To install .NET Framework 3.5 offline on Windows 10 64-bit, you must use the original Windows installation media. Unlike older versions of Windows, the only supported offline method for Windows 10 is through the deployment of "Side-by-Side" (SxS) files found on your OS installation disk or ISO. Prerequisites

She plugged it in. The folder contained a single file: dotnetfx35.exe and a sxs folder. No internet required.

Installing it normally requires an internet connection, but if you're working offline or facing Windows Update errors, you'll need the .NET Framework 3.5 offline installer. Why You Need .NET Framework 3.5

She opened PowerShell as admin and typed:

The Ultimate Guide to .NET Framework 3.5 Offline Installer for Windows 10 64-Bit

Introduction: Why You Still Need .NET Framework 3.5 in the Era of Windows 11 and 10

If you are a Windows 10 64-bit user, you have likely encountered a frustrating pop-up message while trying to install an older game, a legacy business application, or a specialized engineering tool. The message reads: "Your app needs .NET Framework 3.5 (includes 2.0 and 3.0)."

5. Troubleshooting Common Failure Codes

Despite following the procedure, users may encounter error codes

Standalone installers found online for earlier versions of Windows often do not work correctly on Windows 10 without an internet connection. Microsoft Learn Method: Using Installation Media and DISM