MySql and SQlite classes implementing interface
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This shouldnt be hard to answer but I am so desperate and confused. I made interface for executing read query from database

    interface IDatabase
    {
        DataTable ExecuteReaderCommand(IDbCommand command);
        IDbCommand GetNewCommand();
    }

Next, I created two different classes implemetnig above interface.

    class MysqlDatabase : IDatabase
    {
        public DataTable ExecuteReaderCommand(MySqlCommand command)
        {
            DataTable dt = new DataTable();
            // ... read db
            return dt;
        }

        public MySqlCommand GetNewCommand()
        {
            cnn.Open();
            return cnn.CreateCommand();
        }
    }

And

    class SQLiteDatabase : IDatabase
    {
	String dbConnection;
        SQLiteConnection cnn;

        public DataTable ExecuteReaderCommand(SQLiteCommand command)
        {
            DataTable dt = new DataTable();
            // ... read db
            return dt;
        }

        public SQLiteCommand GetNewCommand()
        {
            cnn.Open();
            return cnn.CreateCommand();
        }
    }

But I am getting error that these classes dont implement IDatabase interface:

     MysqlDatabase does not implement interface member 'Database.GetNewCommand()'
     MysqlDatabase.GetNewCommand() cannot implement 'Database.GetNewCommand()' because it does not have the matching return type of 'System.Data.IDbCommand'.

     SQLiteDatabase does not implement interface member Database.ExecuteReaderCommand(System.Data.IDbCommand)
     SQLiteDatabase does not implement interface member 'Database.GetNewCommand()'. SQLiteDatabase.GetNewCommand() cannot implement Database.GetNewCommand() because it does not have the matching return type of 'System.Data.IDbCommand'.

When I look on the `SQLiteCommand` and `MySqlCommand` they both implements `IDbCommand`. 

**How can I use these classes under common interface so I can easilly switch them?**

I am very thankful for any answer.