Factory Pattern Understanding

Question

I have implementated the Factory Pattern as below.

However, since the individual classes are public, nothing prevents someone from instantiating them directly.

Is this correct? How do I ensure that the concrete classes are only created via the Factory?

namespace MRS.Framework
{
    public  abstract class DataSource
    {
        public override string ToString()
        {
            return "DataSource";
        }
    }

    public class XMLDataSource : DataSource
    {

    }

    public class SqlDataSource : DataSource
    {

    }

    public class CSVDataSource : DataSource
    {
        public int MyProperty { get; set; }


        public override string ToString()
        {
            return "CSVDataSource";
        }
    }
}

Factory implementation

namespace MRS.Framework
{
    public abstract class DataSourceFactory
    {
        public abstract DataSource CreateDataSource(DataSourceType datasourcetype);
    }

    public class CSVDataSourceFactory : DataSourceFactory
    {
        public CSVDataSourceFactory()
        {

        }
        public override DataSource CreateDataSource(DataSourceType datasourcetype)
        {
            return new CSVDataSource();
        }
    }


    public class XMLDataSourceFactory : DataSourceFactory
    {
        public override DataSource CreateDataSource(DataSourceType datasourcetype)
        {
            return new XMLDataSource();
        }
    }

    public class SqlDataSourceFactory : DataSourceFactory
    {
        public override DataSource CreateDataSource(DataSourceType datasourcetype)
        {
            return new SqlDataSource();
        }
    }

}

Main

 static void Main(string[] args)
        {
            DataSourceFactory datasourcefactory = new CSVDataSourceFactory();
            CSVDataSource ds = (CSVDataSource)datasourcefactory.CreateDataSource(DataSourceType.CSVDataSource);
            CSVDataSource myds = new CSVDataSource();
            Console.WriteLine(ds.ToString());
            Console.WriteLine(myds.ToString());
            Console.ReadLine();

        }

Answer 1

Yes, your intuition here is correct; if you want to restrict the construction of your CSVDataSourceFactory class then you have your access modifiers wrong.

However, it's not the access modifier of the class you need to fix, it's the access modifier of the constructor. You should mark the default constructor internal so that only other classes within your assembly can construct them. You will, of course, have to enforce your own rules within that assembly but since you have complete control over that code, it shouldn't be an issue.

public class XMLDataSource : DataSource
{
  internal XMLDataSource() { }
}

public class SqlDataSource : DataSource
{
  internal SqlDataSource() { }
}

public class CSVDataSource : DataSource
{
  public int MyProperty { get; set; }

  internal CSVDataSource() { }

  public override string ToString()
  {
      return "CSVDataSource";
  }
}