dotnet CLI/SDK and .NET runtime: Enable major roll-forward after a failed SDK upgrade?

Should Major Roll-Forward Be Enabled by Default?

No. Major roll-forward should not be enabled by default after a failed SDK upgrade. In .NET, major version increments often introduce breaking API changes, behavioral shifts in the Garbage Collector (GC), or JIT compiler optimizations that can lead to MethodNotFoundException or unpredictable performance regressions.

Because .NET runtimes are installed side-by-side, a failed SDK upgrade typically leaves the previous stable runtime intact. Enabling major roll-forward globally to maintain availability risks masking the installation failure and introducing runtime instability across your environment.

Implementation and Verification

If you decide to opt-in to major roll-forward for specific workloads to maintain uptime, use the following scoped methods. This assumes you are using .NET Core 3.0 or later, where the DOTNET_ROLL_FORWARD policy was formalized.

1. Enabling Major Roll-Forward

You can enable this behavior via an environment variable or the application's configuration file:

• Environment Variable: Set DOTNET_ROLL_FORWARD=Major. This tells the host to use the highest installed major version if the targeted version is missing.
• Runtime Config: Add the <RollForward>Major</RollForward> element to the .runtimeconfig.json file of the specific application.

2. Verifying CLI and Runtime Resolution

To ensure the CLI is using the intended SDK and the application is using the expected runtime, execute these commands:

# List all installed SDKs and Runtimes to identify the current state
dotnet --list-sdks
dotnet --list-runtimes

# Verify which SDK is currently active for the current directory
dotnet --version

To verify that a framework-dependent app is actually rolling forward to a newer major version, run the app with the environment variable set and check the logs or use a diagnostic tool to print the runtime version.

Compatibility Testing Requirements

Before deploying a major roll-forward policy, the following testing is required to mitigate risk:

• Regression Testing: Full execution of the test suite against the newer major runtime to detect breaking API changes.
• Performance Baselining: Comparison of memory allocation and CPU usage, as major version jumps often change GC behavior.
• Dependency Audit: Verification that third-party NuGet packages are compatible with the newer runtime version.

Diagnostic Note: To provide a more specific recommendation, please clarify if you are using a global.json file to pin your SDK version, as this overrides the default CLI resolution logic.