MSBuild - Copy file to output directory if the file isn't in the output directory - msbuild

Question

I have a nuget with a .targets file that tells the consuming project to copy all files within a "Dependencies" folder to the output directory.
<ItemGroup>
<Files Include="$(MSBuildThisFileDirectory)/../contentFiles/Dependencies/*.*" />
</ItemGroup>
<Target Name="CopyDependencies" AfterTargets="Build">
<Copy SourceFiles="#(Files)"
DestinationFolder="$(TargetDir)" />
</Target>
This nuget is consumed by two projects: Project A and Project B. For this question, let's say we have a System.Runtime.InteropServices.RuntimeInformation.dll that is one of the dependencies within this nuget. The output directory of Project A does not already have System.Runtime.InteropServices.RuntimeInformation.dll, so it gets copied to the output directory when the project is built. Project B however already contains System.Runtime.InteropServices.RuntimeInformation.dll in the output directory. This causes a runtime issue at startup since the targets file is trying to overwrite the existing DLL of the same name with the System.Runtime.InteropServices.RuntimeInformation.dll file from within the nuget (which is a dependency of other files within the output directory).
So, how can I adjust my .targets file to only copy in files that do not already exist within the output directory based on name, and not size or date modified?

Answer 1

There are several ways but probably the most succinct change to your example code would be the following:
<ItemGroup>
<Files Include="$(MSBuildThisFileDirectory)/../contentFiles/Dependencies/*.*" />
</ItemGroup>
<Target Name="CopyDependencies" AfterTargets="Build">
<Copy SourceFiles="#(Files)" DestinationFolder="$(TargetDir)" Condition="!Exists('$(TargetDir)/%(Filename)%(Extension)')" />
</Target>
The change is adding a Condition on the Copy that is using the metadata of the #(Files) collection to test that the file does not exist in $(TargetDir).
Because of the use of metadata, the Copy is a task batch. Essentially the #(Files) collection is divided into batches by %(Filename)%(Extension) and there is a separate Copy task invoked for each batch.
If there is a large number of files in the Dependencies folder, the following variant may provide better performance.
<ItemGroup>
<Files Include="$(MSBuildThisFileDirectory)/../contentFiles/Dependencies/*.*" />
</ItemGroup>
<Target Name="CopyDependencies" AfterTargets="Build">
<ItemGroup>
<FilesToCopy Include="#(Files)" Condition="!Exists('$(TargetDir)/%(Filename)%(Extension)')" />
</ItemGroup>
<Copy SourceFiles="#(FilesToCopy)" DestinationFolder="$(TargetDir)" />
</Target>
The task batching is moved to the definition of a new ItemGroup collection and the Copy task is invoked once for the set of files. The potential performance improvement is that the implementation of the Copy task tries to parallelize copies, which it can't do when invoked per file.