Silverlight China

A plain-English reference for the Silverlight era — and what to run instead

What Is a XAP File, and Can You Still Open One?

What a XAP file is

A `.xap` file is the deployment package of a Silverlight application — the single file the browser plugin downloaded from the web server and executed. If you're archiving an old project, auditing what a legacy app contains, or scoping a migration, the XAP is often the most complete artifact you have, especially when the original source code has gone missing.

Structurally, a XAP is not exotic: it's a ZIP archive with a different extension. Inside, a typical package contains:

  • AppManifest.xaml — the manifest describing the application's entry point and the assemblies it ships.
  • Compiled .NET assemblies (.dll files) — the application's actual code, built against the Silverlight subset of the .NET framework.
  • Resources — images, fonts, media, localized resource assemblies, and packed XAML for the UI.

Can you still open one?

You can inspect one easily; you can't meaningfully run one anymore. Opening is simple: copy the file, rename `.xap` to `.zip`, and extract it with any archive tool on Windows, macOS, or Linux. No Silverlight runtime is needed for that step, because you're just unzipping.

Running is another matter. Executing a XAP requires the Silverlight browser runtime, and Silverlight reached end of support on October 12, 2021; Microsoft's lifecycle record for Silverlight 5 marks the close of a support window that began in 2011. Modern browsers can't load the plugin at all, for the reasons covered in why Silverlight won't run in modern browsers.

Why inspecting a XAP is genuinely useful

For a migration team, the XAP is a ground-truth inventory of what the application really shipped — as opposed to what people remember it doing. Extracting one tells you:

  • Which assemblies are in play. Every third-party control library and internal shared DLL is sitting right there in the package. Each one is a migration line-item: does a modern equivalent exist, or does that feature get rebuilt?
  • What the UI actually contains. The packed XAML reveals screens, controls, and styles — useful for estimating rebuild effort even before anyone reads code.
  • What the code does, if source is lost. Silverlight assemblies are ordinary .NET metadata-rich binaries, and .NET decompilation tools can reconstruct readable code from them. That's often the difference between "rewrite from a spec nobody has" and "port logic we can actually read." (Check your license terms before decompiling software you don't own.)

This is exactly the evidence-gathering step described in how to inventory a Silverlight app before migrating — the XAP turns the inventory from guesswork into a checklist.

What to do with what you find

Once the package is cataloged, the migration question becomes concrete: business logic in ordinary C# assemblies tends to port forward well, while UI and plugin-specific APIs need rebuilding on a modern stack. For web delivery, Microsoft's current .NET web UI framework is Blazor, which keeps you in C# and reuses the component-based mental model Silverlight developers already have; see whether Blazor is the Silverlight replacement for how that comparison holds up.

One caution: treat an old XAP like any other untrusted binary. Extract and analyze it on a machine you'd be comfortable analyzing unknown software on, and don't go hunting for ways to execute it in a resurrected runtime — a retired, unpatched runtime is a liability, not a tool.

Sources

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