dotnet-multi-targeting

Comprehensive guide for .NET multi-targeting strategies with a polyfill-first approach. This skill consumes the structured output from [skill:dotnet-version-detection] (TFM, C# version, preview flags) and provides actionable guidance on backporting language features, handling runtime gaps, and validating API compatibility across target frameworks.

Out of scope: TFM detection logic (owned by [skill:dotnet-version-detection]), version upgrade lane selection (see [skill:dotnet-version-upgrade]), platform-specific UI frameworks (MAUI, Blazor), cloud deployment configuration.

Cross-references: [skill:dotnet-version-detection] for TFM resolution and version matrix, [skill:dotnet-version-upgrade] for upgrade lane guidance and migration strategies.

---

Decision Matrix: Polyfill vs Conditional Compilation

Use this matrix to select the correct strategy for each type of gap between your highest and lowest TFMs.

Gap Type	Strategy	When to Use	Example
Language/syntax feature	Polyfill (PolySharp)	Compiler needs attribute/type stubs to emit newer syntax on older TFMs	required modifier, init properties, SetsRequiredMembers on net8.0
BCL API addition	Polyfill (SimonCropp/Polyfill) if available, else #if	A newer BCL type or method is missing on older TFMs	System.Threading.Lock on net8.0, Index/Range on netstandard2.0
Runtime behavior difference	Conditional compilation (#if) or adapter pattern	Behavior differs at runtime regardless of compilation	Runtime-async (net11.0 only), different GC modes, SearchValues<T> runtime optimizations
Platform API divergence	Conditional compilation with [SupportedOSPlatform]	API exists only on specific OS targets	Windows Registry APIs, Android-specific intents, iOS keychain

Decision flow:

1. Can a compile-time polyfill satisfy the gap? Use PolySharp or SimonCropp/Polyfill.
2. Is the gap a missing BCL API with no polyfill available? Use #if with TFM-specific code.
3. Is the gap a runtime behavior difference? Use #if or the adapter pattern to isolate divergent code paths.
4. Is the gap platform-specific? Use #if with [SupportedOSPlatform] attributes.

---

PolySharp (Compiler-Synthesized Polyfills)

PolySharp is a source generator that synthesizes the attribute and type stubs the C# compiler needs to emit newer language features when targeting older TFMs. It operates entirely at compile time -- no runtime dependencies are added.

What PolySharp Provides

• required modifier support (C# 11+)
• init property accessors (C# 9+)
• SetsRequiredMembers attribute
• CompilerFeatureRequired attribute
• IsExternalInit type
• CallerArgumentExpression attribute
• StackTraceHidden attribute
• UnscopedRef attribute
• InterpolatedStringHandler attributes
• ModuleInitializer attribute
• Index and Range support types

Setup

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>net8.0;net10.0</TargetFrameworks>
    <!-- Use the highest C# version across all TFMs -->
    <LangVersion>14</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <!-- PolySharp is a source generator; it adds no runtime dependency -->
    <PackageReference Include="PolySharp" Version="1.*">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
  </ItemGroup>
</Project>

How It Works

PolySharp detects which polyfill types are missing for the current TFM and generates source for only those types. On net10.0, where required is natively supported, the generator emits nothing -- zero overhead.

// This compiles on net8.0 WITH PolySharp installed,
// because PolySharp generates the required CompilerFeatureRequired
// and IsExternalInit types that the compiler needs.
public class UserProfile
{
    public required string DisplayName { get; init; }
    public required string Email { get; init; }
    public string? Bio { get; set; }
}

PolySharp Limitations

• PolySharp provides compiler stubs only. It does not backport runtime behavior.
• Features that require runtime support (e.g., runtime-async, SearchValues<T> hardware acceleration) cannot be polyfilled.
• If a feature needs both a compiler attribute AND a BCL API (e.g., collection expressions with Span<T> overloads), you may need both PolySharp and SimonCropp/Polyfill.

---

SimonCropp/Polyfill (BCL API Polyfills)

SimonCropp/Polyfill provides source-generated implementations of newer BCL APIs for older TFMs. Unlike PolySharp (which provides compiler attribute stubs), Polyfill provides actual method and type implementations.

