When attempting to enable .NET Framework 3.5 on Windows 11 22H2, users frequently encounter failures through multiple approaches: - Enabling via Windows Features results in errors

• DISM commands fail with source file errors
• Windows Update downloads consistently fail

Typical Error Messages

Error: 0x800f081f
The source files could not be found.
Use the "Source" option to specify the location of the files needed to restore the feature.

Error: 0x800f0805
The specified package is not a valid Windows package.

Root Cause Analysis

Standard installation methods fail due to three primary reasons: 1. Version Mismatch: Microsoft's official ISO downloads provide the latest 25H2 version, while the target system runs 22H2 (Build 22621) 2. Missing Installation Sources: Windows 11 does not include .NET 3.5 installation files by default 3. Online Download Failures: Windows Update cannot locate a compatible .NET 3.5 package for the specific build version

Solution: Building a Custom ISO with UUP Dump

The most reliable approach involves using UUP Dump to download Windows 11 22H2 components matching your exact system version, then generating an ISO with .NET Framework 3.5 pre-integrated. ### Step 1: Navigate to UUP Dump

Open your web browser and visit: https://uupdump.net/### Step 2: Select Windows Version

Quick Selection Method

1. Locate the "Latest updates" section on the homepage
2. Select Windows 11 → 22H2 (Build 22621)
3. Alternatively, access directly via the 22H2 language selection page

Manual Selection Method

1. Click "Get started" on the homepage
2. Choose "Retail" channel
3. Select "Windows 11"
4. Pick "22H2" (Build 22621)
5. Click "Next"

Step 3: Configure Language and Edition

1. Set language to your preferred option (e.g., English or Chinese Simplified)
2. Select the edition matching your system (Professional recommended)
3. Click "Next"

Step 4: Select Additional Components Maintain the default selections: - ✅ Include Windows Setup

• ✅ Include Windows Recovery Environment (WinRE)
• ✅ Include Network drivers

Click "Next" ### Step 5: Choose Download Method

Select "Create a conversion package using aria2" — this provides a zip archive containing download scripts and conversion utilities. ### Step 6: Retrieve the Conversion Package

1. Click "Download conversion package"
2. Save the file (approximately 8KB — this is expected, as it contains only scripts)

Step 7: Extract and Configure

Extraction

Extract the downloaded archive to a dedicated folder: ``` D:\UUP\22621_build

The extracted contents include: - `uup_download_windows.cmd` — Main download script
- `ConvertConfig.ini` — Configuration file
- `files` — Supporting tools directory

#### Critical Configuration Change

Open `ConvertConfig.ini` in a text editor and locate: ```
NetFx3       =0

Change to: ``` NetFx3 =1

**This modification is essential** — it instructs the converter to integrate .NET Framework 3.5 into the resulting ISO. #### Configuration Reference



| Parameter | Default | Description |
|---|---|---|
| AutoStart | 1 | Automatically begins conversion after download |
| AddUpdates | 1 | Includes cumulative updates |
| NetFx3 | 0 | Set to 1 for .NET 3.5 integration |
| SkipISO | 0 | Set to 1 to skip ISO generation |

### Step 8: Execute the Download Script

#### Run as Administrator

1. Right-click `uup_download_windows.cmd`
2. Select "Run as administrator"
3. A command window will open showing the download progress

#### Download Phases

The script executes in multiple stages: **Phase 1: Converter Download**```
[INFO] Downloading the UUP converter...

Downloads conversion tools (~10-20MB) Phase 2: Microsoft Store Applications``` [INFO] Retrieving aria2 script for Microsoft Store Apps... [INFO] Downloading Microsoft Store Apps...

Downloads store applications (~200-500MB) **Phase 3: Windows Components**```
[INFO] Retrieving aria2 script for the UUP set...
[INFO] Downloading the UUP set...

Downloads Windows 11 core components (~3-5GB) Phase 4: ISO Conversion``` [INFO] Starting conversion... [INFO] Creating ISO...

#### Time Estimates

- Download: 30-60 minutes (varies by connection speed)
- Conversion: 10-20 minutes
- Total: 40-80 minutes

*Note: Do not close the command window during this process.*

### Step 9: Locate the Generated ISO

Upon completion, a ISO file appears in the extraction directory: ```
22621.1.220506-1250.NI_RELEASE_CLIENTPRO_OEMRET_X64FRE_EN-US.ISO

Step 10: Install .NET Framework 3.5

Open Elevated PowerShell

Press Win + X and select "Terminal (Admin)" or "PowerShell (Administrator)" #### Mount the ISO

$isoPath = "D:\UUP\22621_build\22621.1.220506-1250.NI_RELEASE_CLIENTPRO_OEMRET_X64FRE_EN-US.ISO"
Mount-DiskImage -ImagePath $isoPath

Identify the Mounted Drive Letter

$mountedVolume = Get-Volume | Where-Object { $_.DriveType -eq 'CD-ROM' -and $_.Size -gt 3GB }
$driveLetter = $mountedVolume.DriveLetter
Write-Host "ISO mounted to drive: $driveLetter`:"

Verify Source Files Exist

$sourcePath = "$driveLetter`:\sources\sxs"
if (Test-Path $sourcePath) {
    Write-Host "Source directory verified" -ForegroundColor Green
    Get-ChildItem $sourcePath -Filter "*netfx3*" | Select-Object Name, Length
} else {
    Write-Host "Source directory not found" -ForegroundColor Red
}

Execute DISM Installation

DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:"$driveLetter`:\sources\sxs"

Expected Output

Deployment Image Servicing and Management tool
Version: 10.0.22621.1

Image Version: 10.0.22621.2428

Enabling feature(s)
[==========================100.0%==========================]
The operation completed successfully.

Verify Installation

$featureStatus = Get-WindowsOptionalFeature -Online -FeatureName NetFx3
Write-Host "Feature Name: $($featureStatus.FeatureName)"
Write-Host "State: $($featureStatus.State)"

Output should confirm: State : Enabled### Step 11: Cleanup

Unmount the ISO: ``` Dismount-DiskImage -ImagePath $isoPath

Troubleshooting
---------------

#### Download Interrupted

The script supports resumption — simply re-run `uup_download_windows.cmd` as administrator. #### Conversion Fails

Check: - Available disk space (minimum 15GB required)
- Administrator privileges
- Review `aria2_download.log` for detailed errors

#### Generated ISO Usage

The resulting ISO is a complete Windows 11 22H2 installation media suitable for: - Fresh system installation
- In-place upgrades
- Virtual machine deployment
- System repair scenarios

Why This Approach Succeeds
--------------------------

1. **Exact Version Match:** UUP Dump provides components matching Build 22621 exactly
2. **Pre-integration:** Modifying ConvertConfig.ini embeds .NET 3.5 directly into the ISO
3. **Offline Capability:** No dependency on Windows Update servers
4. **Reusability:** The generated ISO can be used indefinitely for multiple installations

Method Comparison
-----------------

| Method | Succcess Rate | Download Required | Time |
|---|---|---|---|
| Windows Features | 0% | No | 5 min |
| DISM + Windows Update | 20% | No | 10 min |
| Microsoft Official ISO | 0% | 5GB | 10 min |
| UUP Dump Custom ISO | 100% | 5GB | 80 min |

Additional Use Cases
--------------------

**Multi-System Deployment:** Copy the ISO to external storage and use it across multiple machines without re-downloading. **System Reinstallation:** If reinstalling Windows, the generated ISO already includes .NET 3.5, eliminating the need for post-install configuration. **Virtualization:** Mount the ISO in virtualization software for development and testing environments requiring .NET 3.5. </div>