This repository demonstrates an issue with the fast up-to-date checks in Visual Studio 2017 15.9.10 for projects which have CopyToOutputDirectory items but no project references.

⚠ Note that this demonstration pertains only to modern, SDK-style C# projects. A similar issue happens with legacy projects, but the root cause seems to be different (the presence of project references doesn't matter) and the workarounds do not apply.

The solution is set up as follows:

• There are four projects:
  • TestProgram - a .NET Core 2.2 app
  • LibraryA - a .NET Standard 2.0 library
  • LibraryB - a .NET Standard 2.0 library
  • Dummy - a .NET Standard 2.0 library
• TestProgram references LibraryA
• LibraryA references LibraryB
• Dummy is only present to demonstrate a workaround for the issue.
• Both LibraryA and LibraryB include a text file which is copied to the output directory with <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>, the contents of these files are printed when TestProgram is run.

Reproducing the problem

(Note: The following assumes you have Visual Studio configured to build out of date projects when you run.)

1. Open the solution in Visual Studio.
2. Select TestProgram as the startup project.
3. Press F5 to start the program. The output is as follows:

   Hello World!
   Hello from ClassA!
   Text file A reporting for duty!
   Hello from ClassB!
   Text file B reporting for duty!
   Done.
   

4. Close the program and add a message TextFileA.txt and press F5 to start the program again. The output includes your message:

   Hello World!
   Hello from ClassA!
   Text file A reporting for duty! Hello, world!
   Hello from ClassB!
   Text file B reporting for duty!
   Done.
   

5. Close the program and add a message to TextFileB.txt instead, press F5 to start the program again. The output will not include your message:

   Hello World!
   Hello from ClassA!
   Text file A reporting for duty! Hello, world!
   Hello from ClassB!
   Text file B reporting for duty!
   Done.
   

6. Observe that the file is updated in LibraryB's output (LibraryB\bin\Debug\netstandard2.0\TextFileB.txt), but not in TestProgram's output (TestProgram\bin\Debug\netcoreapp2.2\TextFileB.txt).

The source of the problem

The problem is caused by the fast up-to-date checks. The fast up-to-date checks do not consider whether or not CopyToOutputDirectory items from referenced projects are up-to-date or not. Instead it is relying on the reference copy marker. This can be observed if you enable verbose logging for up-to-date checks.

In the case of editing TextFileA.txt, TestProgram is updated because the LibraryA.csproj.CopyComplete file is newer than TestProgram's output marker:

4>FastUpToDate: Adding input reference copy markers: (TestProgram)
4>FastUpToDate:     'C:\Development\Playground\UsingTransitiveCopiedContent\LibraryA\obj\Debug\netstandard2.0\LibraryA.csproj.CopyComplete' (TestProgram)
4>FastUpToDate:     'C:\Development\Playground\UsingTransitiveCopiedContent\LibraryB\obj\Debug\netstandard2.0\LibraryB.csproj.CopyComplete' (TestProgram)
4>FastUpToDate: Adding output reference copy marker: (TestProgram)
4>FastUpToDate:     'C:\Development\Playground\UsingTransitiveCopiedContent\TestProgram\obj\Debug\netcoreapp2.2\TestProgram.csproj.CopyComplete' (TestProgram)
4>FastUpToDate: Latest write timestamp on input marker is 3/27/2019 8:44:47 PM on 'C:\Development\Playground\UsingTransitiveCopiedContent\LibraryA\obj\Debug\netstandard2.0\LibraryA.csproj.CopyComplete'. (TestProgram)
4>FastUpToDate: Write timestamp on output marker is 3/27/2019 8:39:52 PM on 'C:\Development\Playground\UsingTransitiveCopiedContent\TestProgram\obj\Debug\netcoreapp2.2\TestProgram.csproj.CopyComplete'. (TestProgram)
4>FastUpToDate: Input marker is newer than output marker, not up to date. (TestProgram)
4>FastUpToDate: Project is not up to date. (TestProgram)

(The above output is printed by Visual Studio when the up-to-date checks logging is set to verbose in Tools > Options > Project and Solutions > .NET Core.)

When you edit only TextFileB.txt, TestProgram is not updated because the LibraryB.csproj.CopyComplete marker doesn't exist:

4>FastUpToDate: Adding input reference copy markers: (TestProgram)
4>FastUpToDate:     'C:\Development\Playground\UsingTransitiveCopiedContent\LibraryA\obj\Debug\netstandard2.0\LibraryA.csproj.CopyComplete' (TestProgram)
4>FastUpToDate:     'C:\Development\Playground\UsingTransitiveCopiedContent\LibraryB\obj\Debug\netstandard2.0\LibraryB.csproj.CopyComplete' (TestProgram)
4>FastUpToDate: Adding output reference copy marker: (TestProgram)
4>FastUpToDate:     'C:\Development\Playground\UsingTransitiveCopiedContent\TestProgram\obj\Debug\netcoreapp2.2\TestProgram.csproj.CopyComplete' (TestProgram)
4>FastUpToDate: Latest write timestamp on input marker is 3/27/2019 8:45:36 PM on 'C:\Development\Playground\UsingTransitiveCopiedContent\LibraryA\obj\Debug\netstandard2.0\LibraryA.csproj.CopyComplete'. (TestProgram)
4>FastUpToDate: Write timestamp on output marker is 3/27/2019 8:45:38 PM on 'C:\Development\Playground\UsingTransitiveCopiedContent\TestProgram\obj\Debug\netcoreapp2.2\TestProgram.csproj.CopyComplete'. (TestProgram)
4>FastUpToDate: Project is up to date. (TestProgram)

(If a file is missing, BuildUpToDateCheck will not consider it. You can see the file is completely missing rather than stale by checking in the intermediate directory for LibraryB.)

The reason the CopyComplete marker doesn't exist is because MSBuild doesn't create it if there's no references copied to the output.

Workarounds

I've found two workarounds to this issue.

Add a dummy reference to LibraryB

One workaround is to add a dummy reference to LibraryB, which is what the Dummy project is for. There's already a commented-out ProjectReference in LibraryB.csproj for this purpose.

The downside to this approach is that there will be a useless Dummy.dll assembly floating around.

Create the copy marker yourself

The other workaround is to create the CopyComplete file yourself when MSBuild doesn't.

I've added a small target to LibraryB that does this:

<Target Name="MissingCopyMarkerWorkaround" AfterTargets="CopyFilesToOutputDirectory" Condition="'@(ReferenceCopyLocalPaths)' == '' or '@(ReferencesCopiedInThisBuild)' == ''">
  <Touch Files="@(CopyUpToDateMarker)" AlwaysCreate="true">
    <Output TaskParameter="TouchedFiles" ItemName="FileWrites" />
  </Touch>
</Target>

Basically this just touches the CopyComplete file in the opposite condition that MSBuild is doing so already.

This avoids the Dummy.dll issue, but it is unclear if this has unintended consequences since skipping the marker is presumably done for a reason.

Misc notes

Note: This repository demonstrates the issue with a reference of a reference, but that is done only because:

• I thought the initial issue only happened with references of rererences.
• I wanted to demonstrate when the issue doesn't happen.

If you remove the reference to LibraryB from LibraryA, the issue will start happening to TextFileA.txt too.