Get number of occurrences of character in an array is even or odd ?

I propose one line to get whether the number of occurrences of 'X' in the string is odd or even. It uses Linq and a conditional assignment.

string s = "DEXWRXWXQOX";
string result = ((s.Where(c => c == 'X').Count() & 1) == 0) ? "even" : "odd";

If you want to return recordset of characters with number of occurences of each of them and information about even/odd, try this:

char[] letters = { 'D', 'E', 'X', 'W', 'R', 'X', 'W', 'X', 'Q', 'O', 'X' };

var result = letters.GroupBy(a=>a)
    .Select(g=>new
        {
            Character = g.Key,
            Count = g.Count(),
            IsEvenOrOdd = g.Count() % 2==0 ? "Even" : "Odd"
        });

Result:

Character Count IsEvenOrOdd
D         1         Odd
E         1         Odd
X         4         Even
W         2         Even
R         1         Odd
Q         1         Odd
O         1         Odd

Just to be different from C. Pallini and Matt T. Heffron :)

My response to the interviewer's question would be: there is no way to determine the number of objects in a set of objects which match some criterion without a test which compares each member using the criterion, and, that, by definition is a test of equality that requires some form of conditional evaluation, i.e., use of 'if.

And, then, perhaps, I would have written a short piece of code using Linq:

// required
// using System.Linq

private bool IsEven(char letter)
{
    return letters.Count(ch => ch == letter)%2 == 0;
}

Limiting to uppercase letters:

public static void Main()
  {
    char[] a = { 'D', 'E', 'X', 'W', 'R', 'X', 'W', 'X', 'Q', 'O', 'X' };
    bool [] odd = new bool[26];
    foreach (char c in a)
      odd[c-'A'] = !odd[c-'A'];
    for (int n = 0; n < 26; ++n)
      Console.WriteLine("{0} occurrences number is {1}. ", (char)(n + 'A'), odd[n] ? "odd" : "even");

  }

Just to be different from CPallini:

public static void Main()
{
  string a = "DEXWRXWXQOX";
  Dictionary<char, bool> odd = new Dictionary<char, bool>();
  foreach (char c in a)
  {
    bool codd;
    odd[c] = odd.TryGetValue(c, out codd) && !codd;
  }
  foreach (char c in odd.Keys)
    Console.WriteLine("{0} occurrences number is {1}. ", c, odd[c] ? "odd" : "even");
}

OR

private static readonly string[] EO = new string[] { "even", "odd" };
public static void Main()
{
  string a = "DEXWRXWXQOX";
  Dictionary<char, int> counts = new Dictionary<char, int>();
  foreach (char c in a)
  {
    int n;
    counts.TryGetValue(c, out n); // you don't HAVE to use the returned Boolean ;-)
    // if there was no existing entry for c in counts then n has been set to 0
    counts[c] = n + 1;
  }
  foreach (char c in counts.Keys)
    Console.WriteLine("{0} occurrences number is {1}. ", c, EO[counts[c] & 1]);  // L.S.B. is 1 if odd
}