TwoSum.cs

public static int[] TwoSum(int[] nums, int target)
{
  if (nums.Length < 2)
      return new int[2];

  var dic = new Dictionary();
  for (var i = 0; i < nums.Length; i++)
  {
      if (dic.ContainsKey(target - nums[i]))
      {
          return new int[] { (int)dic[target - nums[i]], i };
      }
      else
      {
          dic.Add(nums[i], i);
      }
  }
  return new int[2];
}