ReverseInPlace.cs

using System;
using System.Diagnostics;
using System.Linq;
using System.Text;

public class Program
{
  public static string ReverseString(string input)
  {
    if (input == null)
      throw new InvalidOperationException("Cannot reverse a null string");

    var values = input.ToArray<char>();

    for (int i = 0; i < values.Length / 2; i++)
    {
      var start = values[i];
      var end = values[values.Length - 1 - i];

      values[i] = end;
      values[values.Length - 1 - i] = start;
    }

    return new string(values);
  }

  public static string ReverseWordsInString(string input)
  {
    if (input == null)
      throw new InvalidOperationException("Cannot reverse a null string");

    var words = input.Split(' ');
    var reversed = new StringBuilder();

    foreach (var word in words)
    {
      reversed.Append(ReverseString(word)).Append(" ");
    }

    return reversed.ToString().Trim();
  }

  public static string ReverseWordsInStringLinqStyle(string input)
  {
    if (input == null)
      throw new InvalidOperationException("Cannot reverse a null string");

    return input
      .Split(' ')
      .Aggregate(new StringBuilder(), (result, next) => result.Append(ReverseString(next)).Append(" "))
      .ToString()
      .Trim();
  }

  public static void Main(string[] args)
  {
    Debug.Assert(ReverseString("abcd") == "dcba");
    Debug.Assert(ReverseString("abcde") == "edcba");
    Debug.Assert(ReverseString("abcdef") == "fedcba");
    Debug.Assert(ReverseString("abcdefg") == "gfedcba");

    Debug.Assert(ReverseWordsInString("abcdefg") == "gfedcba");
    Debug.Assert(ReverseWordsInString("a b c d") == "a b c d");
    Debug.Assert(ReverseWordsInString("ab bc") == "ba cb");

    Debug.Assert(ReverseWordsInStringLinqStyle("abcdefg") == "gfedcba");
    Debug.Assert(ReverseWordsInStringLinqStyle("a b c d") == "a b c d");
    Debug.Assert(ReverseWordsInStringLinqStyle("ab bc") == "ba cb");
  }
}