[dotnet] Use the default .NET scheme for TargetPlatformVersion for library projects.

Make.versions

@@ -71,6 +71,12 @@ WATCHOS_NUGET_OS_VERSION=11.0
 MACOS_NUGET_OS_VERSION=15.0
 MACCATALYST_NUGET_OS_VERSION=18.0
 
+# The following are the OS versions we first supported with the current .NET version.
+# These versions must *not* change with minor .NET updates, only major .NET releases.
+IOS_TARGET_PLATFORM_VERSION_LIBRARY=18.0
+TVOS_TARGET_PLATFORM_VERSION_LIBRARY=18.0
+MACOS_TARGET_PLATFORM_VERSION_LIBRARY=15.0
+MACCATALYST_TARGET_PLATFORM_VERSION_LIBRARY=18.0
 
 # In theory we should define the default platform version if it's not specified in the TFM. The default should not change for a given .NET version:
 # * We release support for iOS 14.5 with .NET 6
@@ -100,6 +106,14 @@ MACCATALYST_NUGET_OS_VERSION=18.0
 # So we've made the decision that the default target platform version is
 # always the latest target platform version.
 
+# However, this turns out to be somewhat of a complication for library developers,
+# because they typically don't need Xcode to build their projects, and if we auto-
+# update their TargetPlatformVersion to the latest, then all their customers
+# have to also update their workloads, which for some people end up being a rather
+# nasty surprise (because with the above algorithm it happens without developer
+# action). Thus we follow .NET's default platform version scheme for library projects:
+# it won't change in minor .NET releases.
+
 
 #
 # Here we list all the releases we support for each platform.

dotnet/Makefile

@@ -116,7 +116,8 @@ targets/Microsoft.$(1).Sdk.Versions.props: targets/Microsoft.Sdk.Versions.templa
 		-e "s/@PLATFORM@/$(1)/g" \
 		-e "s/@NUGET_VERSION_NO_METADATA@/$$($(2)_NUGET_VERSION_NO_METADATA)/g" \
 		-e "s/@NUGET_VERSION_FULL@/$$($(2)_NUGET_VERSION_FULL)/g" \
-		-e "s/@TARGET_PLATFORM_VERSION@/$$($(2)_NUGET_OS_VERSION)/g" \
+		-e "s/@TARGET_PLATFORM_VERSION_EXE@/$$($(2)_NUGET_OS_VERSION)/g" \
+		-e "s/@TARGET_PLATFORM_VERSION_LIBRARY@/$$($(2)_TARGET_PLATFORM_VERSION_LIBRARY)/g" \
 		-e "s/@CURRENT_BRANCH@/$$(CURRENT_BRANCH_SED_ESCAPED)/g" \
 		-e "s/@CURRENT_HASH_LONG@/$$(CURRENT_HASH_LONG)/g" \
 		-e 's*@VALID_RUNTIME_IDENTIFIERS@*$(foreach rid,$(3),\n\t\t<_XamarinValidRuntimeIdentifier Include="$(rid)" Platform="$(1)" />)*' \

dotnet/targets/Microsoft.Sdk.Versions.template.props

@@ -6,7 +6,8 @@
 
 		<_ShortPackageVersion>@NUGET_VERSION_NO_METADATA@</_ShortPackageVersion>
 		<_PackageVersion>@NUGET_VERSION_FULL@</_PackageVersion>
-		<_XamarinTargetPlatformVersion>@TARGET_PLATFORM_VERSION@</_XamarinTargetPlatformVersion>
+		<_XamarinTargetPlatformVersionExecutable>@TARGET_PLATFORM_VERSION_EXE@</_XamarinTargetPlatformVersionExecutable>
+		<_XamarinTargetPlatformVersionLibrary>@TARGET_PLATFORM_VERSION_LIBRARY@</_XamarinTargetPlatformVersionLibrary>
 		<_XamarinIsPreviewRelease>@XCODE_IS_PREVIEW@</_XamarinIsPreviewRelease>
 		<_XamarinDotNetVersion>@DOTNET_TFM@</_XamarinDotNetVersion>
 		<_XamarinPackSuffix>@DOTNET_TFM@_@NUGET_OS_VERSION@</_XamarinPackSuffix>

dotnet/targets/Xamarin.Shared.Sdk.TargetFrameworkInference.props

@@ -12,6 +12,7 @@
 
   <!-- Set the default TargetPlatformVersion if it wasn't specified in the TargetFramework variable. This must be set after importing Sdk.targets (in Xamarin.Shared.Sdk.targets) -->
   <PropertyGroup>
-    <TargetPlatformVersion Condition="'$(TargetPlatformVersion)' == ''">$(_XamarinTargetPlatformVersion)</TargetPlatformVersion>
+    <TargetPlatformVersion Condition="'$(TargetPlatformVersion)' == '' And '$(OutputType)' == 'Exe'">$(_XamarinTargetPlatformVersionExecutable)</TargetPlatformVersion>
+    <TargetPlatformVersion Condition="'$(TargetPlatformVersion)' == '' And '$(OutputType)' != 'Exe'">$(_XamarinTargetPlatformVersionLibrary)</TargetPlatformVersion>
   </PropertyGroup>
 </Project>