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Static File handling in ASP.NET Core 9.0

I know, I know, .NET 10 is already in preview and I am still catching up on what was added to .NET 9.0. Today while upgrading an older application to .NET 9, I decided to have a look at the new static file handling introduced in .NET 9 through the MapStaticAssets feature.

Static File middleware (before .NET 9)

Before .NET 9, static files(Javascript, CSS, images, …) were handled through the UseStaticFiles middleware

This middleware is still there as the new MapStaticAssets feature does not support all the features that the original middleware had.

From the documentation:

MapStaticAssets (starting from .NET 9)

Now enough about what is NOT supported, let’s talk about why you should use this feature. Start by replacing the usage UseStaticFiles by MapStaticAssets:

MapStaticAssets provides the following advantages compared to UseStaticFiles:

  • Build time compression for all the assets in the app. It uses gzip during development and gzip + brotli during publishing. All assets are compressed with the goal of reducing the size of the assets to the minimum.
  • The ETags for each resource are the Base64-encoded string of the SHA-256 hash of the content. This ensures that the browser only redownloads a file if its contents have changed.
  • Dynamic compression is not need on the web server level simplifying configuration and performance as assets are compressed at build time.

Let’s see it in action…

I first start my application with the original UseStaticFiles implementation. Here is a screenshot of the response headers:


Let’s compare this with the same file served through MapStaticAssets:

Do you notice the differences?

The content-encoding is set to gzip (in dev) and the e-tag is calculated differently using fingerprinting. And of course the size is also smaller.

Nice!

More information

Map static files in ASP.NET Core | Microsoft Learn

What's new in ASP.NET Core 9.0 | Microsoft Learn

Enabling dynamic compression in IIS

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