---
title: "Why does my .NET app fail to start after changing the runtime version?"  
description: "Why does my .NET app fail to start after changing the runtime version?"  
author: "John Smith"  
published: 2026-09-21  
updated: 2026-09-22  
canonical: https://answers.mindstick.com/qa/117258/why-does-my-net-app-fail-to-start-after-changing-the-runtime-version  
category: "troubleshooting"  
tags: ["troubleshooting", "iis", "runtime"]  
reading_time: 2 minutes  

---

# Why does my .NET app fail to start after changing the runtime version?

You upgraded the .NET runtime on your server, but the application pool still points to the older version. The result? A crash on launch with a cryptic "Unable to find the specified file" error.

Open a command prompt and run `dotnet --list-runtimes`. If the version you need is missing, install it via the Microsoft Install Script or your OS package manager. Do not assume the global SDK installation covers the server runtime.

## Answers

### Answer by Rana Sunny

I recently bumped the target runtime for a console application from .NET 6 to .NET 8, rebuilt, and now it throws a cryptic error on launch. The process exits immediately, and the usual diagnostic output isn't showing up. Has anyone run into this before?

The first thing I checked was whether the runtime was actually installed. I ran `dotnet --list-runtimes` and .NET 8 showed up fine. Next, I verified the `TargetFramework` in the project file matched what I expected, and I double-checked that there were no lingering `appsettings.Development.json` overrides pointing to an older SDK.

What usually trips this up is a mismatch between the runtime and the SDK, or the app relying on a native dependency that was compiled against a different ABI. If you have any C++/CLI wrappers, third-party native DLLs, or even certain NuGet packages with native assets, they can refuse to load under a newer runtime. Another common culprit is the `Microsoft.AspNetCore.App` shared framework version drifting out of sync with your target.

A quick way to narrow it down is to run the app under a debugger or with `dotnet run --verbosity diagnostic`. If you see a `BadImageFormatException` or a failure to load a specific assembly, that usually points to an architecture or version mismatch rather than a pure configuration issue.

If you're still stuck, paste the exact exception and the contents of your `.csproj` here. It might also help to confirm whether you're using a self-contained deployment or relying on the shared runtime on the target machine.


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Original Source: https://answers.mindstick.com/qa/117258/why-does-my-net-app-fail-to-start-after-changing-the-runtime-version

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