Question

I am having difficulty in reading data from my SQLite database from MonoTouch.

I can read and write without any difficulty for the first few screens and then suddenly I am unable to create any further connections with the error:

Mono.Data.Sqlite.SqliteException: Unable to open the database file   
at Mono.Data.Sqlite.SQLite3.Open (System.String strFilename, SQLiteOpenFlagsEnum flags, Int32 maxPoolSize, Boolean usePool) [0x0007e] in /Developer/MonoTouch/Source/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLite3.cs:136    
at Mono.Data.Sqlite.SqliteConnection.Open () [0x002aa] in /Developer/MonoTouch/Source/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteConnection.cs:888 

I ensure that i dispose and close every connection each time i use it but still i have this problem. For example:

var mySqlConn = new SqliteConnection(GlobalVars.connectionString);
mySqlConn.Open();
SqliteCommand mySqlCommand = new SqliteCommand(SQL, mySqlConn);
mySqlCommand.ExecuteNonQuery();
mySqlConn.Close();
mySqlCommand.Dispose();
mySqlConn.Dispose();

I’m guessing that I’m not closing the connections correctly. Any help would be greatly appreciated.

Answer 1

I’m pretty sure you guess is right. However it’s pretty hard to guess what went wrong (e.g. what’s defined in your connectionString will affect how Sqlite is initialized and will work).

From your example you seem to be disposing the SqliteConnection correctly but things could still go wrong. E.g. if some code throws an exception (and you catch them somewhere) then the Dispose call might never be called. It would be safer to do something like:

using (var mySqlConn = new SqliteConnection(GlobalVars.connectionString) {
    mySqlConn.Open();
    using (SqliteCommand mySqlCommand = new SqliteCommand(SQL, mySqlConn)) {
        mySqlCommand.ExecuteNonQuery();
        // work with the data
    }
    mySqlConn.Close();
}

That would ensure that the automagically finally clauses will dispose of the instance you create.

Also you might want to consider reusing your (first) connection instance, e.g. opening it once and re-use it everywhere in your application. OTOH you need to be aware of threading in this case (by default, you can change it, each connection is only safe to use on the thread that has created it).

Reusing could help your app performance but it also does not really fix your issue (but it might hide it). So I suggest you try to debug this first:

Using MonoDevelop you can set a breakpoint on line #136 on the /Developer/MonoTouch/Source/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLite3.cs file (which is included with your MonoTouch installation) to see the actual n error code (before it gets translated to a string).

You can also set breakpoints on the dispose code to ensure it gets executed (and does not return errors). The number of connection creations and disposals should match. If not then use the Call Stack to see who’s opening without closing.