Platform dependent file not found

[MichaelRumpler]

Description

I created a new MAUI Class Library with platform dependent files and folders. Now from a file with an *.iOS.cs extension which should only be compiled for iOS and MacCatalyst I want to call a method in a file in Platforms/iOS/iOSHelper.cs which should also only be compiled for iOS and MacCatalyst. But I get an error that the Platforms.iOS namespace does not exist. It looks like VS tries to compile my .iOS.cs file for net6 and for that TargetFramework it really does not exist.

But the project dropdown only shows iOS and MacCatalyst as expected.

The same as the class I want to call.

The funny thing is, that VS does not show an error in intellisense. It even added the using statement itself (added with ctrl-.).

I also have similar code in LabelHandler.Android.cs where I call into MauiLib2.Platforms.Android.AndroidHelper and this works.

Here is a repro project:
MauiLib2.zip

Steps to Reproduce

1. open VS Version 17.2.0 Preview 5.0
2. create a new MAUI Class Library
3. copy the lines from https://docs.microsoft.com/en-us/dotnet/maui/platform-integration/configure-multi-targeting#combine-filename-and-folder-multi-targeting to the .csproj file
4. create a class with an .iOS.cs extension outside the Platforms folder
5. create another class in Platforms/iOS
6. try to call the latter from the former

(or simply open the attached repro project and try to compile it)

Version with bug

Release Candidate 2 (current)

Last version that worked well

Unknown/Other

Affected platforms

Windows

Affected platform versions

Visual Studio Version 17.2.0 Preview 5.0

Did you find any workaround?

No

Relevant log output

The error message in the Output window is:

1>D:\Temp\MauiLib2\MauiLib2\Handlers\LabelHandler.iOS.cs(3,16,3,25): error CS0234: The type or namespace name 'Platforms' does not exist in the namespace 'MauiLib2' (are you missing an assembly reference?)

But the Error List shows it better as this includes the platform for which it tries to compile.

[Eilon]

@MichaelRumpler I think you need to wrap the call to your iOS-specific functionality in #if's like this:

#if IOS
MyPlatformSpecific.SomeIosThing.DoStuff();
#endif

So that the C# compiler will only try to compile this code on that platform. Or do multiple platforms like #if IOS || MACCATALYST.

An alternative is to make sure that you have the API available on all platforms that you support, even if on some platforms the code does nothing:

#if !IOS
void DoStuff() {} // does nothing when not on IOS
#endif

[MichaelRumpler]

Not even a #if IOS works.

The namespace Platforms does not exist, because there are no classes in it for the TargetFramework net6.0. They are all for some platform related frameworks like net6.0-ios.

However, the #if IOS should not be necessary. Both caller (LabelHandler.iOS.cs) and callee (Platforms/iOS/iOSHelper.cs) are compiled for net6.0-ios and net6.0-maccatalyst only. This is configured in the .csproj:

<ItemGroup Condition="$(TargetFramework.StartsWith('net6.0-ios')) != true AND $(TargetFramework.StartsWith('net6.0-maccatalyst')) != true">
	<Compile Remove="**\**\*.iOS.cs" />
	<None Include="**\**\*.iOS.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
	<Compile Remove="**\iOS\**\*.cs" />
	<None Include="**\iOS\**\*.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
</ItemGroup>

As I wrote above, I copied those lines from https://docs.microsoft.com/en-us/dotnet/maui/platform-integration/configure-multi-targeting#combine-filename-and-folder-multi-targeting

[MichaelRumpler]

The Maui Class Library template configured the TargetFrameworks like this:

	<TargetFrameworks>net6.0;net6.0-android;net6.0-ios;net6.0-maccatalyst</TargetFrameworks>
	<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net6.0-windows10.0.19041.0</TargetFrameworks>

If I remove net6.0-maccatalyst from that, then it compiles!

So one might think that it has something to do with using those files for two frameworks, but No. If I put net6.0-maccatalyst back in and remove net6.0-ios instead, then I get the same error as with all platforms.

(And I already wrote in my first post that the same concept works for Android. From LabelHandler.Android.cs call Platforms/Android/AndroidHelper.cs)

Could it be, that VS for Windows cannot build for MacCatalyst? I also always had to build my Xamarin.Froms library for the Mac on the Mac. When the template included it now, I was hoping that I need the Mac less.

[v-longmin]

Verified repro on Windows 10 with VS 17.3.0 Preview 1.0 [32427.505.main]. Repro project:
MauiLib2.zip

[MichaelRumpler]

I updated to VS 17.3.0 Preview 1.0 and it does behave different now.

The file in the Platforms/iOS folder now only shows the TargetPlatform net6.0-ios in the dropdown list.

The file outside of Platforms, but with an .iOS.cs extension still shows both TargetFrameworks in the dropdown list. Therefore there is one more error now.

The additional error is to be expected, if the Platforms/iOS/*.cs file is not compiled for net6.0-maccatalyst, but it should be according to the .csproj:

<ItemGroup Condition="$(TargetFramework.StartsWith('net6.0-ios')) != true AND $(TargetFramework.StartsWith('net6.0-maccatalyst')) != true">
	<Compile Remove="**\**\*.iOS.cs" />
	<None Include="**\**\*.iOS.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
	<Compile Remove="**\iOS\**\*.cs" />
	<None Include="**\iOS\**\*.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
</ItemGroup>

[MichaelRumpler]

It seems like the GA version changed something and the Platforms folder now gets a special handling. Files in Platforms/iOS are ONLY used on iOS and not on MacCatalyst no matter what you configured in your .csproj. I renamed the Platforms folder to PlatformSpecific and then those files were used on both platforms at last.

I added a PR for the docs so that others may not fall in the same trap.