Example of code that's easier to test and code that's harder to test. Instance vs. Static.

acceptable.cs

// Acceptable
// Use of IoC required now in two places: UserService and CoolService


public class UserRepository
{
	public User GetUserByID(long userID)
	{
	    //Get user from database
	}

	public void Save(User user)
	{
		//Save user to database
	}
}

public class UserService
{
	private UserRepository _userRepository;

	public UserService(UserRepository userRepository)
	{
		_userRepository = userRepository
	}

	public User GetUserByID(long userID)
	{
	    return _userRepository.GetUserByID(userID);
	}

	public void Save(User user)
	{
		_userRepository.Save(user);
	}
}

public class CoolService
{
	private UserService _userService;

	public SomethingCool(UserService userService)
	{
		_userService = userService;
	}

	public void MakeUserCool(long userID)
	{
		var user = _userService.GetUserByID(userID)
		user.IsCool = true;
		_userService.Save(user);
	}
}

[TestFixture]
public class TestCoolService
{
	[Test]
	[ExpectedException(typeof(Exception))]
	public void TestUserIsCool
	{
		var userRepositoryMock = new UserRepositoryMock();
		var userService = new UserService(userRepositoryMock);
		var coolService = new CoolService(userService);
		
		//This will throw an exception if user is not cool
		coolService.MakeUserCool(1);
	}

	private class UserRepositoryMock
	{
		public override User GetUserByID(long userID)
		{
			return new User { UserID = userID, IsCool = false };
		}
		
		public override void Save(User user)
		{
			if(!user.IsCool)
				throw new Exception("Expected user to be cool");
		}
	}

}

bad.cs

// Bad
// IoC required at the method level and still needed at the CoolService level.
// Too many extra hoops to jump through and test


public class UserService
{
	public static User GetUserByID(long userID)
	{
	    return GetUserByID(userID, new UserService());
	}

	public static User GetUserByID(long userID, UserService userService)
	{
		return userService.GetUserByID(userID);
	}

	public static void Save(User user)
	{
		Save(user, new UserService());
	}

	public static void Save(User user, UserService userService)
	{
		userService.Save(user);
	}
}

public class CoolService
{
	private UserService _userService;

	public CoolService(UserService userService)
	{
		_userService = userService;
	}

	public void MakeUserCool(long userID)
	{
		var user = UserService.GetUserByID(userID, _userService)
		user.IsCool = true;
		UserService.Save(user, _userService);
	}
}

[TestFixture]
public class TestCoolService
{

	[Test]
	[ExpectedException(typeof(Exception))]
	public void TestUserIsCool
	{
		var userServiceMock = new UserServiceMock();
		var coolService = new CoolService(userServiceMock);
		
		//This will throw an exception if user is not cool
		coolService.MakeUserCool(1);
	}

	private class UserServiceMock
	{
		public override User GetUserByID(long userID)
		{
			return new User { UserID = userID, IsCool = false };
		}
		
		public override void Save(User user)
		{
			if(!user.IsCool)
				throw new Exception("Expected user to be cool");
		}
	}

}

best.cs

// Best
// Use of IoC is relegated only to the calling class.
// Only 1 point of indirection


public class UserService
{
	public virtual User GetUserByID(long userID)
	{
	    //Get user from database
	}

	public virtual void Save(User user)
	{
		//Save user to database
	}
}

public class CoolService
{
	private UserService _userService;

	public SomethingCool(UserService userService)
	{
		_userService = userService;
	}

	public void MakeUserCool(long userID)
	{
		var user = _userService.GetUserByID(userID)
		user.IsCool = true;
		_userService.Save(user);
	}
}

[TestFixture]
public class TestCoolService
{
	[Test]
	[ExpectedException(typeof(Exception))]
	public void TestUserIsCool
	{
		var userServiceMock = new UserServiceMock();
		var coolService = new CoolService(userServiceMock);
		
		//This will throw an exception if user is not cool
		coolService.MakeUserCool(1);
	}

	private class UserServiceMock
	{
		public override User GetUserByID(long userID)
		{
			return new User { UserID = userID, IsCool = false };
		}
		
		public override void Save(User user)
		{
			if(!user.IsCool)
				throw new Exception("Expected user to be cool");
		}
	}

}

Best2.cs

// Best 2
// Use of IoC is relegated only to the calling class.
// Injection of dependency is through an instance variable rather than through the constructor


public class UserService
{
	public virtual User GetUserByID(long userID)
	{
	    //Get user from database
	}

	public virtual void Save(User user)
	{
		//Save user to database
	}
}

public class CoolService
{
	public UserService UserService { get; set; }

	public void MakeUserCool(long userID)
	{
		var user = UserService.GetUserByID(userID)
		user.IsCool = true;
		UserService.Save(user);
	}
}

[TestFixture]
public class TestCoolService
{
	[Test]
	[ExpectedException(typeof(Exception))]
	public void TestUserIsCool
	{
		var userServiceMock = new UserServiceMock();
		var coolService = new CoolService();
		coolService.UserService = userServiceMock;
		
		//This will throw an exception if user is not cool
		coolService.MakeUserCool(1);
	}

	private class UserServiceMock
	{
		public override User GetUserByID(long userID)
		{
			return new User { UserID = userID, IsCool = false };
		}
		
		public override void Save(User user)
		{
			if(!user.IsCool)
				throw new Exception("Expected user to be cool");
		}
	}

}