What Polyfill Provides

Key polyfilled APIs (non-exhaustive):

• System.Threading.Lock (C# 13 / net9.0+)
• String.Contains(char), String.Contains(string, StringComparison)
• String.ReplaceLineEndings()
• HashCode struct
• SkipLocalsInit attribute
• TaskCompletionSource (non-generic)
• Stream.ReadExactly, Stream.ReadAtLeast
• Memory<T> and Span<T> extensions
• IReadOnlySet<T> interface
• Various LINQ additions (TryGetNonEnumeratedCount, DistinctBy, Chunk, etc.)

Setup

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>net8.0;net10.0</TargetFrameworks>
    <LangVersion>14</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <!-- Polyfill is a source generator; no runtime dependency -->
    <PackageReference Include="Polyfill" Version="7.*">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
  </ItemGroup>
</Project>

Usage Example

// System.Threading.Lock is a net9.0+ type.
// With Polyfill installed, this compiles on net8.0.
public class ThrottledProcessor
{
    private readonly Lock _lock = new();

    public void Process(string item)
    {
        lock (_lock)
        {
            // Lock provides better diagnostics than object-based locking
            Console.WriteLine($"Processing: {item}");
        }
    }
}

Combining PolySharp and Polyfill

For maximum compatibility, use both packages together. They are complementary and do not conflict:

<ItemGroup>
  <!-- PolySharp: compiler attribute stubs (required, init, etc.) -->
  <PackageReference Include="PolySharp" Version="1.*">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
  </PackageReference>

  <!-- Polyfill: BCL API implementations (Lock, LINQ additions, etc.) -->
  <PackageReference Include="Polyfill" Version="7.*">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
  </PackageReference>
</ItemGroup>

With both installed, you get full language feature support (PolySharp) and BCL API backporting (Polyfill) on older TFMs.

---

Conditional Compilation

Use conditional compilation (#if) when the gap is a runtime behavior difference or a platform API that cannot be polyfilled at compile time.

TFM-Based Conditionals

The compiler defines preprocessor symbols for each TFM. Use NET8_0_OR_GREATER-style symbols (available since .NET 5) for version range checks:

public static class PerformanceHelper
{
#if NET10_0_OR_GREATER
    // net10.0+ has optimized SearchValues with hardware acceleration
    private static readonly SearchValues<char> s_vowels =
        SearchValues.Create("aeiouAEIOU");

    public static int CountVowels(ReadOnlySpan<char> text)
        => text.Count(s_vowels);
#else
    // Fallback for net8.0: manual loop
    public static int CountVowels(ReadOnlySpan<char> text)
    {
        int count = 0;
        foreach (char c in text)
        {
            if ("aeiouAEIOU".Contains(c))
                count++;
        }
        return count;
    }
#endif
}

Available Preprocessor Symbols

Symbol	True When
NET8_0	Exactly net8.0
NET8_0_OR_GREATER	net8.0 or any higher version
NET9_0_OR_GREATER	net9.0 or any higher version
NET10_0_OR_GREATER	net10.0 or any higher version
NET11_0_OR_GREATER	net11.0 or any higher version
NETSTANDARD2_0	Exactly netstandard2.0
NETSTANDARD2_0_OR_GREATER	netstandard2.0 or higher

When #if Is Correct

1. Runtime behavior gap: The API exists on both TFMs but behaves differently at runtime (e.g., GC.Collect modes, HttpClient connection pooling behavior).
2. No polyfill available: The BCL API is not covered by SimonCropp/Polyfill and cannot be stubbed.
3. Performance-critical path: You want to use a TFM-specific optimized API path (e.g., SearchValues<T>, FrozenDictionary<K,V>).
4. Platform API: The API is available only on a specific OS platform target.

When #if Is Wrong

• Language syntax feature (e.g., required, init): Use PolySharp instead.
• Missing BCL method that has a polyfill (e.g., System.Threading.Lock): Use SimonCropp/Polyfill instead.
• Wrapping entire files in #if blocks -- use TFM-specific source files instead (see below).

