Function Kata "Count characters"

Develop a function that counts the number of characters in a string. The input of the function is a stringthe output of a dictionary of char to int:

IDictionary CountCharacters(string input)

The string "This must not be" returns the following result:

D:1, a:2, s:2, _:3, d:1, r:1, f:1, n:2, i:2, c:1, h:1, e:1, t:1

(The character "_" stands for a space.)

Variation #1

As a variation, upper and lower case letters can be treated equally. The string "This must not be" then provides the following result:

d:2, a:2, s:2, _:3, r:1, f:1, n:2, i:2, c:1, h:1, e:1, t:1