Add NETSDK1235 warning for non-AnyCPU PlatformTarget in modern .NET

[Copilot]

PlatformTarget is a legacy .NET Framework setting that has no effect in modern .NET. Users explicitly setting it to non-AnyCPU values (x64, x86, arm, arm64) should be guided to remove this customization.

Changes

• New warning NETSDK1235: Warns when PlatformTarget is explicitly set to a non-AnyCPU value in .NETCoreApp projects
• Target _CheckForNonAnyCPUPlatformTarget: Only fires when user explicitly sets the property (not when SDK infers it from RuntimeIdentifier)
• Unit tests: Validates warning appears for explicit non-AnyCPU values and does not appear for AnyCPU

Example

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <PlatformTarget>x64</PlatformTarget> <!-- NETSDK1235 warning -->
  </PropertyGroup>
</Project>

Output:

warning NETSDK1235: Setting the PlatformTarget property to 'x64' is not recommended 
for modern .NET applications. The PlatformTarget property is a legacy setting from 
.NET Framework and has no effect in modern .NET. Consider removing PlatformTarget 
or setting it to 'AnyCPU'.

No warning is issued when:

• PlatformTarget is AnyCPU
• PlatformTarget is not set or inferred from RuntimeIdentifier
• Target framework is .NET Framework

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Flag usage of non-AnyCPU values for PlatformTarget in modern .NET applications</issue_title> <issue_description>### Is your feature request related to a problem? Please describe.

The PlatformTarget field (and therefore Platform in MSbuild) only applies to .NET Framework and doesn't apply to modern .NET applications. For these applications, Platform can (should) always be AnyCPU. We should guide users to removing customizations to this property for projects that do not need it.

From Jan Kotas in dotnet/sdk#42344:

PlatformTarget is left-over from .NET Framework. (PlatformTarget can only represent architectures that .NET Framework targets. It does not represent full range of platforms that .NET Core can target.)

If you are targeting current .NET, building everything as AnyCPU (ie hardcoding PlatformTarget to AnyCPU) all the time should work just fine. It is what we are doing in dotnet/runtime repo. We hardcode PlatformTarget to AnyCPU, even for platform-specific binaries.</issue_description>

Comments on the Issue (you are @copilot in this section)

@jkotas Also, you may want to stop inferring the `PlatformTarget` in the SDK when targeting modern .NET applications:

https://github.com/dotnet/sdk/blob/88c64736ff7039536fc2546d5026d8b78e840e49/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RuntimeIdentifierInference.targets#L124-L157</comment_new> <comment_new>@baronfel

@jkotas this issue is for an MSBuild BuildCheck for usage in customer projects, so I extracted your comment to a new issue for the SDK to stop setting it in the first place.</comment_new>

• Fixes dotnet/sdk#42556

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[MattParkerDev]

There is currently a requirement when using CsWin32 to specify a PlatformTarget to generate certain methods, I believe due to x64 vs x86 differences