Introduction:

Build Provider is a new feature introduced in ASP.NET 2.0. You can perform very nifty stuff if you know how to use it correctly. In this article I will demonstrate how to create a custom build provider which will generate custom entity classes. I like to thank Fritz Onion for this post about build providers Jaw-dropping Experience with Custom Build Providers which formed the basis for this article. 

What are Build Providers?

To better understand the build providers let’s first take a look at the App_Code folder in ASP.NET 2.0 applications. The App_Code is a special folder in the application which will build the items inside it. This means that if you put an aspx page inside the App_Code then the file is build using the System.Web.Compilation.PageBuildProvider. This is one of the default providers used by the ASP.NET to compile and build .aspx extension pages. There are many other providers used by ASP.NET when building the application. Take a look at the providers in the code below:

<buildProviders>

   <add extension=".aspx" type="System.Web.Compilation.PageBuildProvider"/>

   <add extension=".ascx" type="System.Web.Compilation.UserControlBuildProvider"/>

   <add extension=".master" type="System.Web.Compilation.MasterPageBuildProvider"/>

   <add extension=".asix" type="System.Web.Compilation.ImageGeneratorBuildProvider"/>

   <add extension=".asmx" type="System.Web.Compilation.WebServiceBuildProvider"/>

   <add extension=".ashx" type="System.Web.Compilation.WebHandlerBuildProvider"/>

   <add extension=".soap" type="System.Web.Compilation.WebServiceBuildProvider"/>

   <add extension=".resx" type="System.Web.Compilation.ResXBuildProvider"/>

   <add extension=".resources" type="System.Web.Compilation.ResourcesBuildProvider"/>

   <add extension=".wsdl" type="System.Web.Compilation.WsdlBuildProvider"/>

   <add extension=".xsd" type="System.Web.Compilation.XsdBuildProvider"/>

   <add extension=".js" type="System.Web.Compilation.ForceCopyBuildProvider"/>

</buildProviders>

Each provider is registered using an extension and the type. Now, let’s take a look at how we can create our own custom providers.

Creating the Mapping File:

The first task is to create a mapping file. The mapping file is the file which will be placed in the App_Code folder and build using our custom build provider. Since, we will be creating entity classes and our classes will be based on the database schema we will call it .map files. Let’s take a look at the .map file which contains the information on how to create the entity classes.

<?xml version="1.0" encoding="utf-8" ?>

<mappings>

  <mapping connectionString="Server=localhost;Database=Northwind;Trusted_Connection=true"

            selectCommand="SELECT * FROM Products" className="Product" namespace="EntityClass.BLL"/>

   <mapping connectionString="Server=localhost;Database=Northwind;Trusted_Connection=true"

             selectCommand ="SELECT * FROM Categories" className="Category" namespace="EntityClass.BLL" />

 </mappings>

The mapping file contains the root called “mappings”. After that each database table is contained inside the “mapping” element. Let’s take a look at the attributes in more details:

connectionString: The connection string used to connect to the database.

selectCommand: The command to execute in order to get the schema.

className: The name of the entity class to be created.

namespace: The name of the namespace.

Creating the Mapping Class:

It is not necessary to create a mapping class but I am doing it over here to organize the code. The only purpose of the mapping class is to hold the information defined in the mapping file. Take a look at the mapping class below:

public class Entity

    {

        private string _connectionString;

        private string _selectCommand;

        private string _className;

        private string _namespace;

        public string ConnectionString

        {

            get { return this._connectionString; }

            set { this._connectionString = value; }

        }

        public string SelectCommand

        {

            get { return this._selectCommand; }

            set { this._selectCommand = value; }

        }

        public string ClassName

        {

            get { return this._className; }

            set { this._className = value; }

        }

        public string Namespace

        {

            get { return this._namespace; }

            set { this._namespace = value; }

        }

    }

Creating the Custom Entity Builder Class:

Now, we are ready to create our custom entity class. Simply, add a new Class Library project to your solution and then add a class named EntityBuilder. The EntityBuilder class should derive from the BuildProvider class. Next, override the GenerateCode method.

Here is the implementation of the GenerateCode method:

public override void GenerateCode(AssemblyBuilder assemblyBuilder)

        {