C# – Repository Pattern, POCO, and Business Entities

asp.net-mvccentity-frameworkentity-framework-4net

I know there are a lot of threads here already on the repository pattern but somehow I feel my question is a little different. Maybe because yesterday was the first time I heard of the word POCO.

My question is this–usually, I have add and save methods in my business entities. Say I am writing a Q/A site, and I have the following entities: questions, answers, and comments. If I wanted to use the repository pattern, I basically need to keep only the properties in my business entities (example, Question), and move my operations to the repository class (example, QuestionRepository), right? If this is true, does POCO mean a business entity with just the properties?

I'm using Entity Framework 4.0, which created my entities in the edmx code behind. If I wanted to use the repository pattern with this, there is no need to write my own business entities (Question, Answer, etc) since they are already generated by EF, right? All I'd need is the Repository to do CRUD? And I'll be having three repositories for this example, one for each entity?

Best Answer

First an observation about Asp.net MVC project template

I must say that there is a tiny misunderstanding with Visual Studio's Asp.net MVC project template. And that is the Model folder. People not knowing MVC pattern would automatically relate this to data model and not MVC application/presentation model. This is fine for simple applications where we don't distinguish between the two but not for anything else.

Let's continue to my answer

When I write business level applications I separate my solution into 4 projects (at least):

presentation tier - Asp.net MVC application but I remove Model folder and have all my views as strong type views to avoid magic strings as much as possible
service tier - business logic processes
data tier - data model ie. EF4 and repositories that access this model
objects tier - this project actually has POCOs that are used for inter-layer communication and any interfaces used by various layers (think of IoC)

My request process usually looks very clean and works this way:

When a request is made my Asp.net MVC controller action validates data (POCO objects), does whatever is necessary for the presentation tier before calling into services.
Service is called that does whatever business process logic requires and normally calls repository to do something with data.
Repository manipulates data in the data model and then creates POCOs from results that will be returned to service layer.
Service layer receives POCOs does additional logic if needed and returns them back to presentation.
Presentation (controller) decides which view to display and provides model for that particular view and returns it. Of course instead of a view it can be any other result as well.

Advantage of using separate MVC model classes in Objects project (you couldn't put them in the Model folder because of circular project reference) is that I can have presentation optimised classes. Or better said: I have business process centric interface instead of data centric.

Let's explain this with an example: Take for instance user registration view. It can't be strong typed to data model's User entity. Why? Because it has two inputs for password. So I can have an application/presentation model class called UserRegistration even though there's nothing similar in data model. Its validation works completely differently compared to data model's User entity. If I'd have my user registration done without strong type I'd have to make my controller action with all parameters of every single field. These wouldn't be automatically validated which would mean that I can have a larger bug surface. Someone might hurry into writing code but forget about certain aspects of validation.

Strong type views returning strong types back at server are the safest way of getting rid of all kinds of obscure bugs that are usually discovered by users especially if you don't do any methodical testing on your project (which is somewhere between 75-90% chance).

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – How to create an Excel (.XLS and .XLSX) file in C# without installing Microsoft Office

You can use a library called ExcelLibrary. It's a free, open source library posted on Google Code:

ExcelLibrary

This looks to be a port of the PHP ExcelWriter that you mentioned above. It will not write to the new .xlsx format yet, but they are working on adding that functionality in.

It's very simple, small and easy to use. Plus it has a DataSetHelper that lets you use DataSets and DataTables to easily work with Excel data.

ExcelLibrary seems to still only work for the older Excel format (.xls files), but may be adding support in the future for newer 2007/2010 formats.

You can also use EPPlus, which works only for Excel 2007/2010 format files (.xlsx files). There's also NPOI which works with both.

There are a few known bugs with each library as noted in the comments. In all, EPPlus seems to be the best choice as time goes on. It seems to be more actively updated and documented as well.

Also, as noted by @АртёмЦарионов below, EPPlus has support for Pivot Tables and ExcelLibrary may have some support (Pivot table issue in ExcelLibrary)

Here are a couple links for quick reference:
ExcelLibrary - GNU Lesser GPL
EPPlus - GNU (LGPL) - No longer maintained
EPPlus 5 - Polyform Noncommercial - Starting May 2020
NPOI - Apache License

Here some example code for ExcelLibrary:

Here is an example taking data from a database and creating a workbook from it. Note that the ExcelLibrary code is the single line at the bottom:

//Create the data set and table
DataSet ds = new DataSet("New_DataSet");
DataTable dt = new DataTable("New_DataTable");

//Set the locale for each
ds.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
dt.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;

//Open a DB connection (in this example with OleDB)
OleDbConnection con = new OleDbConnection(dbConnectionString);
con.Open();

//Create a query and fill the data table with the data from the DB
string sql = "SELECT Whatever FROM MyDBTable;";
OleDbCommand cmd = new OleDbCommand(sql, con);
OleDbDataAdapter adptr = new OleDbDataAdapter();

adptr.SelectCommand = cmd;
adptr.Fill(dt);
con.Close();

//Add the table to the data set
ds.Tables.Add(dt);

//Here's the easy part. Create the Excel worksheet from the data set
ExcelLibrary.DataSetHelper.CreateWorkbook("MyExcelFile.xls", ds);

Creating the Excel file is as easy as that. You can also manually create Excel files, but the above functionality is what really impressed me.

Best Answer

First an observation about Asp.net MVC project template

Let's continue to my answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – How to create an Excel (.XLS and .XLSX) file in C# without installing Microsoft Office

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