Fixes problems in WindowsDesktop SDK due to _TargetFrameworkVersionWithoutV being undefined sometimes

[vatsan-madhavan]

[release/3.0 version of #1707]

Fixes #1651 - Invalid comparison in MSBuild's ignore-conditions mode in outer build of multitargeted project

This change ensures that the WindowsDesktop SDK never deals with undefined numeric values related to TFM.

This solution leverages the fact that Items are evaluated after Properties by MSBuild. See Comparing properties and items

_TargetFrameworkVersionValue - a new Property which is defined for exclusive use within the WindowsDesktop SDK, and which is intended to act as a proxy for _TargetFrameworkVersionWithoutV - is defined in Microsoft.NET.Sdk.WindowsDesktop.targets after TargetFrameworkVersionWithoutV is guaranteed to be defined by Microsoft.NET.Sdk.targets.

Even here, we ensure that a fallback default (0.0) is provided - guaranteeing that _TargetFrameworkVersionValue will always be numeric.

When the Conditions in the various Items in Microsoft.NET.WindowsDesktop.props are evaluated (which will happen strictly after all Properties are evaluated - whether they appear in .props or .targets), _TargetFrameworkVersionValue would have been (a) well-defined and (b) guaranteed to be a numeric value. This would in turn guarantee that errors of the kind "A numeric comparison was attempted on "$(_TargetFrameworkVersionValue)" that evaluates to "" instead of a number," will no longer appear in outer-builds or in any other context.

PS: This is a fix for a regression introduced by #1027 (which shipped in Preview 6) as part of a set of fixes for improving multi-targeting experience, improved warnings and error messages during build, and improvements to project-system integration.

[vatsan-madhavan]

Marking as *NO MERGE* until we receive permission to merge this change into release/3.0 branch.

[ghost]

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[mrward]

@vatsan-madhavan - Do we know which .NET Core 3.0 sdk version will contain this change?

[vatsan-madhavan]

This was merged 4 days ago - I would think that it would be already in the latest builds in https://github.com/dotnet/core-sdk. Have you had a chance to try out the latest build from there yet?

[mrward]

I have tried 3.0.100-preview9-014004 which looks to be more recent than what is on core-sdk and that does not seem to include the fix yet. Will try one of the core-sdk builds.

[mrward]

dotnet/core-sdk seem to have more recent versions of the .NET Core sdk: 3.0.100-rc1-014162

Using https://dotnetcli.blob.core.windows.net/dotnet/Sdk/release/3.0.1xx/dotnet-sdk-latest-osx-x64.pkg the fix does not yet seem to be included.

/usr/local/share/dotnet/sdk/3.0.100-rc1-014162/Sdks/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.props(112,53) : error MSB4086: A numeric comparison was attempted on "$(_TargetFrameworkVersionWithoutV)" that evaluates to "" instead of a number, in condition "'$(_TargetFrameworkVersionWithoutV)' >= '4.0'".

Presumably it will land in a later release.

[mrward]

Tried out 3.0.100-rc2-014198 from dotnet/core-sdk and that includes these changes. Thanks.