AGIBuild/dotnet-version-detection

dotnet-version-detection

Detects .NET version information from project files and provides version-specific guidance. This skill runs first before any .NET development work. All other skills depend on the detected version to adapt their guidance.

Cross-cutting skill referenced by [skill:dotnet-advisor] and virtually all specialist skills. See also [skill:dotnet-file-based-apps] for .NET 10+ file-based apps that run without a .csproj.

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Detection Precedence Algorithm

Read project files in this strict order. Higher-numbered sources are lower priority. Stop falling through once a TFM is resolved.

1. Direct <TargetFramework> in .csproj (highest priority)

Read the nearest .csproj file to the current working file/directory.

<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
</PropertyGroup>

If found and the value is a literal TFM (e.g., net10.0, not $(SomeProperty)), this is the authoritative TFM. Report it and proceed to additional detection (Step 5).

If the value is an MSBuild property expression (starts with $(), skip to Step 4 for unresolved property handling.

2. <TargetFrameworks> in .csproj (multi-targeting)

<PropertyGroup>
  <TargetFrameworks>net8.0;net10.0</TargetFrameworks>
</PropertyGroup>

If found:

• Report all TFMs (semicolon-delimited)
• Guide based on the highest TFM (e.g., net10.0)
• Note polyfill needs for lower TFMs: "Consider [skill:dotnet-multi-targeting] for PolySharp/Polyfill to backport language features to net8.0"
• Proceed to additional detection (Step 5)

3. Directory.Build.props shared TFM

If no <TargetFramework> or <TargetFrameworks> found in the .csproj (or if the .csproj inherits from shared props), read Directory.Build.props in the current directory or any parent directory up to the solution root.

<!-- Directory.Build.props -->
<PropertyGroup>
  <TargetFramework>net10.0</TargetFramework>
</PropertyGroup>

If found, use this as the TFM. Note: per-project .csproj values override Directory.Build.props.

4. MSBuild Property Expressions (fallback with warning)

If the TFM value is an MSBuild property expression rather than a literal:

<TargetFramework>$(MyCustomTfm)</TargetFramework>

Emit warning:

Warning: Unresolved MSBuild property $(MyCustomTfm). Cannot determine TFM statically. Falling back to SDK version from global.json.

Then fall through to global.json SDK version (Step 4a) or dotnet --version (Step 4b).

4a. global.json SDK version

{
  "sdk": {
    "version": "10.0.100"
  }
}

Map SDK version to approximate TFM:

• 8.0.xxx -> net8.0
• 9.0.xxx -> net9.0
• 10.0.xxx -> net10.0
• 11.0.xxx-preview.x -> net11.0

Report: "Inferred TFM from global.json SDK version. Verify actual TFM in project file."

4b. dotnet --version (last resort)

If no global.json exists, use dotnet --version output to infer SDK version. Same mapping as 4a.

Report: "Inferred TFM from installed SDK version. No global.json or .csproj found. Consider creating a project with dotnet new."

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Additional Detection (Step 5)

After resolving the TFM, also check these files for supplementary version information. Always perform these checks regardless of which precedence step resolved the TFM.

5a. global.json SDK Version

Even if TFM was resolved from .csproj, read global.json for:

• sdk.version -- the pinned SDK version
• sdk.rollForward -- the rollForward policy (e.g., latestFeature, latestPatch)

Report the SDK version alongside the TFM. Flag inconsistencies:

Warning: TFM net10.0 but global.json pins SDK 9.0.100. The project targets a newer framework than the pinned SDK. Update global.json or verify the build environment has the correct SDK.

5b. C# Language Version

Check for explicit <LangVersion> in .csproj or Directory.Build.props:

<LangVersion>preview</LangVersion>

• If preview -- report "C# preview features enabled. Unlocks the next C# version available in the installed SDK (e.g., C# 15 preview features with a .NET 11 preview SDK)."
• If latest -- report the default C# version for the detected TFM
• If explicit version (e.g., 12.0) -- report that version, warn if it's below the TFM default
• If absent -- use the default C# version for the TFM (see reference data below)

5c. Preview Feature Detection

Check for these properties in .csproj or Directory.Build.props:

EnablePreviewFeatures:

<EnablePreviewFeatures>true</EnablePreviewFeatures>

Report: ".NET preview features enabled. Access to preview APIs and types."

Runtime-async feature flag (.NET 11+):

<Features>$(Features);runtime-async=on</Features>

Report: "Runtime-async enabled. Async/await uses runtime-level execution instead of compiler state machines."

Note: runtime-async requires <EnablePreviewFeatures>true</EnablePreviewFeatures> as well.

5d. Multi-targeting Details

If multi-targeting was detected (Step 2), also note:

• Which TFMs are LTS vs STS vs Preview
• Which TFMs are approaching end-of-support
• Suggest [skill:dotnet-multi-targeting] for polyfill guidance

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Structured Output Format

After detection, present results in this structured format:

.NET Version Detection Results
==============================
TFM:              net10.0 (or net8.0;net10.0 for multi-targeting)
Highest TFM:      net10.0
C# Version:       14 (default for net10.0)
SDK Version:      10.0.100 (from global.json)
Preview Features: none
Runtime-Async:    not enabled
Warnings:         none

Guidance: This project targets .NET 10 LTS with C# 14. Use modern patterns
including field-backed properties, collection expressions, and primary
constructors. All guidance will target net10.0 capabilities.

