.NET 10 `dotnet run` does not respect DOTNET_ROOT variable

[n00j]

Describe the bug

Setting DOTNET_ROOT doesn't seem to affect all steps in dotnet run. It seem like the restore step works, but when it goes to run the application, it ignores the environment variable and uses a default path.

To Reproduce

I'm not sure how to reproduce this publicly. We have an odd setup with a number of symlinks between the sdk/runtime versions. But below is the output if i try to run a simple helloworld program.

DOTNET_ROOT=/bb/bin/dotnetsdk /bb/bin/dotnet run helloworld8.csproj          
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error You must install .NET to run this application.
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error 
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error App: /bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/bincore/csc
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error Architecture: x64
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error App host version: 10.0.0
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error .NET location: Not found
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error 
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error The following locations were searched:
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error   Application directory:
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error     /bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/bincore/
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error   Environment variable:
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error     DOTNET_ROOT = /bb/bin/package/d/dotnet-muxer/10.0.101.1-1+b20251111T19033819.devise/bin/dotnetsdk
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error 
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error Learn more:
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error https://aka.ms/dotnet/app-launch-failed
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error 
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error Download the .NET runtime:
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error https://aka.ms/dotnet-core-applaunch?missing_runtime=true&arch=x64&rid=linux-x64&os=rhel.8&apphost_version=10.0.0

You can see even with DOTNET_ROOT set, internally its pointing DOTNET_ROOT to another location.

Exceptions (if any)

Further technical details

details of dotnet --info

/bb/bin/dotnet --info .NET SDK: Version: 10.0.100 Commit: b0f34d51fc Workload version: 10.0.100-manifests.e3b0c442 MSBuild version: 18.0.2+b0f34d51f

Runtime Environment:
OS Name: rhel
OS Version: 8
OS Platform: Linux
RID: linux-x64
Base Path: /bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.100
Architecture: x64
Commit: b0f34d51fc

.NET SDKs installed:
8.0.415 [/bb/bin/dotnetsdk/sdk]
10.0.100 [/bb/bin/dotnetsdk/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.21 [/bb/bin/dotnetsdk/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.0 [/bb/bin/dotnetsdk/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 8.0.21 [/bb/bin/dotnetsdk/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.0 [/bb/bin/dotnetsdk/shared/Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
DOTNET_CLI_TELEMETRY_OPTOUT [1]
DOTNET_NUGET_SIGNATURE_VERIFICATION [false]
DOTNET_ROOT [/bb/bin/dotnetsdk]
DOTNET_ROOT_X64 [/bb/bin/dotnetsdk]
DOTNET_SKIP_FIRST_TIME_EXPERIENCE [1]

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

- The IDE (VS / VS Code/ VS4Mac) you're running on, and its version

[marcpopMSFT]

Per the documentation:
These environment variables are used only when running apps via generated executables (apphosts).

There is some discussion as to whether this very limited scope of DOTNET_ROOT should be changed but it's working as originally designed today.

[n00j]

Interesting, seems like this behavior changed since
Net 8.

Is there another environment variable that can be set to tell the adk where to find the root folder?

[stewild]

@marcpopMSFT

There was definitely a change in behavior with .NET 10.

For me, setting DOTNET_ROOT_X64 resolves this issue. It seems the actual behavior differs from what is documented.

DOTNET_ROOT_X64="$DOTNET_ROOT"

[marcpopMSFT]

CC @jkotas @agocke on the host and ROOT env behavior as it's not supposed to affect the behavior of dotnet run or dotnet app.dll.

[jkotas]

/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error App: /bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/bincore/csc
/bb/bin/package/d/dotnet-sdk-10.0/10.0.101.1-1+b20251111T21474698.devise/bin/dotnetsdk/sdk/10.0.100/Roslyn/Microsoft.CSharp.Core.targets(84,5): error Architecture: x64

C# compiler that is part of .NET SDK switched to have an apphost in .NET 10. It was run using dotnet csc.dll before .NET 10 and thus it was not affected by DOTNET_ROOT setting. It is run using csc in .NET 10 and thus it is affected by DOTNET_ROOT setting. This error looks related.

For me, setting DOTNET_ROOT_X64 resolves this issue. It seems the actual behavior differs from what is documented.

Runtime reads both DOTNET_ROOT_ARCH and DOTNET_ROOT env variables in a very tight routine one after another (preferring the ARCH variant if it is set). If setting DOTNET_ROOT_X64 in addition to DOTNET_ROOT makes a difference in your setup, it means that something along the way is clearing DOTNET_ROOT env variable and leaving DOTNET_ROOT_X64 env variable.

[agocke]

Roslyn doesn’t look like it’s doing anything wrong here. The task looks like it’s setting dotnet_root to the directory above itself, i.e. …/bin/dotnetsdk.

Messing with the sdk internal layout like this is unsupported. I believe this behavior is by design.