Is there a best way to insert variables into strings?

Question

I am relatively new to programming but have noticed a lot of people when creating strings using variables do something like the below to put a variable into a string. I am curious at the difference/reason why this seems to be preferred by the senior developers I know and to also get opinions on why to use one over the other.

var x = "world";
Console.WriteLine("Hello {0}",x);

Is there an advantage or specific reason as to why the above is used as opposed to this:

var x = "world";
Console.WriteLine("Hello " + x);

Or this

var x = "world";
Console.WriteLine($"Hello {x}");

Answer 1

Rather than ask, test it:

using System;

public static class Program 
{
    public static void Main() 
    {
        X();
        Y();
        Z();
    }

    public static void X()
    {
        var x = "world";
        Console.WriteLine("Hello {0}", x);
    }

    public static void Y()
    {
        var x = "world";
        Console.WriteLine("Hello " + x);
    }

    public static void Z()
    {
        var x = "world";
        Console.WriteLine($"Hello {x}");    
    }
}

Look at the IL produced from the different approaches to see what difference it makes.

The "Hello " + x approach invokes string.Concat then calls WriteLine. You could then go off and look at the source of WriteLine and you'd find it does a string.Concat anyway.

So in short, there's no real difference and you shouldn't expect there to be: the compiler will tend to sort such things out for you anyway and thus worrying about it is just premature micro-optimising on your part, which is behaviour that should be avoided.

And neither is the ideal way to do it anyway. The third approach in my code above,

var x = "world";
Console.WriteLine($"Hello {x}");    

is by far the neatest and clearest way to express it.

Answer 2

It's simply that it's easier to read (and write) for longer/more complicated strings or strings where the variable is somewhere in the middle of the string.

Console.WriteLine("Hello, {0}. Happy {1}. The time is {2}.", name, dayOfWeek, time);

vs.

Console.WriteLine("Hello, " + name + ". Happy " + dayOfWeek + ". The time is " + time + ".");

For trivial examples where a variable is just appended to a string it makes very little difference.