How to count the number of words in given string using c#?

static int GetWordCount(string inputString, bool checkCaseSensitivity)
{
Dictionary charactersDictinary = new Dictionary();
if (string.IsNullOrEmpty(inputString))
{
throw new InvalidOperationException();
}
if (!checkCaseSensitivity)
{
inputString = inputString.ToLower();
}
string[] inputStringArray = inputString.Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
for (int i = 0; i < inputStringArray.Length; i++)
{
if (charactersDictinary.ContainsKey(inputStringArray[i]))
{
i++;
}
else
{
charactersDictinary.Add(inputStringArray[i], i);
}

}
return charactersDictinary.Count;
}