Question

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.

Answer 1

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

No they don’t. They claim they do, but they don’t actually provide the right methods. Look at IDatabase.GetNewCommand():

IDbCommand GetNewCommand();

and your implementation:

public MySqlCommand GetNewCommand()
{
    ... 
}

They’ve got different return types. Likewise your ExecuteReaderCommand method parameter is IDbCommand in IDatabase, but MySqlCommand in MysqlDatabase.

Options:

• Use explicit interface implementation for the “weakly typed” version, exposing the “strongly typed” versions on the concrete classes. This is what SqlCommand does in the .NET framework, for example.

• Make IDatabase generic in the type of command it creates and uses:

  public interface IDatabase<TCommand> where TCommand : IDbCommand
  {
      DataTable ExecuteReaderCommand(TCommand command);
      TCommand GetNewCommand();
  }