Best way to break long strings in C# source code

Question

I am wondering what is the \"best practice\" to break long strings in C# source code. Is this string

\"string1\"+
\"string2\"+
\"string3\"

con

Answer 1

How about the following extension method (which is inspired by common-tags oneLine method)...

using System;
using System.Text.RegularExpressions;
using static System.Text.RegularExpressions.RegexOptions;

namespace My.Name.Space
{
    public static class StringHelper
    {
        public static string AsOneLine(this string text, string separator = " ")
        {
            return new Regex(@"(?:\n(?:\s*))+").Replace(text, separator).Trim();
        }
    }
}

...in combination with the verbatim string literal used as such:

var mySingleLineText = @"
    If we wish to count lines of code, we should not regard them
    as 'lines produced' but as 'lines spent'.
".AsOneLine();

Note that spaces "inside" the string are kept intact, for example:

// foo      bar hello        world.
var mySingleLineText = @"
    foo      bar
    hello        world.
".AsOneLine();

If you don't want newlines to be substituted with spaces, then pass "" as argument to the extension method:

// foobar
var mySingleLineText = @"
    foo
    bar
".AsOneLine("");

Please note: This form of string concatenation is conducted at run time due to the helper-method involved (in contrast to concatenation via the + operator occurring at compile time, as also stated in the accepted answer). So if performance is an issue, go with the +. If you are dealing with long phrases and readability and "ease of use" is the focus, then the approach suggested above may be worth considering.