System.Data.SQLite

The following code works flawlessly:

        {
            using (SQLiteTransaction transaction = my_connection.BeginTransaction())
            {
                using (SQLiteCommand command = new SQLiteCommand(my_connection))
                {
                    SQLiteParameter item_id = command.CreateParameter();
                    item_id.Value = xitem.my_id;

                    SQLiteParameter item_name = command.CreateParameter();
                    item_name.Value = xitem.my_name;

                    SQLiteParameter item_icon = command.CreateParameter();
                    item_icon.Value = xitem.my_icon;

                    SQLiteParameter item_level = command.CreateParameter();
                    item_level.Value = xitem.my_level;

                    SQLiteParameter item_quality = command.CreateParameter();
                    item_quality.Value = xitem.my_quality;

                    SQLiteParameter item_type = command.CreateParameter();
                    item_type.Value = xitem.my_type;
                   
                    command.CommandText = "insert into items (id, name, icon, level, quality, type) values (?1, ?2, ?3, ?4, ?5, ?6);";
                    command.Parameters.Add(item_id);
                    command.Parameters.Add(item_name);
                    command.Parameters.Add(item_icon);
                    command.Parameters.Add(item_level);
                    command.Parameters.Add(item_quality);
                    command.Parameters.Add(item_type);

                    command.ExecuteNonQuery();
                }
                transaction.Commit();
            }

            return 0;
        }

 If I change the query to either of these two:

                    command.CommandText = "insert into items (id, name, icon, level, quality, type) values (?1, ?2, ?3, ?4, ?5, ?6);";
                    command.CommandText = "insert into items (id, name, icon, level, quality, type) values (:a, :b, :c, :d, :e, :f);";

 Then I get an exception from the ExecuteNonQuery() function call:

 An unhandled exception of type 'System.Data.SQLite.SQLiteException' occurred in System.Data.SQLite.dll

Additional information: SQLite error
Insufficient parameters supplied to the command

(Interestingly enough I had the opposite problem when I was using the C library directly in another project.  I couldn't get queries with only ?'s to work.)
 

This is how an ADO .NET Data Provider should behave. However, the one you say works flawlessly should not work at all (you meant ?, ?, ?... right?).

The three common and supported patterns are:

1.) A set of ? with no name (works by going in order from left to right)

 using (SQLiteCommand command = connection.CreateCommand())
{
    command.CommandText = "INSERT INTO test VALUES (?, ?);";
    command.Parameters.AddWithValue("value1", "val1");
    command.Parameters.AddWithValue("value2", 1);
    command.ExecuteNonQuery();
}

2.) An at signed prefixed name 

using (SQLiteCommand command = connection.CreateCommand())
{
    command.CommandText = "INSERT INTO test VALUES (@value1, @value2);";
    command.Parameters.AddWithValue("value1", "val2");
    command.Parameters.AddWithValue("value2", 2);
    command.ExecuteNonQuery();
}

3.) A colon prefixed name 

using (SQLiteCommand command = connection.CreateCommand())
{
    command.CommandText = "INSERT INTO test VALUES (:value1, :value2);";
    command.Parameters.AddWithValue("value1", "val3");
    command.Parameters.AddWithValue("value2", 3);
    command.ExecuteNonQuery();
}

Why do you get the impression that ?1, ?2 should work? Does another provider do that? And :a, :b, :c, :d is fine, you just did not include those names in the paramaters you added.