How to write to a file in .NET Core?

I want to use the Bluetooth LE functions in .NET Core (specifically, BluetoothLEAdvertisementWatcher) to write a scanner which logs information to a file. This is to run as a desktop application and preferably as a command line app.

Constructors like System.IO.StreamWriter(string) are not available, apparently. How do I create a file and write to it?

I would be just as happy to be able to do a System.Console.WriteLine(string) but that doesn't seem to be available under .NET Core either.

Update: To clarify, if I could have a program that looks like this run without error, I'll be off to the races.

using System;
using Windows.Devices.Bluetooth.Advertisement;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main(string[] args)
        {
            BluetoothLEAdvertisementWatcher watcher = new BluetoothLEAdvertisementWatcher();
            Console.WriteLine("Hello, world!");
        }
    }
}

Update 2: Here's the project.json file:

{
  "dependencies": {
    "Microsoft.NETCore.UniversalWindowsPlatform": "5.0.0"
  },
  "frameworks": {
    "uap10.0": {}
  },
  "runtimes": {
    "win10-arm": {},
    "win10-arm-aot": {},
    "win10-x86": {},
    "win10-x86-aot": {},
    "win10-x64": {},
    "win10-x64-aot": {}
  }
}

The output of the command dotnet -v run contains this error message:

W:\src\dotnet_helloworld>dotnet -v run
...
W:\src\dotnet_helloworld\Program.cs(2,15): error CS0234: The type or namespace name 'Devices' does not exist in the namespace 'Windows' (are you missing an assembly reference?)
...

Solution 1:

This code is the skeleton I was looking for when I posed the question. It uses only facilities available in .NET Core.

var watcher = new BluetoothLEAdvertisementWatcher();

var logPath = System.IO.Path.GetTempFileName();
var logFile = System.IO.File.Create(logPath);
var logWriter = new System.IO.StreamWriter(logFile);
logWriter.WriteLine("Log message");
logWriter.Dispose();

Solution 2:

This is the solution I'm using. It uses fewer lines of code and does the job just as good. It's also very compatible with .NET core 2.0

using (StreamWriter writer = System.IO.File.AppendText("logfile.txt"))
{
    writer.WriteLine("log message");
}

Solution 3:

Even better:

using System.IO;

var logPath = Path.GetTempFileName();
using (var writer = File.CreateText(logPath))
{
    writer.WriteLine("log message"); //or .Write(), if you wish
}

Solution 4:

As of today, with RTM, there seems to be this shorter way as well:

var watcher = new BluetoothLEAdvertisementWatcher();

var logPath = System.IO.Path.GetTempFileName();
var logWriter = System.IO.File.CreateText(logPath);
logWriter.WriteLine("Log message");
logWriter.Dispose();