Please reference following code, it works for me.
'use strict';
var gulp = require('gulp'),
addContentFileToVSProject = require('add-content-file-to-project');
gulp.task('addNewFiles', () => {
return new Promise((resolve, reject) => {
// step 1: get new files
var allNewFiles = geNewFiles(sourcePath, destnationPath);
// step 2: add new files to .csproj
for (var f in allNewFiles) {
var newfile = destnationPath + '/' + allNewFiles[f];
// to .csproj
addContentFileToVSProject.execute(newfile);
}
resolve();
});
});
By the way, I'm using Gulp 4.0 and Visual Studio 2017 latest version. The geNewFiles(s, t) is a function which return all new files by comparing files between source and destination folder. I put all new generated files to destination folder.