist;

    public SqlConnection getConnection(String dbname)
    {
        SqlConnection cnn = (from n in connectionrepositroylist
                             where n.databasename == dbname
                             select n.sqlcnn).Single;

        cnn.Open();
        return cnn;
    }
}

sorry for any code errors i just improvised a small version of the implementation for demonstration purpose.

I'am not closing connections after a command execution because it may be used by another command simultaneously.

The questions are:

• Should i be worried about closing the connections ?

• Does connection close automatically if it is idle for a specific period ?

I have a method in mind to implement a timer in the created Connection Repository and check for idle connections through the Executing ConnectionState Enumeration and close them manually. Any suggestions are welcome .

When i want a specific connection i call the getConnection function in the ConnectionRepository class and pass the database name as a parameter.

PS: I didn't post the complete implemented code because it is quite big and includes the preferences that affect the populating of the connection list.

Answer 1

I would suggest not to return the SQLConnection to the calling method at all. Instead, create a method that will accept an Action<SqlConnection>, create the connection inside a using block, and execute the action inside that block

This way you know that the connection will always be correctly closed and disposed, while giving the using code the freedom to do whatever it needs:

public class RepoItem
{
    public string databasename;
    public SqlConnection sqlcnn;
}

public class DatabaseConnector
{
    private List<RepoItem> connectionrepositroylist;

    private SqlConnection GetConnection(String dbname)
    {
        return (from n in connectionrepositroylist
                where n.databasename == dbname
                select n.sqlcnn).SingleOrDefault();
    }

    public void Execute(String dbname, Action<SqlConnection> action)
    {
        using (var cnn = GetConnection(dbname))
        {
            if (cnn != null) // in case dbname is not in the list...
            {
                cnn.Open();
                action(cnn);
            }
        }

    }
}

Then, to execute an sql statement you can do something like this:

public void ExecuteReaderExample(string dbName, string sql)
{
    Execute("dbName",
    connection =>
    {
        using (var cmd = new SqlCommand(sql, connection))
        {
            using (var reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    // do stuff with data form the database
                }
            }
        }
    });
}

Of course, you can also wrap the SqlCommand in a method like this. I've been working with this approach for quite some time now, and as far as I can tell it's working well. In fact, It's working so well I've published a project on git hub based on this approach.
It saves you a lot of the plumbing when dealing with ado.net, by wrapping the connection, command, reader and adapter much the same way. Feel free to download it and adapt to your needs.

P.S. To answer your questions directly:

Should i be worried about closing the connections ?

Yes, you should.

Does connection close automatically if it is idle for a specific period ?

No, it doesn't.

However, implementing a method like I suggested will handle closing and disposing the connection object for you, so you don't need to worry about it.

Update

As Yahfoufi wrote in his comment, this design has a flaw, since multiple commands are using the same instance of SqlConnection, you are risking closing the connection while other commands are running. However, fixing this design flaw is very easy - instead of holding SqlConnection in RepoItem you can simply hold the connection string:

public class RepoItem
{
    public string DatabaseName {get; set;}
    public string ConnectionString {get; set;}
}

Then you change the GetConnection method like this:

    private SqlConnection GetConnection(String dbname)
    {
        return new SqlConnection(from n in connectionrepositroylist
                where n.databasename == dbname
                select n.sqlcnn).SingleOrDefault());
    }

Now each Execute method is working on it's own individual instance of SqlConnection so you don't need to worry about closing in the middle of some other command executing.

However, While we are on the subject of refactoring, I would suggest removing the RepoItem class all together and instead of using a List<RepoItem> to hold the connection strings simply use a Dictionary<string, string>, where the database name is the key and the connection string is the value. This way you can only have one connection string per database name, and your GetConnection method is simplified to this:

private Dictionary<string, string> connectionrepositroylist;

    private string GetConnectionString(String dbname)
    {
        return connectionrepositroylist.ContainsKey(dbname) ? connectionrepositroylist[dbname] : "";
    }

So, the complete DatabaseConnector class will look like this:

public class DatabaseConnector
{
    private Dictionary<string, string> connectionrepositroylist;

    private string GetConnectionString(String dbname)
    {
        return connectionrepositroylist.ContainsKey(dbname) ? connectionrepositroylist[dbname] : "";
    }

    public void Execute(String dbname, Action<SqlConnection> action)
    {
        var connectionString = GetConnectionString(dbname);
        if(!string.IsNullOrEmpty(connectionString))
        {    
            using (var cnn = new SqlConnection(connectionString))
            {
                cnn.Open();
                action(cnn);
            }
        }
    }

    // Of course, You will need a way to populate your dictionary - 
    // I suggest having a couple of methods like this to add, update and remove items.
    public bool AddOrUpdateDataBaseName(string dbname, string connectionString)
    {
        if(connectionrepositroylist.ContainsKey(dbname))
        {
            connectionrepositroylist[dbname] = connectionString;
        }
        else
        {
            connectionrepositroylist.Add(dbname, connectionString);
        }
    }
}

Answer 2

The good news is that ADO.Net manages your connection pools dynamically, so there's minimal overhead in you dynamically opening and closing connections in code. There's a good document here if you want to look through the detail.

To answer the specific questions you've raised:

Should i be worried about closing the connections ?

Yes, but not for the reasons you may think. Microsoft encourage you to close your connections, so as to return them to the pool for (re)use elsewhere in your code. Closing the connection doesn't actually close it - it merely returns the underlying connection to the pool. Failure to close your connections properly can lead to delays in them being returned to the pool, thus adversely affecting your applications performance as more connections need to be added to the pool to cope with demand.

Does connection close automatically if it is idle for a specific period ?

A connection is only returned to the pool when it's Dispose or Finalise methods get called. If you create a connection and drop it into a static container then it will not be returned to the pool at all. As such, your ConnectionRepository may actually be harming performance.

I have a method in mind to implement a timer in the created Connection Repository and check for idle connections

This is unnecessary - close your connections to allow them to return to the pool. This way they will be available for other threads to use

Personally, I'd suggest that you modify your RepoItem class to store connection strings, rather than connection objects, and let ADO.Net's pooling do all the heavy lifting.

public static class ConnectionRepository
{
    private static readonly Dictionary<string, string> Connections = new Dictionary<string, string>(StringComparer.CurrentCultureIgnoreCase);

    public static bool Contains(string key)
    {
        return Connections.ContainsKey(key);
    }

    public static void Add(string key, string connectionString)
    {
        Connections.Add(key, connectionString);
    }

    public static SqlConnection Get(string key)
    {
        var con = new SqlConnection(Connections[key]);
        con.Open();
        return con;
    }
}

With this in place, you can query the database as follows:

public static void foo()
{
    using (var con = ConnectionRepository.Get("MyConnection"))
    using (var cmd = new SqlCommand("SELECT * FROM MyTable", con))
    {
        var dr = cmd.ExecuteReader();
        //...
    }
}

Once the query has executed and the connection is no longer required, the using() block calls its Dispose() method and releases the underlying connection back to the pool for re-use.