---

Multi-Targeting .csproj Patterns

Basic Multi-Targeting

<PropertyGroup>
  <!-- Semicolon-delimited list of TFMs -->
  <TargetFrameworks>net8.0;net10.0</TargetFrameworks>
  <!-- Use the highest C# version to access all language features -->
  <LangVersion>14</LangVersion>
</PropertyGroup>

Conditional Package References

Some packages are only needed on specific TFMs:

<ItemGroup>
  <!-- Polyfill packages: only needed on older TFMs, but safe to reference
       unconditionally because they emit nothing when features are native -->
  <PackageReference Include="PolySharp" Version="1.*">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
  </PackageReference>

  <!-- TFM-conditional package: only available/needed on specific TFMs -->
  <PackageReference Include="System.Text.Json" Version="9.*"
                    Condition="'$(TargetFramework)' == 'net8.0'" />
</ItemGroup>

TFM-Specific Source Files

For large blocks of TFM-specific code, use dedicated source files instead of #if blocks:

<ItemGroup>
  <!-- SDK-style projects auto-include all *.cs files. Remove TFM-specific
       directories first to avoid NETSDK1022 duplicate compile items. -->
  <Compile Remove="Compatibility\**\*.cs" />

  <!-- Then conditionally include only the files for the current TFM -->
  <Compile Include="Compatibility\Net8\**\*.cs"
           Condition="'$(TargetFramework)' == 'net8.0'" />
  <Compile Include="Compatibility\Net10\**\*.cs"
           Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net10.0'))" />
</ItemGroup>

Directory structure:

MyLibrary/
  Compatibility/
    Net8/
      SearchValuesCompat.cs
    Net10/
      SearchValuesNative.cs
  Services/
    TextAnalyzer.cs          # shared code, references interface
  MyLibrary.csproj

Platform-Specific TFMs

For projects targeting platform-specific TFMs (MAUI, Uno):

<PropertyGroup>
  <!-- Use version-agnostic platform globs where possible -->
  <TargetFrameworks>net10.0;net10.0-android;net10.0-ios;net10.0-windows10.0.19041.0</TargetFrameworks>
</PropertyGroup>

<ItemGroup Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'android'">
  <PackageReference Include="Xamarin.AndroidX.Core" Version="1.*" />
</ItemGroup>

Shared Properties via Directory.Build.props

For multi-project solutions, centralize multi-targeting configuration:

<!-- Directory.Build.props -->
<Project>
  <PropertyGroup>
    <LangVersion>14</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="PolySharp" Version="1.*">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
    <PackageReference Include="Polyfill" Version="7.*">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
  </ItemGroup>
</Project>

This ensures all projects in the solution share the same polyfill setup. Individual projects set their own <TargetFrameworks>.

---

API Compatibility Validation

When publishing a NuGet package that targets multiple TFMs, validate that the public API surface is consistent and that you have not accidentally broken consumers.

EnablePackageValidation

Package validation runs automatically during dotnet pack and checks:

• Baseline validation: Compares the current package against a previous version to detect breaking changes.
• Compatible framework validation: Ensures APIs available on one TFM are available on all compatible TFMs.

<PropertyGroup>
  <TargetFrameworks>net8.0;net10.0</TargetFrameworks>
  <!-- Enable package validation during pack -->
  <EnablePackageValidation>true</EnablePackageValidation>
  <!-- Compare against last published version for breaking change detection -->
  <PackageValidationBaselineVersion>1.2.0</PackageValidationBaselineVersion>
</PropertyGroup>

API Compatibility Workflow

Step 1: Enable validation in .csproj

<PropertyGroup>
  <EnablePackageValidation>true</EnablePackageValidation>
</PropertyGroup>

Step 2: Set baseline version (for existing packages)

<PropertyGroup>
  <!-- The last published stable version to compare against -->
  <PackageValidationBaselineVersion>2.0.0</PackageValidationBaselineVersion>
</PropertyGroup>

Step 3: Pack and check

# Pack triggers validation automatically
dotnet pack --configuration Release

# Success: no output about compatibility issues
# Failure: error messages listing incompatible API changes

Step 4: Interpret results

Result	Meaning	Action
Clean pack	All TFMs expose compatible API surfaces; no breaking changes from baseline	Ship
CP0001	Missing type on a compatible TFM	Add the type to the TFM or use #if to exclude it from the public API
CP0002	Missing member on a compatible TFM	Add the member or suppress if intentional
CP0003	Breaking change from baseline version	Bump major version or revert the change
PKV004	Compatible TFM has different API surface	Ensure conditional APIs are intentional

Suppressing Known Differences

For intentional API differences between TFMs, use a suppression file. Package validation can generate one when suppression generation is enabled:

# Build with suppression-file generation enabled
dotnet pack /p:ApiCompatGenerateSuppressionFile=true
# Creates CompatibilitySuppressions.xml in the project directory

The generated CompatibilitySuppressions.xml contains targeted suppressions:

