After upgrading my project to 4.17 it’s started occasionally failing to compile because UnrealBuildTool encounters a System.ArgumentNullException when it tries to run. The full callstack is:
2>EXEC : error : System.ArgumentNullException: Value cannot be null.
2> Parameter name: source
2> at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
2> at UnrealBuildTool.ExternalExecution.ExecuteHeaderToolIfNecessary(BuildConfiguration BuildConfiguration, UEBuildTarget Target, CppCompileEnvironment GlobalCompileEnvironment, List`1 UObjectModules, FileReference ModuleInfoFileName, ECompilationResult& UHTResult)
2> at UnrealBuildTool.UnrealBuildTool.RunUBT(BuildConfiguration BuildConfiguration, String[] Arguments, FileReference ProjectFile)
I don’t recall encountering this error while testing the preview but that may have simply been luck since I didn’t do more than occasionally make sure all my code still compiled and ran. Doing a full rebuild of my project fixes the error but I’d like to avoid having to do that if possible since it usually takes a few minutes.
Further testing finds that this only occurs when I change a header file. I made a new (blank) code project and encounter the exact same issue and tried it on a different machine so it doesn’t seem to be an issue with either my project or computer.
I have the same issue after updating to 4.17.
I can confirm that both workarounds work, but it is quite cumbersome and time consuming to rebuild every time.
Adding my own voice to this. Added two transient properties to a class header, and was hit by this exact build error. Clean and rebuild got me past it.