Too many requests from a single HTML page may cause significant delays and affect the overall time-to-last-byte metrics for a site. Bundling is therefore the process of grouping distinct resources such as CSS files into a single downloadable resource. In this way, multiple and logically distinct CSS files can be downloaded through a single HTTP request.
Minification, on the other hand, is the process that removes all unnecessary characters from a text-based resource without altering the expected functionality. Minification involves shortening identifiers, renaming functions, removing comments and white-space characters. In general, minification refers to removing everything that’s been added mostly for readability purposes, including long descriptive member names.
Although bundling and minification can be applied together, they remain independent processes. On a production site, there’s usually no reason not to bundle minified CSS and script files. The only exception is for large and very common resources that might be served through a Content Delivery Network (CDN). The jQuery library is a great example.
Bundling requires the Microsoft ASP.NET Web Optimization Framework available as a Nuget package. Downloading the Optimization Framework also adds a few more references to the project. In particular, they are WebGrease and Microsoft Infrastructure. These, in turn, bring their own dependencies for the final graph, shown in below Figure.
Bundles are created programmatically during the application startup in global.asax. Also in this case, you can use the xxxConfig pattern and add some BundlesConfig class to the App_Startup folder. The BundleConfig class contains at least one method with code very close to the following snippet.
public static void RegisterBundles(
BundleCollection bundles)
{
bundles.Add(new StyleBundle("~/Content/Styles")
.Include("~/Content/Styles/bootstrap.css",
"~/Content/Styles/myapp.css"));
}
The code creates a new bundle object for CSS content and populates it with distinct CSS files defined within the project. Note that the Include method refers to physical paths within the project where the source code to bundle is located. The argument passed to the StyleBundle class constructor instead is the public name of the bundle and the URL through which it will be retrieved from pages. There are quite a few ways to indicate the CSS files to bundle. In addition to listing them explicitly, you can use a wildcard expression:
bundles.Add(new Bundle("~/css")
.Include("~/content/styles/*.css");
Once CSS bundles are defined invoking them is as easy as using the Styles object:
@Styles.Render("~/Bundles/Css")
As you can figure from the last two snippets, ASP.NET optimization extensions come with two flavors of bundle classes: the Bundle class and the StyleBundle class. The former only does bundling; the latter does both bundling and minification. Minification occurs through the services of an additional class. The default CSS minifier class is CssMinify and it is based on some logic packaged in WebGrease. Switching to a different minifier is easy too. All you do is using a different constructor on the StyleBundle class. You use the constructor with two arguments, the second of which is your own implementation of IBundleTransform.