[BuildCheck Suggestion]: Handle empty value of TargetFramework

[stan-sz]

Summary

For

<TargetFramework />
<TargetFrameworks>net472;netstandard2.0</TargetFrameworks>

or

<TargetFramework></TargetFramework>
<TargetFrameworks>net472;netstandard2.0</TargetFrameworks>

We get

error BC0107: Project foo.csproj specifies 'TargetFrameworks' property 'net472;netstandard2.0' and 'TargetFramework' property '' at the same time. This will lead to 'TargetFrameworks' being ignored and build will behave as single-targeted.

Consider handling the case of $(TargetFramework) having empty value to be valid in this case.

Background and Motivation

A repo can have a default TF with several projects multi-targeting.

Sample issue or antipattern that the check should be flagging

<TargetFramework />
<TargetFrameworks>net472;netstandard2.0</TargetFrameworks>

or

<TargetFramework></TargetFramework>
<TargetFrameworks>net472;netstandard2.0</TargetFrameworks>

Should be valid usages

Sample output

No response

[YuliiaKovalova]

Hi @stan-sz,

Thank you for the suggestion, it sounds reasonable!
If you are willing, you can try to contribute & pilot our BuildCheck system.
Feel free to use this build-in check as an example: https://github.com/dotnet/msbuild/blob/main/src/Build/BuildCheck/Checks/SharedOutputPathCheck.cs

[KalleOlaviNiemitalo]

I had doubt about this case, where an empty <TargetFramework/> is not the same as omitting it altogether:

<Project Sdk="Microsoft.NET.SDK" TreatAsLocalProperty="TargetFramework">
  <PropertyGroup>
    <TargetFramework/>
    <TargetFrameworks>net8.0;netstandard2.0</TargetFrameworks>
  </PropertyGroup>
</Project>

but well, it causes an error

Microsoft.NET.Sdk.CrossTargeting.targets(66,5): error MSB4006: There is a circular dependency in the target dependency graph involving target "GetAllRuntimeIdentifiers".

so there's no need for the BuildCheck to handle this specially.