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This article describes the MSB3178 diagnostic code.
Message text
MSB3178: Assembly 'value' is incorrectly specified as a file.
Description
This warning occurs when MSBuild tries to process a manifest file, but encountered a file that looks like an assembly that was specified as an ordinary file, instead of using the correct XML structure for an assembly. Assembly manifests used in ClickOnce deployment for .NET applications are XML files that contain an XML-encoded of the contents of an application, which might include assemblies and files. Assemblies are specified using the assembly element, which also includes child elements such as assemblyInformation; plain files are specified using the file element.
In Visual Studio when you generate a ClickOnce manifest using the Publish process, you might get this warning if you specify a DLL or EXE with the Build Action set to Content. In Visual Studio, Content means add something as an ordinary file, not as an assembly. In a project file, Content is an item group. If the item group contains files that look like assemblies, you get this warning.
Resolution
Check the assembly manifest XML file for the assembly given in the text of the error message. For more information, see Assembly manifest. You can generate a binary log to see why a particular file is being passed as an assembly. Use the -bl switch on the MSBuild command line, and then use the MSBuild structured log viewer to open the binary log (.binlog file).
If your project file contains an item group Content or your Visual Studio project specifies the Build Action as Content for some files with the DLL or EXE extension, consider whether this is intentional or not.
If your manifest file was generated by a custom tool or process, check any file elements to see if any of them reference files that should be referenced as assemblies instead (look for file extensions such as .dll or .exe.). If it was generated by a tool or MSBuild task, check the command line syntax, shell expansions, and validate any input files to the tool or task.
You can disable this warning by using the NoWarn property:
<NoWarn>$(NoWarn);MSB3178</NoWarn>
You should only disable the warning if you've verified that including what appear to be assemblies as ordinary files is valid and intentional in your scenario.
Applies to
All versions of MSBuild