abstraction_example.cs

//non unit-testable code
class Saver
{
	void SaveMethod(string filename)
	{
		//do everything else before the following calls
		DateTime dateTime = DateTime.Now();
		FileManager.Save(filename, dateTime.ToString());
	}
}

//unit-testable code
class Saver
{
	IDateTimeProvider dateTime;
	IFileManager fileManager;
	
	void SaveMethod(string filename)
	{
		//do everything else before the following calls
		DateTime dateTime = IDateTime.Now();
		fileManager.Save(filename, dateTime.ToString());
	}
}

interface IDateTimeProvider
{
	DateTime Now();
}

interface IFileManager
{
	void Save();
}

//adapter classes
class DateTimeProvider : IDateTimeProvider
{
	DateTime Now()
	{
		return DateTime.Now();
	}
}

class IFileManager : FileManager
{
	void Save(string filename, string contents)
	{
		FileManager.Save(filename, contents);
	}
}