How to write to a file in .NET Core?

Question

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?)
...

Hypothec

Is it the same as you need to do for WinRT? blog.jerrynixon.com/2012/06/… – Equidistant

Can you share the project.json file? You might be missing some package references. – Cucullate

@VictorHurdugaci I've edited the question to include one of the project.json files I've tried, along with the error message that results. No doubt the references are wrong, I'm kind of flying blind here. This particular project.json was taken from a Visual Studio 2015 project template for a .NET Core project. – Hypothec

Answer 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();

Hypothec

Works great. I'm guessing in about six months this will have several hundred upvotes. – Contain

Answer 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");
}

Critchfield

if you want to replace the text, use System.IO.File.CreateText instead of System.IO.File.AppendText – Valenta

Answer 3

Even better:

using System.IO;

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

If writing fewer lines of code is your thing, the above can be re-written as

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

Margotmargrave

Answer 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();

Hootchykootchy

Answer 5

To write to files in .NET Core, you can use two methods:

1. AppendText()
2. CreateText()

These two methods are static members of the File class in the System.IO namespace. The difference between them is one appends to an existing file while the other overwrites the file.

Usage examples:

AppendText

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

CreateText

using (StreamWriter writer = System.IO.File.CreateText("file.txt"))
{
    writer.WriteLine("message");
}

Jonejonell

Answer 6

You can get the System.IO references by using the corefx git repository. This will make the StreamWrite(string) constructor you are looking for available.

This repository will also give you the Console.WriteLine(string) function you are looking for.

Censor

Sounds good, but I'm still stuck. I've installed and built CoreFX. Now what? Do I add it as a reference in a Visual Studio project? Do I prep and run my program using dotnet commands? I've added a code snippet to my original question to clarify what I'd like to accomplish as a first step. – Hypothec

Once it is built you need to add it as a reference yes, then you will be able to include the System.IO library by putting in a using at the top of the file using System.IO; – Censor

There may be a way to do this, but every attempt I made to include references to CoreFX conflicted with .NET Core. – Hypothec

Answer 7

Here is an async FileWriter class.

using System.IO;
public static class AsyncFileWriter
{
    public static async Task WriteToFile(string content)
    {
        var logPath = @"SOME_PATH\log.txt";
        using (var writer = File.CreateText(logPath))
        {
            await writer.WriteLineAsync(content); 
        }
    }
}

Sirkin