<?xml version="1.0" encoding="utf-8"?>
<Suppressions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Suppression>
    <DiagnosticId>CP0002</DiagnosticId>
    <Target>M:MyLib.PerformanceHelper.CountVowels(System.ReadOnlySpan{System.Char})</Target>
    <Left>lib/net8.0/MyLib.dll</Left>
    <Right>lib/net10.0/MyLib.dll</Right>
  </Suppression>
</Suppressions>

Reference the suppression file in .csproj (automatic when file is at project root):

<PropertyGroup>
  <!-- Explicit path if suppression file is not at project root -->
  <ApiCompatSuppressionFile>CompatibilitySuppressions.xml</ApiCompatSuppressionFile>
</PropertyGroup>

Prefer targeted suppression files over blanket <NoWarn>$(NoWarn);CP0002</NoWarn> -- blanket suppression hides real issues. Commit the suppression file to source control so reviewers can see intentional API differences.

ApiCompat Standalone Tool

For CI pipelines that validate without packing:

# Install as a global tool
dotnet tool install -g Microsoft.DotNet.ApiCompat.Tool

# Global tool invocation (after install -g)
apicompat --left-assembly bin/Release/net8.0/MyLib.dll \
          --right-assembly bin/Release/net10.0/MyLib.dll

# Or install as a local tool (preferred for CI reproducibility)
dotnet new tool-manifest   # if .config/dotnet-tools.json doesn't exist
dotnet tool install Microsoft.DotNet.ApiCompat.Tool

# Local tool invocation
dotnet tool run apicompat --left-assembly bin/Release/net8.0/MyLib.dll \
                          --right-assembly bin/Release/net10.0/MyLib.dll

---

Agent Gotchas

1. Do not use #if for language feature polyfills. If the gap is a compiler attribute or syntax feature (e.g., required, init, SetsRequiredMembers), use PolySharp. #if blocks for language features create unnecessary code duplication and maintenance burden.

2. Do not omit <PrivateAssets>all</PrivateAssets> on polyfill packages. PolySharp and SimonCropp/Polyfill are source generators meant for compile time only. Without PrivateAssets=all, the polyfill types leak into your package's dependency graph and can conflict with consumers' own polyfills.

3. Do not hardcode TFM versions in conditional compilation. Use NET10_0_OR_GREATER-style range symbols instead of NET10_0 exact symbols. Exact symbols break when a new TFM is added (e.g., net11.0 would skip the net10.0-specific path). Range symbols automatically include future TFMs.

4. Do not set <LangVersion> per TFM. Set it once to the highest version needed across all TFMs (e.g., <LangVersion>14</LangVersion>). PolySharp and Polyfill handle the backporting. Per-TFM LangVersion causes confusing syntax errors.

5. Do not skip EnablePackageValidation for multi-targeted NuGet packages. Without it, you can accidentally expose different API surfaces on different TFMs, causing consumer build failures when they switch TFMs.

6. Do not use $(TargetFramework) string equality for range checks in MSBuild conditions. Use $([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net10.0')) for forward-compatible range checks. String equality (e.g., == 'net10.0') misses net11.0 and higher.

7. Do not re-implement TFM detection. This skill consumes the structured output from [skill:dotnet-version-detection]. Never parse .csproj files to determine TFMs -- use the detection skill's output (TFM, C# version, SDK version, warnings).

8. Do not assume polyfills cover runtime behavior. PolySharp and Polyfill provide compile-time stubs and source-generated implementations. Features that require runtime changes (e.g., runtime-async, GC improvements, JIT optimizations) cannot be polyfilled -- use #if for these.

9. Do not use version-specific TFM globs for platform targets. Use net*-android pattern matching (version-agnostic) instead of net10.0-android in documentation and tooling to avoid false negatives when users target different .NET versions.

---

Prerequisites

• .NET 8.0+ SDK (multi-targeting requires the highest targeted SDK installed)
• PolySharp NuGet package (for language feature polyfills)
• Polyfill NuGet package by Simon Cropp (for BCL API polyfills)
• Microsoft.DotNet.ApiCompat.Tool (optional, for standalone API compatibility checks)
• Output from [skill:dotnet-version-detection] (TFM, C# version, SDK version)

---

References

Last verified: 2026-02-12

• PolySharp - Source Generator for Polyfill Attributes
• SimonCropp/Polyfill - Source-Only BCL Polyfills
• .NET Target Framework Monikers
• Multi-Targeting in .NET
• C# Preprocessor Directives
• Package Validation Overview
• API Compatibility Overview
• MSBuild Target Framework Properties