Practical Design Patterns in C# – Factory Method

The factory method is exactly what is says on the label – a method that produces object instances on demand. Ergo, a factory method. This method separates the creation of object instances from the consumption. It adds very little value to a code-base on its own in most cases. However, the factory method is the foundation to the abstract factory, which offers much more utility.

In this example, we look at an application that has to write data to one of two different file formats. Separate file writer classes are required for each format, but both must share a common interface for this pattern to work. The correct class is instantiated and the Write method invoked upon it to perform the save operation.

All writer classes are required to implement the IFileWriter interface.

public interface IFileWriter
{
    void Write(byte[] data, string filename);
}

This interface is implemented by each writer class. The example below shows the JpegWriter and PngWriter classes.

public class JpegWriter : IFileWriter
{
    void Write(byte[] data, string filename) { … }
}
public class PngWriter : IFileWriter
{
    void Write(byte[] data, string filename) { … }
}

Then a factory interface is declared, and a factory class created for each file type.

public interface IFileWriterFactory
{
    public IFileWriter Create();
}
public class JpegWriterFactory : IFileWriterFactory
{
    public Create()
    {
        return new JpegWriter();
    }
}
public class PngWriterFactory : IFileWriterFactory
{
    public Create()
    {
        return new PngWriter();
    }
}

Usage

The application determines the file type that is required, then invokes the Create method on the appropriate writer factory, which in turn returns a concrete implementation of the IFileWriter class.

The application then invokes the Write method on the writer instance, which handles format-specific details such as endianness, file headers, encoding and more.

static class Program
{
    private static _jpegWriterFactory = new JpegWriterFactory();

    private static _pngWriterFactory = new PngWriterFactory();

    static void Main()
    {
        // Image created and serialized into byte[]
        …

        IFileWriter writer;
        if ("jpg" == args[1])
        {
            writer = _jpegWriterFactory.Create();
        }
        else
        {
            writer = _pngWriterFactory.Create();
        }

        writer.Write(data, "lena");
    }
}