Extending ASP.NET 2.0 Part I: Creating Custom Build Providers

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Introduction

ASP.NET is a great managed environment that supports dynamic compilation for its registered resources. The aim of this article is to show how one can take advantage of dynamic compilation through the use of custom build providers.

The BuildProvider Class

The System.Web.Compilation.BuildProvider abstract class provides a set of methods and properties that allows ASP.NET to generate source code. To use this class, one must implement it and add an element under the system.web/compilation/buildProviders section under the web.config (or machine.config for your web server).

This list outlines how the ASP.NET build environment uses your instance of BuildProvider:

• Build environment looks at the registered build providers within web.config.
• An instance of your provider is created when the environment encounters a file extension within the App_Code special folder that matches the extension attribute of your configuration entry.
• An AssemblyBuilder object is passed into your builder's GenerateCode method.
• Your provider uses the AssemblyBuilder and other System.Web.Compilation classes to add source code for your file into the overall assembly.

To find out more about the BuildProvider class, please see the References section.

A ZipCode BuildProvider

This article uses a simple build provider that parses a file containing zip code information. The provider creates a state class for each line in the file and exposes the zip codes as properties. The custom file and the generated class look like this:

Contents of file All.zipcode:

Iowa|IA|50311;51442;50111;50021;50023
Nebraska|NE|12345;67890
Texas|TX|99999;12345

Generated state class:

As previously stated, the provider must be registered within web.config in order for the runtime to hook up your provider. The entry for the ZipCodeBuildProvider looks like:

<buildProviders>
    <add appliesTo="Code" 
        extension=".zipCode" 
        type="Lozanotek.Examples.ZipCodeBuildProvider, BuildProvider"/>
</buildProviders>

In order to generate source code that will be compiled into an assembly, you must make use of the classes under the System.Web.Compilation namespace to create an abstract syntax tree (AST) to give to the runtime through the AssemblyBuilder class. The CreateClass, CreateField and CreateProperty methods are used to create a custom state class by parsing a line in the file and creating their corresponding compilation units.

/// <summary>
/// Creates a class by parsing the given line
/// </summary>
/// <param name="line">Line to parse</param>
/// <returns>A class declaration for a custom state class</returns>
private CodeTypeDeclaration CreateClass(string line)
{
    // Parse out the tokens
    string[] values = line.Split(new string[1] { "|" },          
                            StringSplitOptions.RemoveEmptyEntries);

    // Set the name of the class
    string className = values[0];
    CodeTypeDeclaration cls = new CodeTypeDeclaration(className);

    // Set an internal field and its value
    string fieldName = values[1];
    CodeMemberField abbrField = CreateField(fieldName);
    cls.Members.Add(abbrField);

    // Get the zip codes listed for the states
    string[] zipCodes = values[2].Split(new string[1] { ";" }, 
                               StringSplitOptions.RemoveEmptyEntries);

    // Create a property for each of the zip codes found
    foreach (string zipCode in zipCodes)
    {
        string tempZip = zipCode.Trim();
        CodeMemberField zipField = CreateField(tempZip);
        CodeMemberProperty zipProperty = CreateProperty(tempZip);

        cls.Members.Add(zipField);
        cls.Members.Add(zipProperty);
     }

     return cls;
}

/// <summary>
/// Creates a private string field to store the value for the zip code
/// </summary>
/// <param name="fieldName">Name of the fild to create</param>
/// <returns>A unit representing an private string field</returns>
private CodeMemberField CreateField(string fieldName)
{
    // Specify that you want to create a string field with the given name
    CodeMemberField field = new CodeMemberField(typeof(String), 
                                  string.Format("_{0}", fieldName));

    // Make the field private and static
    field.Attributes = MemberAttributes.Private | MemberAttributes.Static;

    // Assign the value of the field to the zip code
    field.InitExpression = 
      new CodeSnippetExpression(string.Format("\"{0}\"", fieldName));

    return field;
}

/// <summary>
/// Creates a property that returns the value of a string field
/// </summary>
/// <param name="propertyName">Name of the property to create</param>
/// <returns>A unit representing a public static property</returns>
private CodeMemberProperty CreateProperty(string propertyName)
{
    CodeMemberProperty prop = new CodeMemberProperty();
    prop.Name = string.Format("zip{0}", propertyName);
    prop.Type = new CodeTypeReference(typeof(String));

    prop.Attributes = MemberAttributes.Public | MemberAttributes.Static;

    // Build the body of the property by assigning it a get statement
    prop.GetStatements.Add(new CodeMethodReturnStatement(
                    new CodeFieldReferenceExpression(null,
                    string.Format("_{0}", propertyName))));

    return prop;
}

Conclusion

I hope this article has shown you, the developer, how to take advantage of the extensibility of the ASP.NET runtime. In my next Extending ASP.NET 2.0 article, I will demonstrate how to extend ASP.NET even further by creating custom expression builders.

Please feel free to make any comments or suggestions to this article by using the forums below.

References

During the writing of this article, I used the following resources:

• MSDN documentation for BuildProvider