The OCP or “The Open Closed Principle” is the popular “O” from the SOLID principles of Object-Oriented Design.

As an architect consultant working in London, I get a question about SOLID, every other interview. As usually the interview is about an hour, I answer the question by an out-of-the-textbook type of answer, otherwise, the interview will span for hours. I keep my opinions for myself and I am a happy puppy.

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Unit testing per se is a controversial topic, what you should unit test is another controversial topic. The topic of this post started by a short conversation during a .NET London Group meeting back in 2010, I thought about it and then gave it more thoughts, I started using it in my projects, then now I have enough experience to share my thoughts. Here is my opinion, if you agree or disagree, then please do comment.

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Let me first define the terms that I will be referring to:

Presentation Code: Is the minimal code in a view that is necessary to display visual elements, but does not take any business decision. This is a fictional simple example:

// Business Logic Code. This code understands how the business works
// (don't do this)
if (Model.IsMonthly && !Model.IsPaid && Model.Payment >= 12 && ...)
{
  <p>Some warning message</p>
}

// Presentation Code. This code is only concerned with whether to
// show or hide a UI element.
if (Model.ShouldWarn)
{
  <p>Some warning message</p>
}

Fat-Razor: I’ve coined this term to define ASP.NET Razor views with a lot of Razor/C# code that is not presentation code. This is about the Razor/C# code specifically and not about HTML/CSS/JS.

Model: This is an overloaded term. However, in this context, it is an object of data representation such as a record in the database.

Viewmodel: They are data transfer objects that are meant to be view-specific and to carry exactly what the view needs, no less and no more. Generally, viewmodels are non-reusable and each vm is meant to to be tightly coupled to one view, however, there are exceptions depending on what you are doing.

To give you a flavour of a model and a viewmodel:

// An example of a model
public class Person
{
    public Guid PersonId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public short YearOfBirth { get; set; }
    public DateTime LastAuth { get; set; }
    // This might represent a one-to-many db relationship
    public Order[] Orders { get; set; }
}

// An example of a viewmodel
public class ProfileViewModel
{
    // Doesn't necessarily need an id

    // Combined first and last name
    public string Name { get; set; }
    // Calculated from YearOfBirth
    public short Age { get; set; }
    // E.g. "One day ago" and constructed from LastAuth
    public string LastSeen { get; set; }
    // Calculated by projections on Orders
    public short NumberOfAllOrders { get; set; }
    public short NumberOfUndeliveredOrders { get; set; }
}

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N.B.Throughout this post I am using Excel 2010 and Visual Studio 2010.

Writing a UDF in VBA to be exposed to Excel cells is straight forward, just write the function in a VBA module and Bob’s your uncle. However, it is slightly trickier to expose your functions to Excel in a managed language, such as C# or F#.

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Charity Hack is an annual event held and sponsored by PayPal UK at their venue in Richmond, London on a Saturday and a Sunday. It is an event where developers from different backgrounds are invited and introduced to different charity-relaled organisations APIs where developers are encouraged to hack useful apps using these APIs.

We introduced, as JustGiving, our new RESTful APIs and demonstrated how to use them.