Could not load file or assembly 'Microsoft.Build, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

[Filip Navara]

watch : System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.Build, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.

File name: 'Microsoft.Build, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at Microsoft.DotNet.Watcher.Program.MainInternalAsync(IReporter reporter, CommandLineOptions options, CancellationToken cancellationToken)
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.Start[TStateMachine](TStateMachine& stateMachine)
   at Microsoft.DotNet.Watcher.Program.MainInternalAsync(IReporter reporter, CommandLineOptions options, CancellationToken cancellationToken)
   at Microsoft.DotNet.Watcher.Program.HandleWatch(CommandLineOptions options) in /Users/filipnavara/Projects/dotnet-hotrewatch/dotnet-watch/Program.cs:line 202
watch : An unexpected error occurred

I am aware of this happening for the packages distributed on NuGet. It happens for Release builds of the tool but doesn't seem to happen for Debug builds. I didn't identify the root cause yet.

[Filip Navara]

Workaround is now in place. In case you grabbed a version before the workaround was implemented, you can run:

dotnet tool uninstall -g dotnet-hotrewatch
dotnet tool install -g dotnet-hotrewatch --version "*-*"

On macOS you would need to re-sign the updated version:

codesign -s "-" ~/.dotnet/tools/dotnet-hotrewatch

[Filip Navara]

The root cause of the problem:

dotnet-hotrewatch/dotnet-watch/Program.cs

Lines 70 to 76 in 558678b

 #if DEBUG 
             // In the usual case, use the SDK that contains the dotnet-watch. However during local testing, it's 
             // much more common to run dotnet-watch from a different SDK. Use the ambient SDK in that case. 
             MSBuildLocator.RegisterDefaults(); 
 #else 
             MSBuildLocator.RegisterMSBuildPath(sdkRootDirectory); 
 #endif 

Patches are welcome.

[Will Dean]

Is what's needed here a "MSBuild finder"? ISTR I have written such a thing in the past, so I can dig out the code for that.

[Filip Navara]

The existing locator code works and it's quite stable. It was hard-coded for release build to assume an installation location within the SDK bundle. The assumption is not true anymore when deployed as global tool.

Simplest solution would be to remove the non-DEBUG lines linked above.

What is not immediately clear is how the tool behaves with global.json file present that forces a non-default SDK version. It's probably worth investigating.