WinAppSDK Runtime Issue on Windows 11

[tajbender]

WinAppSDK Runtime Issue on Windows 26300 + Visual Studio 2026 Preview

This project failed to start with:

System.Runtime.InteropServices.COMException
Class not registered (0x80040154 — REGDB_E_CLASSNOTREG)

The error occurred before any WinUI code executed, during:

Microsoft.UI.Xaml.Application.Start(...)

Root Cause

Windows Insider build 26300 and Visual Studio 2026 Preview ship with Windows App SDK Runtime 1.8 preinstalled. This environment blocks installation of older WinAppSDK runtimes (1.6, 1.7) via MSIX or Add‑AppxPackage:

0x80073D06 — A higher version of this package is already installed.

As a result:

WinAppSDK 1.6 cannot be installed

DDLM / Main / Singleton packages for 1.6 cannot be deployed

The project still references Microsoft.WindowsAppSDK 1.6.x

WinUI tries to activate the 1.6 runtime

Windows only exposes 1.8

→ ActivationFactory fails

→ REGDB_E_CLASSNOTREG

This is a version mismatch, not a project bug.

Why Add‑AppxPackage Fails

Windows 26300 already contains:

Microsoft.WindowsAppRuntime.1.8 (x64 + x86)

Microsoft.WindowsAppRuntime.CBS.1.8

Microsoft.WindowsAppRuntime.1.7

Microsoft.WindowsAppRuntime.1.6 (Store variant, not Redist)

No DDLM / Main / Singleton packages for 1.6

When attempting to install the Redist 1.6 packages:

A higher version of this package is already installed.

Windows Insider builds do not allow downgrading WinAppSDK frameworks.

Solution

Upgrade the project to WinAppSDK 1.8

In your .csproj:

Then:

Clean the solution

Rebuild

Run

The app will now use the system‑installed 1.8 runtime, which is fully compatible with Windows 26300 and VS 2026 Preview.

Notes for Other Developers

Windows Insider builds may ship newer WinAppSDK runtimes ahead of NuGet defaults.

These builds block installation of older MSIX runtime packages.

If your WinUI 3 app suddenly fails with REGDB_E_CLASSNOTREG on startup, check:

Get-AppxPackage WindowsAppRuntime

If the system has a newer runtime than your project references → upgrade your NuGet package.

[tajbender]

Badges, just for Reference:

Runtime, Environment:

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Build, Status:

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Project Metadata:

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[tajbender]

Setup-Script: Create blank WinUi 3 project solution:

# ================================
# Clean WinUI 3 Setup Script
# ================================

param(
    [string]$SolutionName = "MyApp",
    [string]$ProjectName = "MyApp"
)

Write-Host "Creating clean WinUI 3 solution..." -ForegroundColor Cyan

# 1. Create solution
dotnet new sln -n $SolutionName

# 2. Create WinUI 3 project (packaged)
dotnet new winui3 -n $ProjectName

# 3. Add project to solution
dotnet sln $SolutionName.sln add "$ProjectName/$ProjectName.csproj"

# 4. Force WinAppSDK 1.8
Write-Host "Setting WinAppSDK 1.8..." -ForegroundColor Cyan
$csprojPath = "$ProjectName/$ProjectName.csproj"

# Replace existing WinAppSDK reference
(Get-Content $csprojPath) `
    -replace 'Microsoft.WindowsAppSDK" Version="[^"]+"', 'Microsoft.WindowsAppSDK" Version="1.8.0"' `
    | Set-Content $csprojPath

# 5. Restore + Build
Write-Host "Restoring packages..." -ForegroundColor Cyan
dotnet restore

Write-Host "Building project..." -ForegroundColor Cyan
dotnet build

Write-Host "Done. Clean WinUI 3 project created successfully." -ForegroundColor Green

Build.yml

name: Build WinUI 3 App

on:
  push:
    branches: [ main ]
  pull_request:
    branches: [ main ]

jobs:
  build:
    runs-on: windows-latest

    steps:
    - name: Checkout repository
      uses: actions/checkout@v4

    - name: Setup .NET
      uses: actions/setup-dotnet@v4
      with:
        dotnet-version: 8.0.x

    - name: Restore dependencies
      run: dotnet restore

    - name: Build solution
      run: dotnet build --configuration Release --no-restore

    - name: Publish (optional for unpackaged)
      if: ${{ false }} # set to true if you want publish output
      run: dotnet publish -c Release