How to count unique characters in a string using c#?

static int GetCharacterCount(string inputString)
{
Dictionary charactersDictinary = new Dictionary();
if (string.IsNullOrEmpty(inputString))
{
throw new NullReferenceException();
}
for (int i = 0; i < inputString.Length; i++)
{
if (charactersDictinary.ContainsKey(inputString[i]))
{
i++;
}
else
{
charactersDictinary.Add(inputString[i], i);
}

}
return charactersDictinary.Count;
}