Firstly you don't need to call ToCharArray as a string can already be indexed as a char array, so this will save you an allocation.
The next optimisation is to use a StringBuilder to prevent unnecessary allocations (as strings are immutable, concatenating them makes a copy of the string each time). To further optimise this we pre-set the length of the StringBuilder so it won't need to expand its buffer.
public string Reverse(string text)
{
if (string.IsNullOrEmpty(text))
{
return text;
}
StringBuilder builder = new StringBuilder(text.Length);
for (int i = text.Length - 1; i >= 0; i--)
{
builder.Append(text[i]);
}
return builder.ToString();
}
Edit: Performance Data
I tested this function and the function using Array.Reverse with the following simple program, where Reverse1 is one function and Reverse2 is the other:
static void Main(string[] args)
{
var text = "abcdefghijklmnopqrstuvwxyz";
// pre-jit
text = Reverse1(text);
text = Reverse2(text);
// test
var timer1 = Stopwatch.StartNew();
for (var i = 0; i < 10000000; i++)
{
text = Reverse1(text);
}
timer1.Stop();
Console.WriteLine("First: {0}", timer1.ElapsedMilliseconds);
var timer2 = Stopwatch.StartNew();
for (var i = 0; i < 10000000; i++)
{
text = Reverse2(text);
}
timer2.Stop();
Console.WriteLine("Second: {0}", timer2.ElapsedMilliseconds);
Console.ReadLine();
}
It turns out that for short strings the Array.Reverse method is around twice as quick as the one above, and for longer strings the difference is even more pronounced. So given that the Array.Reverse method is both simpler and faster I'd recommend you use that rather than this one. I leave this one up here just to show that it isn't the way you should do it (much to my surprise!)