Fixes & Performance

Killing Floor 2 .NET Framework 3.5 Error on Windows 10 - Fix

A common first-launch problem on new Windows 10 PCs: you download Killing Floor 2 on Steam, press Play, and get an error saying .NET Framework 3.5 cannot be installed. The game never starts. In the Steam bug-report thread this page is based on, the player solved it in minutes. The cause was not the game at all, but a disabled Windows Update.

Killing Floor 2 key art Killing Floor 2. The .NET error appears before the game gets a chance to start.

Why it happens

  • On the first launch, Steam runs the game's setup step and installs the components it depends on.
  • Windows 10 and 11 do not ship .NET Framework 3.5 enabled by default. It is an optional Windows feature.
  • When something asks for it, Windows downloads it through Windows Update. If Windows Update is disabled, paused or blocked, the download fails and you get the "cannot be installed" message.

Fix 1: Turn Windows Update back on (what solved it in the thread)

  1. Open Settings > Update & Security > Windows Update (Windows 11: Settings > Windows Update).
  2. If updates are paused, click Resume updates.
  3. If you disabled the service, press Win + R, type services.msc, find Windows Update, set Startup type to Manual or Automatic, and click Start.
  4. Launch Killing Floor 2 from Steam again and let the setup install .NET 3.5.

Fix 2: Enable .NET Framework 3.5 by hand

If you would rather install it before launching the game:

  1. Press Win + R, type optionalfeatures, press Enter. This opens Turn Windows features on or off.
  2. Tick .NET Framework 3.5 (includes .NET 2.0 and 3.0).
  3. Click OK and choose Let Windows Update download the files for you.
  4. Restart the PC if asked, then start the game.

This route also needs Windows Update to work, so do Fix 1 first if it fails.

Fix 3: Install it from the command line

Open Command Prompt as administrator and run:

DISM /Online /Enable-Feature /FeatureName:NetFx3 /All

Wait for it to reach 100%, then reboot. If you have Windows installation media, you can point DISM at its sources\sxs folder with /Source: and /LimitAccess, which skips the online download.

If the game still will not start

  • Make sure the install finished: check Turn Windows features on or off again. The .NET 3.5 box should be filled.
  • Verify the game files in Steam: right-click Killing Floor 2 > Properties > Installed Files > Verify integrity of game files.
  • For other launch problems, see Killing Floor 2 not launching on Steam and KF2 slow startup.

FAQ

Why does Killing Floor 2 ask for .NET Framework 3.5?
The first time you start the game, Steam runs a setup step that installs the components it needs. On a fresh Windows 10 PC, .NET Framework 3.5 is usually not installed yet, so Windows tries to download it.
What fixed the error in the original Steam thread?
Turning Windows Update back on. Windows downloads .NET 3.5 through Windows Update, so the install fails while the update service is disabled.
Can I install .NET 3.5 manually?
Yes. Open Turn Windows features on or off and tick .NET Framework 3.5, or run DISM /Online /Enable-Feature /FeatureName:NetFx3 /All in an administrator command prompt.
Do I need to reinstall the game after fixing it?
No. Once .NET 3.5 is installed, start Killing Floor 2 from Steam again and the first-launch setup should finish normally.