In my current project, I need to add a third party library with source files. Before this, I only have expericence that adding prebuilt library such as .lib files. This time, I create a new folder and module files, i.e. IXXLib.h， XXLib.cpp and XXLib.build.cs, under the path ‘UnrealEngine\Engine\Plugins\MyPlugin\Source\ThirdParty\XXLib’ because I don’t want to expose it to other modules explicitly. But After I regenerating the project file, I didn’t find any files under the folder except those three module files.

In default, all source files will be discovered recursively. There should be no reason to be empty. So I debuged into the UnrealBuildTool([see details here]) and found a wordless things that UBT will check if the module path contains ‘ThirdParty’ in order to determine to add source files or not. The related codes are in ProjectFileGenerator.cs, Line 1961:

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// We'll keep track of whether this is an "engine" or "external" module.  This is determined below while loading module rules.
bool IsThirdPartyModule = CurModuleFile.ContainsNam("ThirdParty", UnrealBuildTool.RootDirectory);

and Line 1995:

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bool SearchSubdirectories = !IsThirdPartyModule || bGatherThirdPartySource;

if( bGatherThirdPartySource )
{
    Log.TraceInformation( "Searching for third-party source files..." );
}

// Find all of the source files (and other files) and add them to the project
List<FileReference> FoundFiles = SourceFileSearch.FindModuleSourceFiles( CurModuleFile, SearchSubdirectories:SearchSubdirectories );

Hence, we have two ways to solve this problem. The intuitive way is modifying the path of the third party library such as replacing ‘ThirdParty’ to ‘External’ and promising that there are no more ‘ThirdParty’ words in the path.
The other ways is adding an argument -THIRDPARTY when executing GenerateProjectFiles.bat. You can find the source at Line 1127:

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case "-THIRDPARTY":
    bGatherThirdPartySource = true;
    break;

The disadvantage of the later method is that it will import all source files under Thirdparty folders and make the project file very big.

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