/usr/lib/mono/xbuild/Microsoft/Portable/v4.0/Microsoft.Portable.CSharp.targets is in mono-xbuild 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<Import Project="..\Microsoft.Portable.Core.props" />
<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
<Import Project="..\Microsoft.Portable.Core.targets" />
</Project>
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