For multi-targeting:

.NET Version Detection Results
==============================
TFMs:             net8.0;net10.0
Highest TFM:      net10.0
C# Version:       14 (default for highest TFM)
SDK Version:      10.0.100 (from global.json)
Preview Features: none
Warnings:         net8.0 reaches end of support Nov 2026

Guidance: Multi-targeting net8.0 and net10.0. Guide on net10.0 patterns.
For net8.0 compatibility, use PolySharp/Polyfill for language features.
See [skill:dotnet-multi-targeting] for detailed polyfill guidance.

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Edge Cases

No .csproj Found

If no .csproj exists in the workspace:

• Check for .sln or .slnx files and look for referenced projects
• If no project files found, continue the fallback chain:
  1. Read global.json for SDK version (Step 4a) and infer TFM from it
  2. If no global.json, use dotnet --version (Step 4b) to infer TFM
  3. Only if both inference methods fail: "No .NET project or SDK detected. Defaulting guidance to net10.0 (current LTS). Use dotnet new to create a project."

MSBuild Property Indirection

If <TargetFramework> contains $(PropertyName):

• Emit: "Unresolved property: $(PropertyName). Cannot determine TFM from static analysis."
• Fall back to global.json SDK version, then dotnet --version
• Recommend verifying TFM via dotnet --list-sdks or dotnet msbuild -getProperty:TargetFramework

Inconsistent Files

If .csproj says net10.0 but global.json pins SDK 9.0.100:

• Prefer the .csproj TFM (higher precedence)
• Warn about the mismatch
• Suggest updating global.json to match

No SDK Installed

If dotnet --version fails or is not found:

• Report: "No .NET SDK detected on this system. Install from https://dot.net"
• If project files exist, still report the TFM from project files
• Note that builds will fail without the SDK

.fsproj and .vbproj

The same detection logic applies to F# (.fsproj) and VB.NET (.vbproj) projects. The <TargetFramework> element is identical across all .NET project types.

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Caching Behavior

Version detection results should be cached per-project (per .csproj path). Re-detect when:

• The user switches to a different project within the solution
• The user explicitly asks to re-detect
• A .csproj, Directory.Build.props, or global.json file is modified

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Version-to-Feature Reference Data

Last updated: 2026-02-11

This reference data maps .NET versions to their C# language versions, key features, and support lifecycle. This section is separate from the detection logic above -- detection determines which version is in use; this data maps that version to available features.

Version Matrix

| .NET Version | Status | C# Version | TFM | Support End | Notes | |-------------|--------|-------------|-----|-------------|-------| | .NET 8 | LTS (active) | C# 12 | net8.0 | Nov 2026 | Approaching end of support | | .NET 9 | STS | C# 13 | net9.0 | May 2026 | Approaching end of support | | .NET 10 | LTS (current) | C# 14 | net10.0 | Nov 2028 | Recommended for new projects | | .NET 11 | Preview 1 | C# 15 (preview) | net11.0 | TBD (expected STS end: ~May 2028 if Nov 2026 GA) | Preview only -- not for production |

C# Version Feature Highlights

C# 12 (net8.0)

• Primary constructors for classes/structs
• Collection expressions ([1, 2, 3])
• ref readonly parameters
• Default lambda parameters
• Alias any type with using
• Inline arrays
• Interceptors (experimental)

C# 13 (net9.0)

• params collections (any collection type)
• Lock type (System.Threading.Lock)
• New escape sequence \e
• Method group natural type improvements
• Implicit indexer access in object initializers
• ref and unsafe in iterators/async
• Partial properties

C# 14 (net10.0)

• Field-backed properties (field contextual keyword)
• nameof for unbound generic types
• Extension improvements (extension blocks)
• First-class Span<T> in more contexts
• allows ref struct anti-constraint for generics

C# 15 preview (net11.0)

• Collection expression arguments (with() syntax for capacity/comparers):

  List<int> nums = [with(capacity: 32), 0, ..evens, ..odds];
  HashSet<string> names = [with(comparer: StringComparer.OrdinalIgnoreCase), "Alice"];
  

• Additional features expected as .NET 11 progresses through preview

.NET 11 Preview 1 Notable Features

These features are available when net11.0 TFM is detected with preview features enabled:

• Runtime-async: Async/await at runtime level (requires <EnablePreviewFeatures>true</EnablePreviewFeatures> + <Features>$(Features);runtime-async=on</Features>)
• Zstandard compression: System.IO.Compression.Zstandard (2-7x faster than Brotli/Deflate)
• BFloat16: System.Numerics.BFloat16 for AI/ML workloads
• Happy Eyeballs: ConnectAlgorithm.Parallel for dual-stack networking
• CoreCLR for Android: Default runtime for MAUI Android Release builds
• XAML source generation: Default in MAUI (replaces XAMLC)
• EF Core: Complex types + JSON columns with TPT/TPC inheritance
• CoreCLR on WASM: Experimental alternative to Mono for Blazor WASM

Support Lifecycle Guidance

When reporting version information, include lifecycle context:

• End-of-support approaching (within 6 months): Warn and suggest [skill:dotnet-version-upgrade]
• Preview/RC: Warn "not for production use" unless user explicitly opted in
• STS reaching end: Note shorter support window compared to LTS
• Current LTS: Confirm as recommended target for new projects