How to assign C# output message to a string variable?

var s = "Hello";
Console.WriteLine(s);

or

Func<string,string> ConsoleWriteLine = input => {Console.WriteLine(input); return input;};

and then

var x = ConsoleWriteLine("Hello"); // x == "Hello" now.

Let me ask you though: why? What are you trying to accomplish? We might help you with that instead ;)

EDIT: asker said he wants the Console.WriteLine to write to the file (I'm guessing: as well).

A way to accomplish that might be to have a custom TextWriter set to System.Console.Out, as described here: https://msdn.microsoft.com/en-us/library/system.console.out%28v=vs.110%29.aspx

The actual setting is using Console.SetOut (https://msdn.microsoft.com/en-us/library/system.console.setout%28v=vs.110%29.aspx).

The less insane way to do it would be to use (e.g.) NLog - then just write Log.Info("message") and have both FileAppender and ConsoleAppender set up :)

Check the NLog here: http://nlog-project.org/

Relevant config example from https://github.com/NLog/NLog/wiki/Tutorial#multiple-targets:

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

    <targets>
        <target name="logfile" xsi:type="File" fileName="file.txt" />
        <target name="console" xsi:type="Console" />
    </targets>

    <rules>
        <logger name="*" minlevel="Trace" writeTo="logfile" />
        <logger name="*" minlevel="Info" writeTo="console" />
    </rules>
</nlog>

You dont want to store it that way. First create the string variable:

string val

Then use the string for whatever you want:

Console.WriteLine(val);

If you want to log all things written to Console.Out, you can create your own LoggingConsoleWriter that extends TextWriter and writes all text to multiple TextWriters, one of which can be a log-file. Then, you can set the standard output to an instance of this class using the Console.SetOut()-method:

// This class writes to two different outputs, one of which can be the log
public class LoggingConsoleWriter : TextWriter
{
    private readonly TextWriter _output;
    private readonly TextWriter _log;

    public LoggingConsoleWriter(TextWriter output, TextWriter log)
    {
        _output = output;
        _log = log;
    }

    public override void Write(char value)
    {
        _output.Write(value);
        _log.Write(value);
    }

    public override Encoding Encoding => _output.Encoding;
} 

class Program
{
    static void Main(string[] args)
    {
        var logOutput = new StreamWriter(File.OpenWrite("log.txt"));
        var newOutput = new LoggingConsoleWriter(Console.Out, logOutput);

        // Redirect all Console.WriteX calls to write to newOutput
        Console.SetOut(newOutput);

        // This call will write to logOutput and the original Console.Out
        // through the LoggingConsoleWriter
        Console.WriteLine("Hello");
    }
}

Alternatively, if you just want to log to a string variable. You can instantiate a StringWriter instead, like so:

class Program
{
    static void Main(string[] args)
    {

        var sw = new StringWriter();
        var newOutput = new LoggingConsoleWriter(Console.Out, logOutput);

        // Redirect all Console.WriteX calls to write to newOutput
        Console.SetOut(newOutput);

        // This call will write to the StringWriter and the original Console.Out
        // through the LoggingConsoleWriter
        Console.WriteLine("Hello");

        // Retrieve the currently written values
        string output = sw.GetStringBuilder().ToString();
    }
}

Here's a quick and dirty way to accomplish what you're trying to do using TraceListeners:

public static void WriteToLog(string _logstring)
{
            string filepath = @"C:\myFilePath\fileName"

            Trace.Listeners.Clear();

            TextWriterTraceListener twtl = new TextWriterTraceListener(filepath + ".log");
            ConsoleTraceListener ctl = new ConsoleTraceListener(false);

            Trace.Listeners.Add(twtl);
            Trace.Listeners.Add(ctl);
            Trace.AutoFlush = true;
            Trace.WriteLine(_logstring);
}

Then elsewhere in your code you can call this method like this:

WriteToLog("This will be written to a file and print on the console");