Why does Nuget package pick the wrong platform?

I have a nuget package http://www.nuget.org/packages/Tavis.UriTemplates/ that has the following nuspec file,

<?xml version="1.0"?>
<package xmlns="http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd">
  <metadata>
    <id>Tavis.UriTemplates</id>
    <version>0.4</version>
    <authors>Darrel Miller</authors>
    <requireLicenseAcceptance>false</requireLicenseAcceptance>
        <title>URI Template resolution library</title>
    <description>Implementation of RFC 6570</description>
    <tags>http</tags>
        <releaseNotes>Added PCL version</releaseNotes>
        <projectUrl>https://github.com/tavis-software/UriTemplates</projectUrl>
        </metadata>
    <files>
        <file src="Packages\temp\UriTemplates\lib\Net35\*.*" target="lib\Net35" />
        <file src="Packages\temp\UriTemplates\lib\portable\*.*" target="lib\Portable-Net40+WinRT45+WP71+sl4" />
    </files>
</package>

If I install this package in a project that is .net40 or .net45 it selects the .net 35 DLL. Anyone have any idea why it doesn't select the PCL library?

Solution

NuGet treats PCL assemblies as less compatible than assemblies that target a specific version of the .NET framework.

For your NuGet package you can either add a .NET 4.0 and a .NET 4.5 assembly or remove the .NET 3.5 assembly.

Looking at the NuGet source code, in the VersionUtility class, a PCL assembly's weighting will be halved compared to an assembly that references the full version of .NET.

// we divide by 2 to ensure Portable framework has less compatibility value than specific framework.
return GetCompatibilityBetweenPortableLibraryAndNonPortableLibrary(projectFrameworkName, packageTargetFrameworkName) / 2;

With your NuGet package, even though the PCL assembly's compatiblity is higher based on the value returned from the GetCompatibilityBetweenPortableLibraryAndNonPortableLibrary method, the division by two makes the .NET 3.5 assembly more compatible.