Did you know? – NuGetGallery Build (part 5)

In the previous post we left off trying to figure out how NuGetGallery downloads its required packages during the build process. We arrived at the file that contains the targets to do this NuGet.targets (simplified):

<Project ToolsVersion="4.0">
    <Import Project="NuGet.settings.targets"/>
    <Target Name="RestorePackages" DependsOnTargets="CheckPrerequisites">
        <Exec Command="$(RestoreCommand)"
              LogStandardErrorAsError="true"
              Condition="Exists('$(PackagesConfig)')" />
    </Target>
</Project>

Excellent, there’s the definition of a “RestorePackages” target that runs an Exec task which presumably will take care of downloading the packages. The “RestoreCommand” is defined in yet another targets file which is imported at the beginning, NuGet.settings.targets (simplified):

<Project ToolsVersion="4.0">
    <PropertyGroup>
        <NuGetToolsPath>$(SolutionDir).nuget</NuGetToolsPath>
        <NuGetExePath>$(NuGetToolsPath)\nuget.exe</NuGetExePath>
        <PackagesConfig>$(ProjectDir)packages.config</PackagesConfig>
        <PackagesDir>$(SolutionDir)packages</PackagesDir>

        <!-- Commands -->
        <RestoreCommand>"$(NuGetExePath)" install "$(PackagesConfig)" -o "$(PackagesDir)"</RestoreCommand>

        <!-- Make the build depend on restore packages -->
        <BuildDependsOn Condition="$(RestorePackages) == 'true'">
            RestorePackages;
            $(BuildDependsOn);
        </BuildDependsOn>
    </PropertyGroup>   
 </Project>

A lot of things happening here:
- There is a property definition to the location of NuGet.exe (which is included in the source)
- There is a property definition to a packages.config, which contains the list of packages that it needs to download.
- There is a property definition to a directory where the packages will be downloaded into.
- Then there is a property definition for the actual restore command, which is a call to Nuget.exe with the “install” parameter.

The next line is probably the most important one, it’s where the target defined previously is glued into the build process. “BuildDependsOn” is a special MSBuild property used to override the sequence of steps during build. In this case it adds the “RestorePackages” target at the beginning.

Don’t get tripped up by the condition included in the “BuildDependsOn” which uses a property with the same name as the target “RestorePackages” (this confused me for a while). Basically we are only going to download the references if the “RestorePackages” property is set to true, which if you remember it was set at the beginning of Website.csproj.