how to generate css file from scss file dotnet core

I solved this issue from another angle as I was unable to get the above running with a dotnetcore 2.0 application.

Initially I tried the recommended approach from Microsoft: https://learn.microsoft.com/en-us/aspnet/core/client-side/using-gulp?view=aspnetcore-2.1 and convert the project.json file with the changes suggested by @Scott Addie into an XML syntax supported in the csproj file. Looks something like this:

<Target Name="MyPreCompileTarget" BeforeTargets="Build">
    <Exec Command="gulp frontend" ContinueOnError="false" />
</Target>

But, depending on the logic inside your gulpfile.js, this sometimes refused to run. In my case it seemed to fail on filewatchers like this one:

gulp.watch('./frontend/source/javascript/**/*.js', ['compileJs']);

So I ended up apporaching it the other way around. Start the sass compilation tool from my gulpfile.js and then from within the gulpfile.js start the dotnet process. Something like this:

/**
 * NodeJS Gulp tasks for "compiling" frontend files and start potentially start the dotnetcore application
 */

const gulp = require('gulp');
const plumber = require('gulp-plumber');
const sass = require('gulp-sass');
const runSequence = require('run-sequence').use(gulp);
const minifyCss = require('gulp-minify-css');
const notifier = require('node-notifier');
const browserify = require('browserify');
const source = require('vinyl-source-stream');
const buffer = require('vinyl-buffer');
const sourcemaps = require('gulp-sourcemaps');
const path = require('path');
const uglify = require('gulp-uglify');
const rename = require('gulp-rename');
const livereload = require('gulp-livereload');
var spawn = require('child_process').spawn;
var fancyLog = require('fancy-log');
var beeper = require('beeper');

// Port that livereload is listening to
const liveReloadPort = 5812;

/**
 * Initialize the basic tasks for compiling front-end assets
 */
gulp.task('frontend', function () {
    return runSequence(
        'compileJs',
        'compileSass',
        'watch',
        'livereload')
})

/**
 * Initialize tasks for compiling front-end assets and runs dotnet in watch mode as well
 */
gulp.task('dotnet', function () {
    return runSequence(
        'compileJs',
        'compileSass',
        'watch',
        'livereload',
        function () {

            //
            // Start the dotnet core process
            //
            const child = spawn("dotnet", ["watch", "run"], {
                    cwd: process.cwd()
            })
            let stdout = '';
            let stderr = '';

            child.stdout.setEncoding('utf8');

            child.stdout.on('data', function (data) {
                stdout += data;
                console.log(data);
            });

            child.stderr.setEncoding('utf8');
            child.stderr.on('data', function (data) {
                stderr += data;
                fancyLog.error(data);
                beeper();
            });

            child.on('close', function (code) {
                fancyLog("Done with exit code", code);
                fancyLog("You access complete stdout and stderr from here"); // stdout, stderr
            });

        });
})

/**
 * Starts the livereload tool
 */
gulp.task('livereload', function () {
    // Start livereload
    console.log('* Starting livereload on port: ' + liveReloadPort);
    livereload.listen(5812);
})

/**
 * Watches the filesystem and initializes the tasks when a file has been changed
 */
gulp.task('watch', function () {

    // 1) Compile the client-side JavaScript file when one of the source files changes
    gulp.watch('./frontend/source/javascript/**/*.js', ['compileJs']);

    // 2) Compile the client-side CSS file when one of the source SASS files changes
    gulp.watch('./frontend/source/stylesheets/*.scss', ['compileSass']);
});

/**
 * Compiles SASS files and stores the result into the public folder
 */
gulp.task('compileSass', function () {

    return gulp.src('./frontend/source/stylesheets/main.scss')
        .pipe(plumber())
        .pipe(sass().on('error', function (err) {
            console.log('SASS compile error: ', err.toString());

            notifier.notify({
                'title': 'SASS Compile Error',
                'message': err.message
            });

            this.emit("end");
        }))
        .pipe(gulp.dest('./frontend/public/stylesheets'))

        .pipe(minifyCss({
            compatibility: 'ie8'
        }))
        .pipe(rename({
            suffix: '.min'
        }))
        .pipe(gulp.dest('./frontend/public/stylesheets'))
        .pipe(livereload());
});

/**
 * Compiles the Javascript files and stores the result in the public folder
 */
gulp.task('compileJs', function () {

    return browserify(path.resolve('./frontend/source/javascript', 'app.js'))
        .bundle()
        .on('error', function (err) {
            console.log('JS/Browserify error:', err.toString());

            notifier.notify({
                'title': 'JS Compile Error',
                'message': err.message
            });

            this.emit("end");
        })
        .pipe(source('app.js'))
        .pipe(buffer())
        .pipe(gulp.dest('./frontend/public/javascript'))

        // Create minified version of JS
        .pipe(buffer())
        .pipe(uglify())
        .pipe(sourcemaps.init({
            loadMaps: true
        }))
        .pipe(sourcemaps.write('./maps'))
        .pipe(rename({
            suffix: '.min'
        }))
        .pipe(gulp.dest('./frontend/public/javascript'))

        .pipe(livereload());
});

Now I can start my dotnetcore application with my frontend compilation tools using gulp dotnet

To be complete, this is my package.json file:

{
  "name": "",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "repository": {
    "type": "git",
    "url": "..."
  },
  "author": "...",
  "license": "...",
  "homepage": "...",
  "devDependencies": {
    "babel-core": "^6.26.3",
    "babel-loader": "^7.1.4",
    "babel-preset-es2015": "^6.24.1",
    "babelify": "^8.0.0",
    "beeper": "^1.1.1",
    "browserify": "^16.2.2",
    "fancy-log": "^1.3.2",
    "gulp": "^3.9.1",
    "gulp-livereload": "^3.8.1",
    "gulp-minify-css": "^1.2.4",
    "gulp-plumber": "^1.2.0",
    "gulp-rename": "^1.2.3",
    "gulp-sass": "^4.0.1",
    "gulp-sourcemaps": "^2.6.4",
    "gulp-uglify": "^3.0.0",
    "node-notifier": "^5.2.1",
    "path": "^0.12.7",
    "run-sequence": "^2.2.1",
    "vinyl-buffer": "^1.0.1",
    "vinyl-source-stream": "^2.0.0",
    "webpack": "^4.10.2"
  },
  "browserify": {
    "transform": [
      [
        "babelify",
        {
          "presets": [
            "es2015"
          ]
        }
      ]
    ]
  }
}